Negative electrode active material, negative electrode, non-aqueous electrolyte secondary battery, and method for manufacturing negative electrode active material

Abstract

To improve at least one of the initial efficiency, the battery capacity and the cycle characteristics of a non-aqueous electrolyte secondary battery.SOLUTION: A negative electrode active material for a non-aqueous electrolyte secondary battery according to an embodiment comprises porous composite particles containing silicon and silicon oxide SiOx (0<x≤2), where some or all of the pores of the porous composite particles are filled with carbon, and some or all of the surfaces of the porous composite particles are coated with carbon.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] Embodiments of the present invention relate to a negative electrode active material, a negative electrode, a non-aqueous electrolyte secondary battery, and a method for manufacturing a negative electrode active material.

Background Art

[0002] In recent years, a technique of adding silicon oxide to a negative electrode active material of a lithium-ion secondary battery has been developed. For example, Patent Document 1 describes a technique of adding silicon oxide pre-doped with lithium to a negative electrode active material.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in a negative electrode material to which silicon oxide is added, the initial efficiency often decreases due to the lithium silicate phase formed during charging. Further, when lithium is pre-doped into silicon oxide as in Patent Document 1, the battery capacity may decrease due to the formation of the lithium silicate phase.

[0005] The problem to be solved by the present invention is to provide a negative electrode active material, a negative electrode, a non-aqueous electrolyte secondary battery, and a method for manufacturing a negative electrode active material that can improve at least one of the initial efficiency, battery capacity, and cycle characteristics of a non-aqueous electrolyte secondary battery.

Means for Solving the Problems

[0006] The inventor conceived that by reducing silicon oxide with metallic magnesium Mg, the amount of Si contributing to the battery capacity could be increased, and at the same time, could magnesium oxide MgO generated simultaneously be easily removed? As a result of intensive studies, the inventor has found that by going through a predetermined process starting with the reduction treatment with metallic magnesium Mg, the initial efficiency, battery capacity, and / or cycle characteristics of a non-aqueous electrolyte secondary battery can be improved.

[0007] The present invention may include the following aspects. [1] Porous composite particles containing silicon and silicon oxide SiO x (0 < x ≦ 2), wherein part or all of the pores of the porous composite particles are filled with carbon, and part or all of the surface of the porous composite particles is coated with carbon, a negative electrode active material for a non-aqueous electrolyte secondary battery. [2] The negative electrode active material according to [1], wherein the porous composite particles further contain magnesium silicate oxide. [3] The negative electrode active material according to [2], wherein the magnesium silicate oxide contains at least one of MgSiO3 and Mg2SiO4. [4] The negative electrode active material according to [2], wherein the molar ratio of Si atoms to Mg atoms (Si / Mg) in the negative electrode active material is 10 or more and 50 or less. [5] The negative electrode active material according to any one of [1] to [4], wherein the porous composite particles do not contain magnesium oxide MgO at least on the surface. [6] The total amount of carbon filling the pores and carbon coating the surface of the porous composite particles is 5% by mass or more and 20% by mass or less based on the porous composite particles, the negative electrode active material according to any one of [1] to [5]. [7] The average particle diameter of the porous composite particles is 100 nm or more and 20 μm or less, the negative electrode active material according to any one of [1] to [6]. [8] A negative electrode current collector, a negative electrode active material layer formed on the negative electrode current collector and containing the negative electrode active material according to any one of [1] to [7], and a negative electrode comprising the same. A non-aqueous electrolyte secondary battery comprising the negative electrode described in [9] and [8].

[10] Silicon oxide SiO a A reduction step of reacting (0 < a ≤ 2) with metallic magnesium Mg, An acid treatment step of treating the material after the reaction with an acid, A carbon deposition step of depositing carbon on the material after the acid treatment, A method for producing a negative electrode active material for a non-aqueous electrolyte secondary battery, including:

[11] The method according to

[10] , wherein in the acid treatment step, porous composite particles having pores on the surface and inside are formed.

[12] The method according to

[11] , wherein in the carbon deposition step, a part or all of the pores are filled with carbon.

[13] The method according to any one of

[10] to

[12] , wherein in the reduction step, the molar ratio of Si atoms to Mg atoms (Si / Mg) is 0.1 or more and 3 or less.

[14] The method according to any one of

[10] to

[13] , wherein in the acid treatment step, the amount of substance of the acid added to 1 g of the material obtained in the reduction step is 1 mmol or more and 100 mmol or less.

[15] The method according to any one of

[10] to

[14] , wherein the carbon deposition step is performed by vapor-depositing carbon at 500°C or higher and 900°C or lower. [Effects of the Invention]

[0008] According to the present invention, there can be provided a negative electrode active material, a negative electrode, a non-aqueous electrolyte secondary battery, and a method for producing a negative electrode active material, which can improve at least one of the initial efficiency, battery capacity, and cycle characteristics of a non-aqueous electrolyte secondary battery in a silicon oxide-based negative electrode active material. [Brief Description of the Drawings]

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Mode for Carrying Out the Invention

[0010] Hereinafter, the negative electrode active material, negative electrode, non-aqueous electrolyte secondary battery, and method for manufacturing the negative electrode active material of the embodiment will be described. Note that the following embodiments show one aspect of the present invention, do not limit the present invention, and can be arbitrarily changed within the scope of the technical idea of the present invention. In addition, each configuration and each feature of the embodiment can be arbitrarily combined.

[0011] [Non-aqueous Electrolyte Secondary Battery] One embodiment of the present invention relates to a non-aqueous electrolyte secondary battery. The non-aqueous electrolyte secondary battery according to this embodiment includes a negative electrode, a positive electrode, a separator interposed between the negative electrode and the positive electrode, and a non-aqueous electrolyte. Specific examples of the secondary battery include lithium-ion secondary batteries having advantages such as high energy density, discharge voltage, and output stability.

[0012] Hereinafter, mainly taking a lithium-ion secondary battery as an example, the present invention is not limited to lithium-ion secondary batteries and can be applied to various non-aqueous electrolyte secondary batteries.

[0013] The lithium-ion secondary battery according to an embodiment of the present invention includes a negative electrode, a positive electrode, a separator interposed between the negative electrode and the positive electrode, and a non-aqueous electrolyte. Further, the lithium-ion secondary battery may selectively include a battery case that houses an electrode assembly composed of the negative electrode, the positive electrode, and the separator, and a sealing member that seals the battery case.

[0014] [Negative electrode] The negative electrode includes a negative electrode current collector and a negative electrode active material layer formed on one or both surfaces of the negative electrode current collector. The negative electrode active material layer may be formed over the entire surface of the negative electrode current collector or only on a part thereof.

[0015] (Negative electrode current collector) The negative electrode current collector used for the negative electrode is not particularly limited as long as it does not induce a chemical change in the battery and has conductivity. For example, as the negative electrode current collector, copper; stainless steel; aluminum; nickel; titanium; fired carbon; a material obtained by surface treatment of copper or stainless steel with carbon, nickel, titanium, silver, etc.; an aluminum-cadmium alloy, etc. can be used.

[0016] The negative electrode current collector may have a thickness of 3 μm or more and 500 μm or less. Fine irregularities can also be formed on the surface of the negative electrode current collector to enhance the adhesive force with the negative electrode active material. The negative electrode current collector can have various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven fabric body, etc.

[0017] (Negative electrode active material layer) The negative electrode active material layer may contain a negative electrode active material, a binder, a conductive agent, and an optional additive. For example, the negative electrode active material layer is formed by applying a negative electrode active material slurry in which a mixture of a negative electrode active material, a binder, and a conductive agent is dissolved or dispersed in a solvent to the negative electrode current collector, followed by drying and rolling, or by casting the above negative electrode active material slurry on another support and then laminating a film obtained by peeling the support on the negative electrode current collector. The above mixture may further contain a dispersant, a filler, and other optional additives as necessary.

[0018] (Negative electrode active material) In the lithium ion secondary battery according to the embodiment, the negative electrode active material includes porous composite particles containing silicon and silicon oxide SiO x (0 < x ≤ 2). Part or all of the pores of the porous composite particles are filled with carbon, and part or all of the surface of the porous composite particles is coated with carbon. Note that the negative electrode active material layer may further include a negative electrode active material different from the porous composite particles. In this specification, all substances contributing to the charge-discharge reaction in the negative electrode are referred to as "negative electrode active material".

[0019] In this specification, "coated with carbon" means that the outer surface of the particle is at least partially physically covered with carbon. Another material (for example, another coating layer) may exist between the outer surface of the particle and the carbon covering it. In this specification, "filled with carbon" means that carbon exists in the pores inside the particle. It is not necessarily the case that the entire inside of the pores is densely filled with carbon, and there may be a portion where the inner wall of the pores does not contact the carbon. Also, another material (for example, a layer covering the inner wall of the pores) may exist between the inner wall of the pores and the carbon.

[0020] The negative electrode active material may be contained in an amount of 80% by mass or more and 99% by mass or less based on the total mass of the negative electrode active material layer.

[0021] (Porous composite particles) The porous composite particles are particles composed of a plurality of types of materials and have a large number of pores inside. However, as described above, part or all of the pores of the porous composite particles are filled with carbon. Therefore, the porous composite particles may not contain substantial voids inside (for example, when all of the pores are filled with carbon, the porous composite particles do not contain voids inside).

[0022] The porous composite particles (excluding the carbon on the surface and in the pores) may be contained in an amount of, for example, 0.1% by mass or more and 99% by mass or less, 1% by mass or more and 90% by mass or less, 5% by mass or more and 50% by mass or less, or 10% by mass or more and 30% by mass or less based on the total mass of the negative electrode active material.

[0023] Silicon (Si) and silicon oxide (SiO) contained in the porous composite particles x are, for example, in the form of Si fine particles dispersed in a matrix of amorphous silicon oxide (SiO) x having a composite structure. The ratio x of oxygen to silicon in silicon oxide (SiO) is 0 < x ≤ 2, preferably 0.5 ≤ x ≤ 1.6, more preferably 0.8 ≤ x ≤ 1.5. x

[0024] Since the silicon (Si) fine particles have a lithium storage capacity, they can be responsible for the charge and discharge reactions in the negative electrode. The silicon oxide matrix that wraps the Si fine particles can alleviate the volume expansion and contraction of the Si fine particles accompanying the lithium storage and discharge.

[0025] The total amount of the silicon phase and the silicon oxide phase contained in the porous composite particles is, for example, 50% by mass or more and 99% by mass or less, 60% by mass or more and 90% by mass or less, or 70% by mass or more and 80% by mass or less based on the total mass of the porous composite particles. If it is 50% by mass or more, a sufficient battery capacity can be obtained.

[0026] The porous composite particles may further contain magnesium silicate oxide. For example, the magnesium silicate oxide includes, but is not limited to, at least one of MgSiO3 and Mg2SiO4.

[0027] The total amount of the magnesium silicate oxide contained in the porous composite particles is, for example, 0.1% by mass or more and 20% by mass or less, 0.5% by mass or more and 10% by mass or less, or 1% by mass or more and 5% by mass or less based on the total mass of the porous composite particles. If it is 0.1% by mass or more, the magnesium silicate oxide may alleviate the volume expansion of the silicon phase. If it is 20% by mass or less, the effect of sufficient increase in battery capacity by the silicon phase can be obtained.

[0028] ​When the porous composite particles contain magnesium silicate oxide, the molar ratio of Si atoms to Mg atoms (Si / Mg) in the negative electrode active material is, for example, 10 or more and 50 or less, preferably 15 or more and 40 or less, and more preferably 20 or more and 30 or less. If it is 10 or more, the amount of Si is relatively large, so a large energy density can be obtained. If it is 50 or less, a sufficient reduction reaction is considered to occur.

[0029] The porous composite particles may further contain magnesium oxide MgO and / or metallic magnesium Mg. These may correspond to the components remaining after the reduction treatment or acid treatment described later. However, preferably, the porous composite particles do not contain magnesium oxide MgO and / or metallic magnesium Mg at least on the surface. Here, the "surface" means the surface of the porous composite particles located under the carbon coating. That is, "not containing magnesium oxide MgO and / or metallic magnesium Mg on the surface" means that magnesium oxide MgO and / or metallic magnesium Mg do not exist immediately under the carbon coating of the porous composite particles. Preferably, the porous composite particles do not contain magnesium oxide MgO and / or metallic magnesium Mg in contact with the carbon coating. More preferably, the porous composite particles do not contain magnesium oxide MgO and / or metallic magnesium Mg.

[0030] The amount of magnesium oxide MgO contained in the porous composite particles is, for example, 5% by mass or less, 1% by mass or less, 0.1% by mass or less, or 0.01% by mass or less based on the total mass of the porous composite particles. The amount of metallic magnesium Mg contained in the porous composite particles is, for example, 5% by mass or less, 1% by mass or less, 0.1% by mass or less, or 0.01% by mass or less based on the total mass of the porous composite particles.

[0031] The porous composite particles can preferably be composed of Si, O, and Mg elements. However, the porous composite particles may further contain materials other than the above (including inevitable impurities).

[0032] The amount of silicon atoms contained in the porous composite particles (the total amount of Si atoms in all silicon-containing materials such as a silicon phase, a silicon oxide phase, and a magnesium silicate oxide phase) is, based on the total mass of the porous composite particles, for example, 10% by mass or more and 90% by mass or less, 30% by mass or more and 80% by mass or less, or 50% by mass or more and 70% by mass or less.

[0033] The amount of magnesium atoms contained in the porous composite particles (the total amount of Mg atoms in all magnesium-containing materials such as a magnesium silicate oxide phase, a magnesium oxide MgO phase, and a metallic magnesium Mg phase) is, based on the total mass of the porous composite particles, for example, 0.1% by mass or more and 20% by mass or less, 0.5% by mass or more and 10% by mass or less, or 1% by mass or more and 5% by mass or less.

[0034] The average particle size of the porous composite particles is, for example, 100 nm or more and 20 μm or less, preferably 200 nm or more and 15 μm or less, and more preferably 500 nm or more and 10 μm or less. In the present specification, the "average particle size" means the particle size at the integrated value of 50% in the particle size distribution measured by the laser diffraction scattering method, that is, the median diameter (D50). The pores have an irregular cross-sectional shape, and some are close to spherical, while others are elongated (with a large aspect ratio). The size of the pores is not particularly limited. For example, when they are close to spherical, the average diameter is 1 nm or more and 500 nm or less, and when the pores have a large aspect ratio, the length of the short side is 1 nm or more and 500 nm or less.

[0035] The arrangement of the pores in the porous composite particles does not have to be spatially uniform. For example, since the pores can be formed by removing a component constituting a part of the composite particles by an acid treatment described later, the pores inside the particles are connected to the particle surface. As a result, more pores inside the particles can exist closer to the surface than to the center of the particles. However, since the arrangement and shape of the pores can vary depending on the production conditions of the porous composite particles including the acid treatment, the above examples are not limiting, and the pores can have any arrangement and shape.

[0036] (Carbon Deposition on the Anode Active Material) As described above, the surface of the porous composite particles is at least partially coated with carbon, and the pores of the porous composite particles are at least partially filled with carbon. In this specification, these carbons are collectively referred to as "deposited carbon".

[0037] The deposited carbon can impart electrical conductivity to the porous composite particles. Further, by coating the surface of the porous composite particles with the deposited carbon, side reactions between the porous composite particles and other materials (such as an electrolyte) can be suppressed. Furthermore, by filling the pores of the porous composite particles with carbon, the possibility of cracks starting from the pores due to the volume expansion and contraction of silicon accompanying charge and discharge can be reduced.

[0038] The carbon material constituting the deposited carbon can include graphites such as natural graphite and artificial graphite; carbon fibers such as mesocarbon microbeads (MCMB), carbon nanotubes, and carbon nanofibers; carbon blacks such as ketjen black, Denka black, and acetylene black; graphene or graphene oxide; or mixtures thereof, etc. However, in order to fill the pores deep inside the fine particles with carbon, it is preferable that it is a film or a deposit using a chemical vapor deposition method (CVD) with acetylene gas or methane gas as a raw material.

[0039] The amount of the deposited carbon formed on the surface of the porous composite particles, that is, the total amount of carbon filling the pores and coating the surface of the porous composite particles, is, based on the porous composite particles (excluding the deposited carbon portion), for example, 5% by mass or more and 20% by mass or less, preferably 6% by mass or more and 18% by mass or less, more preferably 8% by mass or more and 15% by mass or less. If it is 5% by mass or more, it is considered that the filling of the pores with carbon proceeds suitably. If it is 20% by mass or less, a certain level of battery capacity can be obtained, and aggregation between the porous composite particles via the deposited carbon can be suppressed. However, the amount of carbon is not limited to the above range, and even outside the above range, even if the battery performance slightly deteriorates, it can function as a secondary battery.

[0040] The deposited carbon covers, for example, 50% or more and 100% or less, 60% or more and 99% or less, or 70% or more and 95% or less of the surface area of the porous composite particles (excluding the pores inside the particles). The deposited carbon fills, for example, 50% by volume or more and 100% by volume or less, 60% by volume or more and 99% by volume or less, or 70% by volume or more and 95% by volume or less of the total volume of the pores inside the porous composite particles.

[0041] The average thickness of the deposited carbon formed on the surface of the porous composite particles can be, for example, 10 nm or more and 10 μm or less, 50 nm or more and 5 μm or less, 100 nm or more and 2 μm or less, or 200 nm or more and 1 μm or less.

[0042] (Carbon material) The negative electrode active material layer may further contain a carbon material as a negative electrode active material different from the above-mentioned porous composite material on which the deposited carbon is formed. For example, the carbon material may be contained in the negative electrode active material layer in the form of powder. When the carbon material is included in the negative electrode active material, the carbon material may generally include any carbon material that can be used as a negative electrode active material of a non-aqueous electrolyte secondary battery. For example, the carbon material can include one or a mixture of two or more of natural graphite, artificial graphite, graphitized carbon fiber, and amorphous carbon, but is not limited thereto. A composite with an element other than carbon can also be used. Note that the carbon material may be any of low-crystalline carbon and high-crystalline carbon. Representative low-crystalline carbons are soft carbon and hard carbon, and representative high-crystalline carbons are amorphous, plate-like, flaky, spherical, or fibrous natural graphite or artificial graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitch, and high-temperature calcined carbon such as petroleum and coal-based cokes.

[0043] When the negative electrode active material includes a carbon material in addition to the above-mentioned porous composite particles, the mass ratio of the porous composite particles to the carbon material in the negative electrode active material can be, for example, 1:99 to 50:50, preferably 5:95 to 30:80, and more preferably 8:92 to 20:80.

[0044] (Binder) The binder is added as a component that promotes the binding between the active material and the conductive agent, and the binding between the current collector and the like. Examples of the binder include polyvinylidene fluoride (PVdF), polyvinyl alcohol (PVA), polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), polyacrylic acid, acrylamide, polyimide, fluororubber, various copolymers thereof, etc. A mixture of one or more of these can be used, but it is not limited thereto.

[0045] The content of the binder can be 0.1% by mass or more and 30% by mass or less based on the total mass of the negative electrode active material layer. The content of the binder is preferably 0.5% by mass or more and 20% by mass or less, and more preferably 1% by mass or more and 10% by mass or less. When the content of the binder polymer satisfies the above range, sufficient adhesion within the electrode can be imparted while preventing the deterioration of the capacity characteristics of the battery.

[0046] (Conductive agent) The conductive agent is not particularly limited as long as it is an electrically conductive material that does not induce chemical changes. Examples of the conductive agent include carbon-based materials such as artificial graphite, natural graphite, single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, carbon black, acetylene black, ketjen black, denka black, thermal black, channel black, furnace black, lamp black, and carbon fibers; metal powders and metal fibers such as aluminum, tin, bismuth, silicon, antimony, nickel, copper, titanium, vanadium, chromium, manganese, iron, cobalt, zinc, molybdenum, tungsten, silver, gold, lanthanum, ruthenium, platinum, and iridium; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyaniline, polythiophene, polyacetylene, polypyrrole, and polyphenylene derivatives. One or a mixture of two or more of these can be used, but it is not limited thereto.

[0047] The content of the conductive agent can be 0.1% by mass or more and 30% by mass or less based on the total mass of the negative electrode active material layer. The content of the conductive agent is preferably 0.5% by mass or more and 15% by mass or less, and more preferably 0.5% by mass or more and 10% by mass or less. When the content of the conductive agent satisfies the above range, it is advantageous in that sufficient conductivity can be imparted and the battery capacity can be ensured without reducing the amount of the negative electrode active material.

[0048] (Thickening agent) The negative electrode active material slurry when applying the negative electrode active material to the negative electrode current collector can contain a thickening agent. Specifically, the thickening agent can be a cellulose-based compound such as carboxymethyl cellulose (CMC). The thickening agent can be contained in an amount of, for example, 0.5% by mass or more and 10% by mass or less based on the total mass of the negative electrode active material layer.

[0049] (Solvent) The solvent used in the negative electrode active material slurry is not particularly limited as long as it is generally used in the production of the negative electrode. Examples of the solvent include N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), isopropyl alcohol, acetone, water, etc., and one or a mixture of two or more of these can be used, but it is not limited thereto.

[0050] [Method for manufacturing negative electrode] The method for manufacturing a negative electrode for a lithium ion secondary battery according to the embodiment includes the following steps. (1) Step of manufacturing a negative electrode active material (2) Step of manufacturing a negative electrode active material slurry from the negative electrode active material (3) Step of manufacturing a negative electrode from the negative electrode active material slurry

[0051] (1) Manufacturing a negative electrode active material When the manufacturing step (1) of the negative electrode active material is subdivided as the "method for manufacturing the negative electrode active material", it includes the following steps. (a) Reduction step: React silicon oxide SiO a (0 < a ≤ 2) with metallic magnesium Mg. (b) Acid treatment step: Treat the material after the reaction in (1) with an acid. (c) Carbon deposition step: Deposit carbon on the material after the acid treatment in (b).

[0052] Figure 1 is a schematic diagram showing the method for manufacturing a negative electrode active material according to this embodiment. Hereinafter, description will be made with reference to Figure 1, but Figure 1 is merely a conceptual diagram for easy understanding and does not accurately represent the microscopic state of actual particles.

[0053] (a) Reduction step In the reduction step, metallic magnesium Mg acts as a reducing agent. Specifically, it is considered that the following chemical reaction proceeds between silicon oxide SiO a and metallic magnesium Mg. SiO a + aMg → Si + aMgO

[0054] The silicon oxide used as a raw material can be, for example, a powder of SiO a (0 < a ≤ 2). SiO a can have, for example, a structure in which Si fine particles are dispersed in the form of microcrystals or amorphous in an amorphous silicon oxide matrix as shown in Fig. 1(a). In the description of the above porous composite particles, for convenience, the Si fine particles and the silicon oxide SiO x matrix are separately described, but here the Si fine particles and the silicon oxide matrix are combined and collectively referred to as silicon oxide SiO a . That is, a is a value averaged over the entire Si fine particles and silicon oxide matrix. Here, 0 < a ≤ 2, preferably 0.5 ≤ a ≤ 1.6, and more preferably 0.8 ≤ a ≤ 1.5. For example, the raw material silicon oxide is silicon monoxide SiO (a = 1) or silicon dioxide SiO2. Also, the raw material silicon oxide powder may contain only SiO a having a specific value of a, or may be a mixture of two or more SiO a powders having different values of a.

[0055] The average particle diameter of the silicon oxide particles of the raw material is, for example, 100 nm or more and 20 μm or less, preferably 200 nm or more and 10 μm or less, more preferably 500 nm or more and 5 μm or less or 500 nm or more and 1 μm or less. When the average particle diameter is small, advantageously, the reaction points of the charge-discharge reaction relatively increase, and the battery capacity may increase, and the insertion and desorption of lithium ions in the particles can be easily performed, improving the life characteristics of the battery. If the average particle diameter is 100 nm or more, a sufficient specific surface area of the porous composite particles can be ensured, having suitable reactivity with the electrolyte, and the energy density of the battery can be increased to a certain extent. If the average particle diameter is 20 μm or less, the life characteristics due to volume expansion can be suppressed.

[0056] In the reduction step, the molar ratio of Si atoms to Mg atoms (Si / Mg) is 0.1 or more and 3 or less, preferably 0.3 or more and 2.5 or less, more preferably 0.5 or more and 2 or less, still more preferably 0.6 or more and 1.8 or less, and even more preferably 0.8 or more and 1.5 or less. If it is 0.1 or more, a certain amount of Si serving as a charge-discharge reaction point is contained, so that a certain energy density can be obtained. If it is 3 or less, a sufficient reduction reaction occurs and the battery capacity is sufficiently improved.

[0057] In addition to the above, magnesium silicate oxide can be formed as a by-product by the reaction of silicon oxide with metallic magnesium Mg or the generated magnesium oxide MgO. Examples of magnesium silicate oxide include MgSiO3 and Mg2SiO4.

[0058] The reaction conditions are arbitrary as long as the above reaction proceeds. In terms of simplicity of treatment, a method of mixing silicon oxide powder and metallic magnesium Mg powder and heating them is preferable, but it is not limited thereto. For example, these materials may be reacted in a liquid. Since a reduction reaction is carried out, it is preferably carried out under an inert gas (nitrogen, argon, etc.) atmosphere or a reducing atmosphere. The reaction temperature is, for example, 500°C or more and 1000°C or less, preferably 600°C or more and 900°C or less, more preferably 650°C or more and 850°C or less, and still more preferably 700°C or more and 800°C or less. The reaction pressure is not particularly limited. The reaction time is, for example, 10 minutes or more and 3 hours or less, preferably 30 minutes or more and 2 hours or less.

[0059] The above reduction reaction is presumed to proceed by, for example, the following mechanism. However, the following is only a presumption and does not restrict the present invention by theory. The raw material silicon oxide particles react with metallic magnesium Mg, and the Si fine particles in the silicon oxide matrix increase. At the same time, magnesium oxide MgO and magnesium silicate oxide are formed. As a result, as shown in Fig. 1(b), the silicon fine particles are silicon oxide SiO dispersed in the silicon oxide matrix. x, composite particles in which magnesium oxide MgO, magnesium silicate oxide, and unreacted metallic magnesium Mg are integrated and / or aggregated are obtained.

[0060] (b) Acid treatment step In the acid treatment step, the magnesium oxide MgO and unreacted metallic magnesium Mg produced in the reduction step react with protons to form magnesium ions Mg 2+ . Since the magnesium ions Mg 2+ can be removed by washing with water, as a result, magnesium oxide MgO and unreacted metallic magnesium Mg can be removed. MgO + 2H + → Mg 2+ + H2O Mg + 2H + → Mg 2+ + H2 The acid does not need to include those that can strongly dissolve silicon oxide (SiO x ) like hydrogen fluoride (HF).

[0061] The acid treatment step includes adding an acid to the product of the reduction step. The acid is, for example, an acid aqueous solution. The acid aqueous solution is, for example, an aqueous solution of a strong acid. The strong acid may be an inorganic acid or an organic acid, and examples include hydrochloric acid, nitric acid, sulfuric acid, hydrogen bromide, hydrogen iodide, sulfonic acid, etc. These may be arbitrarily combined. That is, the acid treatment step may include a step of reacting the material after the reduction step with one or more acids selected from the group consisting of hydrochloric acid, nitric acid, sulfuric acid, hydrogen bromide, hydrogen iodide, and sulfonic acid. For example, when hydrochloric acid is used, the following chemical reactions occur between magnesium oxide MgO, metallic magnesium Mg, and hydrochloric acid. MgO + 2HCl → MgCl2+ H2O Mg + 2HCl → MgCl2+ H2

[0062] The concentration of the aqueous acid solution is, for example, 0.1 M or more and 10 M or less, preferably 0.5 M or more and 5 M or less. If the concentration is 0.1 M or more, impurities can be sufficiently removed. If the concentration is 10 M or less, sufficient production efficiency can be maintained. The amount of the aqueous acid solution added to 1 g of the composite particles obtained in the reduction step is, for example, 1 mL or more and 100 mL or less, preferably 5 mL or more and 80 mL or less, and more preferably 10 mL or more and 50 mL or less. When the concentration of the aqueous acid solution is 1 M, the amount of the acid substance added to 1 g of the composite particles is, for example, 1 mmol or more and 100 mmol or less, preferably 5 mmol or more and 80 mmol or less, and more preferably 10 mmol or more and 50 mmol or less.

[0063] The reaction conditions are arbitrary as long as the above reaction proceeds. The reaction temperature is, for example, 10°C or more and 90°C or less, preferably room temperature or more and 50°C or less. The reaction pressure is not particularly limited. The reaction time is, for example, 1 hour or more and 48 hours or less, preferably 6 hours or more and 36 hours or less, and more preferably 12 hours or more and 30 hours or less.

[0064] The acid treatment of the composite particles is presumed to proceed by, for example, the following mechanism. However, the following is merely a presumption and does not restrict the present invention by theory. The acid that has come into contact with the composite particles after the reduction step first reacts with magnesium oxide MgO or metallic magnesium Mg on or near the surface of the composite particles. As a result of the magnesium oxide MgO or metallic magnesium Mg that has reacted with the acid dissolving out from the composite particles, pores are formed in the portions from which the magnesium oxide MgO or metallic magnesium Mg has escaped from the composite particles. The acid enters into these pores and further reacts with the magnesium oxide MgO or metallic magnesium Mg inside the particles. In this way, the magnesium oxide MgO or metallic magnesium Mg in the composite particles is decomposed and removed. As a result of the acid treatment, pores are formed inside the composite particles, so that, as shown in Fig. 1(c), porous composite particles having pores on the surface and inside are obtained.

[0065] Since magnesium silicate oxide has low reactivity with acids, at least a part of it can remain even after acid treatment. However, depending on the raw materials, acids, and reaction conditions used, part or all of the magnesium silicate oxide may be decomposed.

[0066] After adding an acid and reacting, substances dissolved in the acid can be removed by general washing. Through the washing and drying steps, a porous composite material can be obtained.

[0067] (c) Carbon deposition step In the carbon deposition step, carbon is deposited on the obtained porous composite particles to coat the surface of the porous composite particles with carbon and fill the inside of the pores of the porous composite particles with carbon. The carbon deposition treatment can be carried out by any generally used carbon deposition method, and the carbon deposition method is not particularly limited. The carbon deposition method may be either a dry method or a wet method. Examples of the former include methods of depositing carbon on a porous composite material by chemical vapor deposition (CVD). Examples of the latter include methods of mixing a carbon source and a porous composite material in a liquid and heating and drying.

[0068] For example, when performing carbon deposition such as the CVD method, in order to sufficiently fill the inside of the pores with carbon, it is preferably carried out at a relatively low temperature for a long time. For example, the temperature for carbon deposition is 500 °C or higher and 900 °C or lower, preferably 600 °C or higher and 800 °C or lower. For example, the time for carbon deposition is 10 minutes or longer and 24 hours or shorter, preferably 30 minutes or longer and 10 hours or shorter, more preferably 30 minutes or longer and 2 hours or shorter.

[0069] By forming a deposited carbon layer on the surface and inside pores of the porous composite particles, electrical conductivity is imparted to the porous composite particles and the surface of the porous composite particles is protected, so that at least one of the battery capacity, initial efficiency, and cycle characteristics can be improved.

[0070] Due to carbon deposition, as shown in Fig. 1(d), part or all of the surface of the porous composite particles is coated with carbon, and part or all of the pores inside the particles are filled with carbon. The porous composite particles thus formed with deposited carbon are used as the negative electrode active material.

[0071] Note that the method for manufacturing the negative electrode active material according to this embodiment does not require a step of actively forming pores in the above composite material. Instead, pores will be formed automatically as long as magnesium oxide MgO and metallic magnesium Mg are removed by the acid treatment step. For example, the method for manufacturing the negative electrode active material according to this embodiment does not include a known pore formation step (for example, a step of vaporizing the target material and depositing it on a metal plate).

[0072] (2) Manufacturing a negative electrode active material slurry from the negative electrode active material A solvent is added to the negative electrode active material obtained in (1) above. At this time, a conductive agent, a binder, a thickener, etc. may be added as necessary. By dissolving or dispersing the negative electrode active material, the conductive agent, the binder, the thickener, etc. in the solvent, a negative electrode active material slurry is obtained.

[0073] (3) Manufacturing a negative electrode from the negative electrode active material slurry After applying the negative electrode active material slurry to the negative electrode current collector and then drying and rolling, a negative electrode with a negative electrode active material layer formed on the negative electrode current collector can be manufactured.

[0074] As another method, for example, after casting the above negative electrode active material slurry on another support and then peeling the film obtained from the support, the film may be laminated on the negative electrode current collector to manufacture the negative electrode. Also, the negative electrode active material layer may be formed on the negative electrode current collector using any other arbitrary method.

[0075] [Positive electrode] In the lithium-ion secondary battery according to the embodiment, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on one or both surfaces of the positive electrode current collector. The positive electrode active material layer may be formed over the entire surface of the positive electrode current collector or only on a part thereof.

[0076] (Positive current collector) The positive current collector used for the positive electrode is not particularly limited as long as it does not induce chemical changes in the battery and has conductivity. For example, as the positive current collector, stainless steel; aluminum; nickel; titanium; fired carbon; those obtained by surface treatment with carbon, nickel, titanium, silver, etc. on the surface of aluminum or stainless steel can be used.

[0077] The positive current collector can have a thickness of 3 μm or more and 500 μm or less. Fine irregularities can also be formed on the surface of the positive current collector to enhance the adhesion to the positive electrode active material. The positive current collector can have various forms such as a film, sheet, foil, net, porous body, foam, non-woven fabric, etc.

[0078] (Positive electrode active material layer) The positive electrode active material layer can be formed, for example, by applying a positive electrode active material slurry in which a mixture of a positive electrode active material, a conductive agent, and a binder is dissolved and dispersed in a solvent to a positive current collector, and then drying and rolling. The above mixture can further contain a dispersant, a filler, and other optional additives as required.

[0079] The positive electrode active material can be contained in an amount of 80% by mass or more and 99% by mass or less based on the total mass of the positive electrode active material layer.

[0080] (Positive electrode active material) As the positive electrode active material, a compound capable of reversible insertion (intercalation) and desorption (deintercalation) of lithium can be used. Specific examples include, for example, lithium metal composite oxides containing one or more metals such as cobalt, manganese, nickel, copper, vanadium, and aluminum and lithium. More specifically, such lithium metal composite oxides include lithium-manganese-based oxides (for example, LiMnO2, LiMnO3, LiMn2O3, LiMn2O4, etc.); lithium-cobalt-based oxides (for example, LiCoO2, etc.); lithium-nickel-based oxides (for example, LiNiO2, etc.); lithium-copper-based oxides (for example, Li2CuO2, etc.); lithium-vanadium-based oxides (for example, LiV3O8, etc.); lithium-nickel-manganese-based oxides (for example, LiNi 1-z Mn z O2 (0 < z < 1), LiMn 2-z Ni z O4 (0 < z < 2), etc.); lithium-nickel-cobalt-based oxides (for example, LiNi 1-y Co y O2 (0 < y < 1), etc.); lithium-manganese-cobalt-based oxides (for example, LiCo 1-z Mn z O2 (0 < z < 1), LiMn 2-y Co y O4 (0 < y < 2), etc.); lithium-nickel-manganese-cobalt-based oxides (for example, Li(Ni x Co y Mn z )O2 (0 < x < 1, 0 < y < 1, 0 < z < 1, x + y + z = 1), Li(Ni x Co y Mn z )O4 (0 < x < 2, 0 < y < 2, 0 < z < 2, x + y + z = 2), etc.); lithium-nickel-cobalt-metal (M) oxides (for example, Li(Ni x Co y Mn z M w)O2 (M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, 0 < x < 1, 0 < y < 1, 0 < z < 1, 0 < w < 1, x + y + z + w = 1), etc.); examples include compounds in which the transition metal elements in these compounds are partially substituted with one or more other metal elements. The positive electrode active material layer can contain any one or more of these compounds. However, it is not limited to only these.

[0081] In particular, in terms of improving the capacity characteristics and stability of the battery, LiCoO2, LiMnO2, LiMn2O4, LiNiO2, lithium nickel manganese cobalt oxide (for example, Li(Ni 1 / 3 Mn 1 / 3 Co 1 / 3 )O2, Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.4 Mn 0.3 Co 0.3 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15 )O2, Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, etc.), lithium nickel cobalt aluminum oxide (for example, Li(Ni 0.8 Co 0.15 Al 0.05 )O2, etc.) are preferred.

[0082] (Binder and conductive agent) The types and contents of the binder and conductive agent used in the positive electrode active material slurry can be the same as those described for the negative electrode.

[0083] (Solvent) The solvent used in the positive electrode active material slurry is not particularly limited as long as it is generally used in the production of the positive electrode. Examples of the solvent include amine solvents such as N,N-dimethylaminopropylamine, diethylenetriamine, and N,N-dimethylformamide (DMF), ether solvents such as tetrahydrofuran, ketone solvents such as methyl ethyl ketone, ester solvents such as methyl acetate, amide solvents such as dimethylacetamide and 1-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), etc. A mixture of one or more of these can be used, but it is not limited thereto.

[0084] The amount of the solvent used is sufficient as long as it can dissolve or disperse the positive electrode active material, the conductive material, and the binder, and has a viscosity that can exhibit excellent thickness uniformity when applied to the positive electrode current collector, taking into account the coating thickness of the slurry and the production yield.

[0085] [Method for manufacturing positive electrode] The method for manufacturing a positive electrode for a lithium-ion secondary battery according to the embodiment includes a step of obtaining a positive electrode active material slurry by dissolving or dispersing a positive electrode active material in a solvent together with a binder, a conductive agent, a thickener, etc. as necessary, and a step of obtaining a positive electrode by forming a positive electrode active material layer on a positive electrode current collector by applying the positive electrode active material slurry on the positive electrode current collector in the same manner as the method for manufacturing a negative electrode.

[0086] [Separator] In the lithium ion secondary battery according to the embodiment, the separator separates the negative electrode and the positive electrode and provides a migration path for lithium ions, and can be used without particular limitation as long as it is usually used as a separator in a lithium ion secondary battery. In particular, those having a small resistance to ion migration of the electrolyte and excellent moisture retention ability of the electrolyte are preferable. For example, a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof can be used as the separator. Also, a normal porous nonwoven fabric, for example, a nonwoven fabric made of high melting point glass fibers or polyethylene terephthalate fibers can be used. Further, a separator coated with a ceramic component or a polymer substance may be used to ensure heat resistance or mechanical strength.

[0087] [Non-aqueous electrolyte] In the non-aqueous electrolyte secondary battery according to the embodiment, examples of the non-aqueous electrolyte include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid electrolytes, etc. that can be used in the manufacture of secondary batteries.

[0088] The non-aqueous electrolyte can contain an organic solvent and a lithium salt, and can further contain additives as necessary. Hereinafter, the liquid electrolyte is also referred to as "electrolyte solution".

[0089] The organic solvent can be used without particular limitation as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether and tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethyl alcohol and isopropyl alcohol; nitrile solvents such as R-CN (where R is a hydrocarbon group having a linear, branched, or cyclic structure of C2 to C20 and may contain a double bond, aromatic ring, or ether bond); amide solvents such as dimethylformamide; dioxolane solvents such as 1,3-dioxolane; sulfolane solvents, etc. One or a mixture of two or more of these can be used, but it is not limited thereto.

[0090] The lithium salt can be used without particular limitation as long as it can provide lithium ions used in the lithium-ion secondary battery. Examples of the lithium salt include 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. One or a mixture of two or more of these can be used, but it is not limited thereto. The lithium salt can be contained in the electrolyte, for example, at a concentration of 0.1 mol / L or more and 2 mol / L or less. When the concentration of the lithium salt is within this range, since the electrolyte has appropriate conductivity and viscosity, it can exhibit excellent electrolyte performance and lithium ions can move effectively.

[0091] Additives can be used as needed for the purposes of improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity. Examples of additives include haloalkylene carbonate-based compounds such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC), pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, triamide hexaline, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride, etc. One or a mixture of two or more of these can be used, but it is not limited thereto. The additive can be contained, for example, in an amount of 0.1% by mass or more and 15% by mass or less based on the total mass of the electrolyte.

[0092] [Method for manufacturing a non-aqueous electrolyte secondary battery] The non-aqueous electrolyte secondary battery according to the embodiment can be manufactured by interposing a separator (for example, a separation membrane) and an electrolytic solution between the negative electrode manufactured as described above and the positive electrode manufactured as described above. More specifically, a separator is disposed between the negative electrode and the positive electrode to form an electrode assembly, and after the electrode assembly is placed in a battery case such as a cylindrical battery case or a rectangular battery case, an electrolyte can be injected for manufacturing. Alternatively, after the above electrode assemblies are stacked, the resultant obtained by impregnating them with an electrolyte can be placed in a battery case and sealed for manufacturing.

[0093] The above battery case can adopt those commonly used in the art. The shape of the battery case can be, for example, cylindrical using a can, rectangular, pouch-shaped, or coin-shaped, etc.

[0094] The lithium-ion secondary battery according to the embodiment can be used not only as a power source for small devices but also as a unit cell of a medium- to large-sized battery module including a large number of battery cells and the like. Preferred examples of such medium- to large-sized devices include, but are not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, power storage systems, and the like.

[0095] [Effect] In recent years, silicon-based materials have often been considered for use in the negative electrode for increasing the capacity of lithium-ion secondary batteries. However, pure silicon Si has a significant reduction in life characteristics because the electron conduction path is interrupted due to cracks caused by expansion during charging. Therefore, a negative electrode active material in which a small amount of silicon monoxide is mixed with graphite has been put into practical use. Silicon monoxide functions to mitigate the expansion of Si and has a composite structure in which Si phases are dispersed in the matrix of the SiO x phase. However, when using silicon monoxide SiO, a lithium silicate phase formed during the first charge of the battery is irreversibly generated, resulting in a significant decrease in the discharge capacity relative to the charge capacity, and thus the initial efficiency is very low, at 65 - 70%. For this reason, it is difficult to balance with the initial efficiency of the positive electrode material when increasing the amount of silicon monoxide SiO in the negative electrode material.

[0096] To solve the above problems, a Li pre-doped SiO x material has been proposed. However, as a result of forming lithium silicate phases with various compositions depending on the doping amount, there have been cases where the capacity decreases, gas is generated due to elution of Si by a basic slurry, or the viscosity increases. For this reason, even if there are no problems in the processability of battery manufacturing at the laboratory level, there are problems such as being unable to coat on a mass production scale or pinholes being formed after drying of the electrode even if it can be coated, and practical application is still difficult.

[0097] According to the negative electrode active material according to the embodiment, the amount of silicon Si that contributes to increasing the capacity by reducing silicon oxide as a raw material is increased, and carbon is deposited in the pores remaining after removing the surface of the porous composite particles and the MgO phase and Mg phase, thereby imparting electrical conductivity to the porous composite particles and suppressing side reactions between the porous composite particles and the electrolyte. As a result, the battery capacity, charge-discharge efficiency, and cycle characteristics can be improved. Further, since all the materials constituting the porous composite particles are stable in water, there is an advantage that gas is not generated as in the conventional case.

[0098] When the negative electrode active material contains magnesium silicate oxide, the magnesium silicate oxide functions as a matrix of Si together with silicon oxide SiO x phase and can relieve the volume expansion of Si. Therefore, the cycle characteristics can be further improved.

Examples

[0099] Hereinafter, examples and comparative examples will be described, but the present invention is not limited thereto. Further, the considerations described below are merely exemplary speculations for assisting the understanding of the invention and do not limit the present invention in any way.

[0100] [Example 1] (Manufacture of negative electrode active material) Silicon monoxide SiO powder with an average particle size of 5 μm and metallic magnesium Mg powder with an average particle size of 300 μm were mixed at a mass ratio of 1:1. This mixture was put into a plasma sintering apparatus (manufactured by Microphase) and held at 800° C. for 1 hour in an inert gas atmosphere to advance the reduction reaction of SiO powder by Mg powder. By X-ray diffraction pattern (XRD) measurement and elemental analysis, it was confirmed that the product contained silicon Si, silicon oxide SiO x (0 < x ≦ 1), metallic magnesium Mg, magnesium oxide MgO, and magnesium silicate oxides MgSiO3 and Mg2SiO4.

[0101] 10 g of the above product was added to 200 mL of 2 M hydrochloric acid and stirred for 24 hours to remove the Mg phase and MgO phase. Thereafter, washing with water and drying were performed. Next, CVD treatment using ethylene gas was performed while rotating the furnace tube at 800 °C. From the mass ratio before and after the CVD treatment, it was confirmed that the product after the treatment contained 7% by mass of carbon. The particle size of these particles was adjusted by appropriately pulverizing and sieving so that the average particle size became 8 μm to obtain a negative electrode active material.

[0102] (Manufacture of coin cell) The obtained negative electrode active material, an aqueous dispersion of carbon black (CB) and single-walled carbon nanotubes (SWCNT) (solid content: 0.4%) as a conductive material, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were prepared at a mass ratio of 88.8:3.3:0.25:3.65:4.0. First, the negative electrode active material, CB, and CMC were mixed, and the SWCNT dispersion and water were added and kneaded. Finally, SBR was added and mixed to produce a negative electrode active material slurry.

[0103] The obtained slurry was uniformly coated on a copper foil, vacuum dried at 110 °C for 10 hours, cut out into a circle with a diameter of 13 mm, and a 2016-type coin cell using lithium metal as a counter electrode was manufactured.

[0104] [Example 2] A negative electrode active material was produced in the same manner as in Example 1 except that the concentration of hydrochloric acid with respect to the reduction product was twice that of Example 1 and washing with water was performed 5 times, and a coin cell was produced in the same manner.

[0105] [Example 3] A negative electrode active material was produced in the same manner as in Example 2 except that the mass ratio of silicon monoxide SiO powder to magnesium Mg powder was 2:1, and a coin cell was produced in the same manner.

[0106] [Example 4] A negative electrode active material was produced in the same manner as in Example 2, except that the mass ratio of silicon monoxide SiO powder to magnesium Mg powder was 4:1, and a coin cell was produced in the same manner.

[0107] [Example 5] A negative electrode active material was produced in the same manner as in Example 3, except that the amount of carbon coated by CVD treatment was increased to 11% by mass, and a coin cell was produced in the same manner.

[0108] [Example 6] A negative electrode active material was produced in the same manner as in Example 3, except that silicon monoxide SiO powder with an average particle size of 1 μm was used as a raw material, and a coin cell was produced in the same manner.

[0109] [Example 7] A negative electrode active material was produced in the same manner as in Example 6, except that the temperature during reduction using metallic magnesium Mg was changed from 800 °C to 680 °C, and a coin cell was produced in the same manner.

[0110] [Comparative Example 1] A negative electrode active material was produced in the same manner as in Example 1, except that 5% by mass of carbon was deposited on the silicon monoxide SiO powder used in Example 1 and used as the negative electrode active material, and a coin cell was produced in the same manner.

[0111] [Evaluation Example 1: SEM Observation and EDS Analysis] The negative electrode active material particles obtained in Example 5 were observed with a scanning electron microscope (SEM) capable of energy dispersive X-ray analysis (EDS). Fig. 2 is an SEM image of a cross-section of the negative electrode active material particles of Example 5. Figs. 3 to 6 are maps of Si atoms, C atoms, O atoms, and Mg atoms obtained by EDS analysis of the cross-section of the negative electrode active material particles of Example 5, respectively. In Figs. 3 to 6, the target atoms are present in the white portions. In the lightly colored portions of Fig. 2, mainly silicon Si or silicon monoxide SiO XOn the other hand, the dark-colored part corresponds to the pores generated by the acid treatment, and it is considered that mainly carbon C or magnesium silicate oxide is located there. From the comparison between Fig. 3 (Si map) and Fig. 4 (C map), it is considered that a large amount of C exists in the part where Si does not exist. From the comparison between Fig. 3 (Si map) and Fig. 6 (Mg map), Mg is scattered throughout the particles. Considering that MgO and Mg have been removed by the acid treatment, it is considered that magnesium silicate oxide is scattered throughout the particles. Note that the upper left region and the lower right region of the image are the resin of the base material. From this SEM image and EDS analysis results, the following were confirmed. (1) It was confirmed that composite particles in which a silicon-based material and a magnesium-based material were integrated by a reduction reaction were formed. (2) By removing Mg or MgO from the composite particles by acid treatment, pores were formed inside the composite particles. (3) By CVD treatment, not only the surface of the composite particles was coated with carbon, but also carbon penetrated into the pores of the composite particles and filled the pores.

[0112] [Evaluation Example 2: XRD Measurement and Elemental Analysis] By XRD measurement and elemental analysis by X-ray fluorescence analysis (XRF), it was confirmed that the negative electrode active material obtained in each example contains silicon Si, silicon oxide SiO x (0 < x ≤ 1), and magnesium silicate oxides MgSiO3 and Mg2SiO4. According to XRF, the molar ratio of Si atoms to Mg atoms (Si / Mg) in Example 1 was 29.5.

[0113] [Evaluation Example 3: Initial Characteristics of Battery] Regarding the coin cells of each example and comparative example, charge and discharge were performed at a constant current of 0.2C with a cut-off voltage of 1.5V. The "initial capacity" is defined as follows as the value obtained by dividing the discharge capacity in this first charge and discharge process by the mass (g) of the negative electrode active material powder used in each example and comparative example.

Equation

[0114] Also, the charge-discharge efficiency (hereinafter referred to as "initial efficiency") in this first charge-discharge process is defined by the following formula.

Equation

[0115] The results of Evaluation Example 3 were summarized in the following table together with the above manufacturing conditions.

Table 1

[0116] [Evaluation Example 4: Battery Life Characteristics] For the coin cells manufactured according to each of the examples and comparative examples, following the first charge-discharge process performed in Evaluation Example 1, after performing charge-discharge one more time under the same conditions, charge-discharge under the same conditions was repeated 48 times at a constant current of 0.5C. That is, a total of 50 charge-discharge processes were repeated including the first and second charge-discharge processes. FIG. 7 is a diagram showing the cycle characteristics of the coin cells of each of the examples and comparative examples. Specifically, FIG. 7 is a graph plotted with the number of cycles of the charge-discharge process on the horizontal axis and the discharge capacity of the coin cells of each of the examples and comparative examples on the vertical axis.

[0117] [Consideration of Evaluation Results of Examples and Comparative Examples] The evaluation results described above will be considered below. However, the following considerations are a hypothesis at the present time and do not restrict the present invention by theory. Examples 1 to 6 in which Mg was introduced to reduce SiO had superior initial capacity, initial efficiency, and cycle characteristics for at least about the first 20 cycles compared to Comparative Example 1 in which no reduction treatment was performed. As a result of the reduction treatment increasing the Si portion having lithium storage characteristics, it is considered that the battery capacity of the coin cell was improved. Also, due to the reduction treatment, silicon oxide SiO that generates a lithium silicate phase, which is an irreversible component, in the first charge-discharge xSince it decreases, it is considered that the initial efficiency of the coin cell has improved. In particular, in Example 6, since the particle size of the raw material SiO was reduced, it is considered that the reduction and acid treatment proceeded faster, resulting in an increase in the initial capacity and the initial efficiency. Also, since the particle size of SiO is small, it is considered that the insertion and desorption of lithium ions are smoothly performed, and as a result, the life characteristics of the battery are also improved. In Example 7 as well, although 1 μm of SiO, the same as in Example 6, is used as the raw material, since the reduction temperature is low, the degree of reduction of SiO is lower than that in Example 6, and as a result, the initial capacity and the initial efficiency are considered to be lower than those in Example 6.

[0118] The following reasons are considered for the relatively stable cycle characteristics in Examples 1 to 7 as well. (1) The generated magnesium silicate oxide phase and the remaining silicon oxide phase act as a matrix for silicon Si to relieve the expansion and contraction during charge and discharge. (2) By filling the pores formed by the acid treatment with carbon, the side reaction with the electrolyte is suppressed and conductivity is imparted, and as a result, the deterioration of the negative electrode active material is suppressed.

Claims

1. Silicon and silicon oxide SiO ｘ including porous composite particles containing (0 < x ≤ 2), Part or all of the pores of the porous composite particles are filled with carbon, and part or all of the surface of the porous composite particles is coated with carbon, A negative electrode active material for a non-aqueous electrolyte secondary battery.

2. The porous composite particles further contain magnesium silicate oxide, The negative electrode active material according to Claim 1.

3. The magnesium silicate oxide contains at least one of MgSiO ３ and Mg ２ SiO ４ . The negative electrode active material according to Claim 2.

4. The molar ratio (Si / Mg) of Si atoms to Mg atoms in the negative electrode active material is 10 or more and 50 or less, The negative electrode active material according to Claim 2.

5. The porous composite particles do not contain magnesium oxide MgO, at least on the surface, The negative electrode active material according to any one of Claims 1 to 4.

6. The total amount of carbon filling the pores and carbon coating the surface of the porous composite particles is 5% by mass or more and 20% by mass or less based on the porous composite particles, The negative electrode active material according to any one of Claims 1 to 4.

7. The average particle diameter of the porous composite particles is 100 nm or more and 20 μm or less, The negative electrode active material according to any one of Claims 1 to 4.

8. The porous composite particles are composed of Si, O, and Mg elements, The negative electrode active material according to any one of Claims 1 to 4.

9. A negative electrode current collector, A negative electrode active material layer formed on the negative electrode current collector and containing the negative electrode active material according to any one of Claims 1 to 4, A negative electrode comprising:

10. A non-aqueous electrolyte secondary battery comprising the negative electrode according to Claim 9.

11. Silicon oxide SiO ａ a reduction step of reacting (0 < a ≤ 2) with metallic magnesium Mg, An acid treatment step of treating the material after the reaction with an acid, A carbon deposition step of depositing carbon on the material after the acid treatment, A method for producing a negative electrode active material for a non-aqueous electrolyte secondary battery, comprising:

12. In the acid treatment step, porous composite particles having pores on the surface and inside are formed, The method according to Claim 11.

13. In the carbon deposition step, part or all of the pores are filled with carbon, The method according to Claim 12.

14. In the reduction step, the molar ratio (Si / Mg) of Si atoms to Mg atoms is 0.1 or more and 3 or less, The method according to any one of Claims 11 to 13.

15. In the acid treatment step, the amount of acid added per 1 g of the material obtained in the reduction step is 1 mmol or more and 100 mmol or less, The method according to any one of Claims 11 to 13.

16. The carbon deposition step is performed by depositing carbon at a temperature of 500°C or higher and 900°C or lower. The method according to any one of claims 11 to 13.

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