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

Abstract

A negative electrode active material capable of improving cycle characteristics of a secondary battery, a negative electrode, a non-aqueous electrolyte secondary battery, and a method for manufacturing the negative electrode active material are provided. A method for manufacturing a negative electrode active material according to an embodiment comprises: a reduction step of reacting silicon oxide (SiOa, 0<a≤2) with metal magnesium (Mg); an acid treatment step of acid-treating a material after the reaction; and a mechanical alloying step of performing mechanical alloying on the material after the reaction until a crystallite size evaluated from a powder X-ray diffraction pattern becomes 10 nm or less.

Description

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

[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.

[0002] This application claims priority based on Japanese application No. 2024-231224 filed on December 26, 2024, and all contents disclosed in the specification of said application are incorporated into this application.

[0003] Recently, a technology for adding silicon oxide to the negative electrode active material of a lithium-ion secondary battery is being developed. For example, Patent Document 1 describes a technology for adding silicon oxide pre-doped with lithium to the negative electrode active material.

[0004] [Prior Art Literature]

[0005] [Patent Literature]

[0006] Patent Document 1: Japanese Patent Publication No. 2018-152250

[0007] However, in the case of anode materials with added silicon oxide, the initial efficiency is often lowered due to the lithium silicate phase generated during charging. In addition, as described in Patent Document 1, if lithium is pre-doped into silicon oxide, there is a possibility that the battery capacity will be lowered due to the formation of the lithium silicate phase.

[0008] In response to this, the inventors have carefully examined the matter and found that capacity and initial efficiency can be increased by reducing silicon oxide to reduce the silicate phase and generate the Si phase. However, at the same time, the inventors also found a problem in which the crystallinity of the Si phase increases due to the reduction treatment, resulting in significant expansion of the electrode and consequently deterioration of the battery's cycle characteristics.

[0009] The problem that the present invention aims to solve is to provide a negative electrode active material capable of improving the cycle characteristics of a secondary battery, a negative electrode, a non-aqueous electrolyte secondary battery, and a method for manufacturing a negative electrode active material.

[0010] The present invention may include the following forms.

[0011] [1] Composite particles containing Si phase and magnesium silicide oxide, and

[0012] A cathode active material having a crystallite size on the Si phase and a crystallite size evaluated from a powder X-ray diffraction pattern for at least one phase included in magnesium silicide oxide, which is 10 nm or less.

[0013] [2] The magnesium silicide oxide described above is a negative electrode active material described in [1], comprising at least one of Mg2SiO4 and MgSiO3.

[0014] [3] A cathode active material described in [1] or [2], wherein at least one of the Si phase and the magnesium silicide oxide contained in the composite particle is amorphous.

[0015] [4] A negative electrode active material described in any one of [1] to [3], wherein the amount of Si contained in the composite particles is 20 mass% or more and 70 mass% or less based on the total mass of the composite particles.

[0016] [5] The above composite particles are cathode active materials described in any one of [1] to [4] that do not substantially contain MgO.

[0017] [6] The above composite particle is a cathode active material described in any one of [1] to [5], the surface of which is coated with a carbon-based material.

[0018] [7] The amount of the carbon-based material covering the composite particles is 5 mass% or more and 20 mass% or less based on the amount of the composite particles (excluding the carbon-based material), as described in [6].

[0019] [8] Cathode current collector, and

[0020] A cathode having a cathode active material layer formed on the above-mentioned cathode current collector, comprising a cathode active material described in any one of [1] to [7].

[0021] [9] A non-aqueous electrolyte secondary battery having the negative electrode described in [8].

[0022]

[0010] Silicon oxide (SiO₂) a A reduction step in which , 0<a≤2) reacts with metallic magnesium (Mg),

[0023] An acid treatment step for acid treating the material after the above reaction, and

[0024] A method for manufacturing a cathode active material, comprising a mechanical alloying step for the material after the above reaction, wherein mechanical alloying is performed until the crystallite size evaluated from the powder X-ray diffraction pattern becomes 10 nm or less.

[0025]

[0011] A method for manufacturing a cathode active material as described in

[0010] , wherein the above mechanical alloying is performed by one or more means selected from the group consisting of a planetary mill, a rotary mill, a vibrating mill, an electric mill and a stirring mill.

[0026]

[0012] The above mechanical alloying is performed under an inert atmosphere. A method for manufacturing a cathode active material as described in

[0010] or

[0011] .

[0027]

[0013] A method for manufacturing a negative electrode active material as described in any one of

[0010] to

[0012] , wherein MgO is removed by reacting with acid through the acid treatment step above.

[0028]

[0014] In the above reduction step, silicon oxide (SiO₂) a A method for manufacturing a cathode active material as described in any one of

[0010] to

[0013] , wherein at least a portion of ) is reduced to Si.

[0029]

[0015] A method for manufacturing a cathode active material as described in any one of

[0010] to

[0014] , further comprising a coating step of coating the material after the above reaction with a carbon-based material.

[0030] According to the present invention, a negative electrode active material capable of improving the cycle characteristics of a secondary battery, a negative electrode, a non-aqueous electrolyte secondary battery, and a method for manufacturing the negative electrode active material can be provided.

[0031] Figure 1 is a diagram showing the powder X-ray diffraction patterns of Example 1, Example 2, and Comparative Example 1.

[0032] FIG. 2 is a diagram showing the cycle characteristics of the coin cells of Example 1, Comparative Example 1, and Comparative Example 2.

[0033] Figure 3 is a diagram showing the cycle characteristics of the coin cells of Example 2, Comparative Example 3, and Comparative Example 4.

[0034] Hereinafter, a negative electrode active material, a negative electrode, a non-aqueous electrolyte secondary battery, and a method for manufacturing the negative electrode active material according to embodiments will be described. Meanwhile, the following embodiments represent one embodiment of the present invention and are not intended to limit the present invention, and may be modified arbitrarily within the scope of the technical concept of the present invention. Furthermore, each configuration and each feature of the embodiments may be combined arbitrarily.

[0035] [1. Non-aqueous electrolyte secondary battery]

[0036] One embodiment of the present invention relates to a non-aqueous electrolyte secondary battery. A non-aqueous electrolyte secondary battery according to the present embodiment comprises 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 a lithium-ion secondary battery having advantages such as high energy density, discharge voltage, and output stability.

[0037] Hereinafter, the components (anode, cathode, separator, and non-aqueous electrolyte) are described, primarily using lithium-ion secondary batteries as examples. However, the present invention is not limited to lithium-ion secondary batteries and can be applied to various non-aqueous electrolyte secondary batteries.

[0038] [1-1. Cathode]

[0039] [1-1-1. Composition of the Cathode]

[0040] The cathode includes a cathode current collector and a cathode active material layer formed on one or both sides of the cathode current collector. The cathode active material layer may be formed on the entire surface of the cathode current collector or only on a part thereof.

[0041] (Cathode current collector)

[0042] The negative current collector used for the negative electrode is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, copper; stainless steel; aluminum; nickel; titanium; calcined carbon; copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc.; aluminum-cadmium alloy, etc. may be used as the negative current collector.

[0043] The negative current collector may have a thickness of 3 μm or more and 500 μm or less. Fine irregularities may be formed on the surface of the negative current collector to increase adhesion with the negative active material. The negative current collector may have various forms, such as, for example, a film, sheet, foil, net, porous body, foam, or nonwoven fabric.

[0044] (Cathode active material layer)

[0045] The negative active material layer may comprise a negative active material, a binder, a conductive agent, and optional additives. For example, the negative active material layer may be formed by applying a negative active material slurry, in which a mixture of the negative active material, binder, and conductive agent is dissolved or dispersed in a solvent, onto a negative current collector and then drying and rolling, or by casting the negative active material slurry onto another support and then peeling it off from the support to obtain a film, which is then laminated onto the negative current collector. The mixture may further comprise a dispersant, a filler, or any other optional additives as needed.

[0046] (Cathode active material)

[0047] In a lithium-ion secondary battery according to an embodiment, the negative electrode active material comprises composite particles containing a Si phase and magnesium silicide oxide. The crystallite size of the Si phase and the crystallite size evaluated from a powder X-ray diffraction (XRD) pattern for at least one phase included in the magnesium silicide oxide are both 10 nm or less.

[0048] The negative electrode active material layer may further include a negative electrode active material (e.g., a carbon-based material) different from the composite particles mentioned above. In this specification, any material that contributes to the charge-discharge reaction at the negative electrode is referred to as a "negative electrode active material."

[0049] The cathode active material may be included in an amount of 80 mass% or more and 99 mass% or less based on the total mass of the cathode active material layer. The composite particles (excluding the carbon coating described below) may be included in an amount of, for example, 0.1 mass% or more and 99 mass% or less, 1 mass% or more and 90 mass% or less, 5 mass% or more and 50 mass% or less, or 10 mass% or more and 30 mass% or less based on the total mass of the cathode active material.

[0050] Silicon (Si) has the ability to absorb lithium, so it can be responsible for the charge and discharge reaction at the cathode.

[0051] The amount of Si phase contained in the composite particles is 20 mass% or more and 70 mass% or less, based on the total mass of the composite particles. When the amount of Si phase is 20 mass% or more, sufficient battery capacity can be obtained. When the amount of Si phase is 70 mass% or less, the expansion of the Si phase can be sufficiently mitigated by magnesium silicide oxide.

[0052] The crystallite size of the Si phase contained in the composite particle is 10 nm or less, and may be, for example, 1 nm or more and 9 nm or less, 2 nm or more and 8 nm or less, 3 nm or more and 7 nm or less, or 4 nm or more and 6 nm or less. In this specification, the crystallite size of the Si phase is a value calculated by the Scherrer equation from the full width at half of the (111) peak of the Si phase in the powder XRD pattern.

[0053] When the crystallite size is 10 nm or less, the crystallites on the Si phase are sufficiently small, so the expansion of the entire electrode during charging and discharging can be suppressed. As a result, the occurrence of cracks caused by expansion and contraction can be suppressed, so the cycle characteristics of the secondary battery can be improved.

[0054] As described above, the composite particles may further contain magnesium silicide oxide. For example, the magnesium silicide oxide includes at least one of MgSiO3 and Mg2SiO4, but is not limited to these. Preferably, the composite particles consist only of the Si phase and magnesium silicide oxide (provided that they may contain unavoidable impurities).

[0055] The total amount of magnesium silicide contained in the composite particles is, based on the total mass of the composite particles, for example, 0.1 mass% or more and 20 mass% or less, 0.5 mass% or more and 10 mass% or less, or 1 mass% or more and 5 mass% or less. If it is 0.1 mass% or more, the magnesium silicide may mitigate the volume expansion of the Si phase. If it is 20 mass% or less, the effect of sufficient battery capacity increase due to the Si phase is obtained.

[0056] When the composite particles contain magnesium silicide oxide, the ratio of the amount of Si atoms to Mg atoms in the cathode active material (Si / Mg) 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 becomes relatively large, so a large energy density can be obtained. If it is 50 or less, it is thought that a sufficient reduction reaction is taking place.

[0057] The crystallite size of the MgSiO3 phase contained in the composite particle is 10 nm or less, and may be, for example, 0.5 nm or more and 8 nm or less, 1 nm or more and 5 nm or less, 1.5 nm or more and 4 nm or less, or 2 nm or more and 3 nm or less. In this specification, the crystallite size of the MgSiO3 phase is a value calculated by the Scherrer equation from the full width at half of the (310) peak of the MgSiO3 phase in the powder XRD pattern.

[0058] When the crystallite size is 10 nm or less, the crystallites of the MgSiO3 phase are sufficiently small, so the interface (grain boundary) between the Si phase and the MgSiO3 phase becomes smaller, and the MgSiO3 phase can suppress the expansion of the Si phase. As a result, the occurrence of cracks due to expansion and contraction can be suppressed, so the cycle characteristics of the secondary battery can be improved.

[0059] At least one of the Si phase and magnesium oxide silicide contained in the composite particles may be amorphous. For example, at least one of the Si phase, MgSiO3 phase, and Mg2SiO4 phase is amorphous. For example, the MgSiO3 phase may be amorphous. Also, the Mg2SiO4 phase may be amorphous. When the Si phase is amorphous, the expansion of the Si phase can be sufficiently suppressed. When the MgSiO3 phase and / or Mg2SiO4 phase are amorphous, the grain boundaries between them and the Si phase are substantially eliminated, so these magnesium silicide phases can absorb the expansion of the Si phase and suppress damage to the electrode, such as cracks caused by the expansion of the Si phase.

[0060] The composite particles may further contain magnesium oxide (MgO) and / or metallic magnesium (Mg). These may correspond to the components remaining from the reduction or acid treatment described below. However, preferably, the composite particles do not substantially contain magnesium oxide (MgO) and / or metallic magnesium (Mg). In this specification, "substantially not" means that peaks attributable to the target material cannot be clearly identified in the powder XRD pattern of the composite particles.

[0061] The amount of magnesium oxide (MgO) contained in the composite particles is, for example, 5 mass% or less, 1 mass% or less, 0.1 mass% or less, or 0.01 mass% or less, based on the total mass of the composite particles. The amount of metallic magnesium (Mg) contained in the composite particles is 0.1 mass% or less or 0.01 mass% or less, based on the total mass of the composite particles.

[0062] The composite particles may further contain materials other than those mentioned above (including unavoidable impurities).

[0063] The amount of silicon atoms contained in the composite particles (the total amount of Si atoms in all silicon-containing materials, such as Si phase, silicon oxide phase, magnesium silicide phase, etc.) is, for example, 20 mass% or more and 95 mass% or less, 30 mass% or more and 80 mass% or less, or 50 mass% or more and 70 mass% or less, based on the total mass of the composite particles.

[0064] The amount of magnesium atoms contained in the composite particles (the total amount of Mg atoms in all magnesium-containing materials, such as magnesium silicide oxide phase, magnesium oxide (MgO) phase, metallic magnesium (Mg) phase, etc.) is, for example, 0.1 mass% or more and 20 mass% or less, 0.5 mass% or more and 10 mass% or less, or 1 mass% or more and 5 mass% or less, based on the total mass of the composite particles.

[0065] The average particle size of the 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 this specification, 'average particle size' means the particle size at 50% of the cumulative value in the particle size distribution measured by laser diffraction scattering, i.e., the median diameter (D50).

[0066] (Carbon coating)

[0067] The surface of the composite particle may be coated with a carbon-based material. Hereinafter, this coating of carbon-based material is referred to as "carbon coating." In this specification, "coated with carbon-based material" means that the outer surface of the particle is physically covered at least partially with a carbon-based material. Another material (e.g., another coating layer) may exist between the outer surface of the particle and the carbon-based material covering it.

[0068] The carbon coating can impart electrical conductivity to the composite particles. Furthermore, by coating the surface of the composite particles with a carbon-based material, side reactions between the composite particles and other materials (e.g., electrolytes) can be suppressed. Additionally, the carbon-based coating can absorb the expansion of the Si phase, thereby suppressing the occurrence of cracks.

[0069] Carbon-based materials constituting the carbon coating may include graphites such as natural graphite and artificial graphite; carbon fibers such as mesocarbon microbeads (MCMB), carbon nanotubes, and carbon nanofibers; carbon black such as Ketjen black, Denka black, and acetylene black; graphene or graphene oxide; or mixtures thereof.

[0070] The amount of carbon coating formed on the composite particles is, for example, 5 mass% or more and 20 mass% or less based on the amount of composite particles (excluding the carbon coating), preferably 6 mass% or more and 18 mass% or less, and more preferably 8 mass% or more and 15 mass% or less. If it is 5 mass% or more, side reactions between the composite particles and other materials can be suppressed. If it is 20 mass% or less, not only can a certain battery capacity be obtained, but aggregation of composite particles through deposited carbon can also be suppressed. However, the amount of carbon is not limited to the above range, and even if it is outside the above range, it can function as a secondary battery, even if the battery performance is somewhat reduced.

[0071] The carbon coating 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 composite particles. The average thickness of the carbon coating formed on the surface of the composite particles may 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.

[0072] (bookbinder)

[0073] A binder is added as a component that promotes the bonding between the active material and the conductive agent, or the bonding with the current collector. Examples of binders 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, and various copolymers thereof, and one or more of these may be used, but are not limited to these.

[0074] The content of the binder may be 0.1 mass% or more and 30 mass% or less based on the total mass of the negative electrode active material layer. The content of the binder may preferably be 0.5 mass% or more and 20 mass% or less, and more preferably 1 mass% or more and 10 mass% or less. When the content of the binder polymer satisfies the above range, sufficient adhesion within the electrode can be provided while preventing a decrease in the capacity characteristics of the battery.

[0075] (Challenge System)

[0076] The conductive agent is not particularly limited as long as it is an electrically conductive material that does not cause chemical changes. Examples of conductive agents include carbon-based materials such as artificial graphite, natural graphite, single-layer carbon nanotubes, multi-layer carbon nanotubes, graphene, carbon black, acetylene black, Ketjen black, Denka black, thermal black, channel black, furnace black, lamp black, and carbon fibers; metal powders or 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 more of these may be used, but are not limited to these.

[0077] The content of the conductive agent may be 0.1 mass% or more and 30 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 mass% or more and 15 mass% or less, and more preferably 0.5 mass% or more and 10 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 battery capacity can be secured because the amount of the negative electrode active material is not reduced.

[0078] (Thickening agent)

[0079] The cathode active material slurry used when coating the cathode active material onto the cathode current collector may contain a thickener. Specifically, the thickener may be a cellulose-based compound such as carboxymethyl cellulose (CMC). The thickener may be contained in an amount of, for example, 0.5 mass% or more and 10 mass% or less based on the total mass of the cathode active material layer.

[0080] (menstruum)

[0081] The solvent used in the cathode active material slurry is not particularly limited as long as it is generally used in the manufacture of the cathode. Examples of solvents include N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), isopropyl alcohol, acetone, water, etc., and one or more of these may be used, but are not limited to these.

[0082] [1-1-2. Method for Manufacturing the Cathode]

[0083] A method for manufacturing a negative electrode for a lithium-ion secondary battery according to an embodiment comprises the following steps.

[0084] (1) Step of manufacturing a negative electrode active material

[0085] (2) Step of preparing a cathode active material slurry from a cathode active material

[0086] (3) Step of manufacturing a cathode from a cathode active material slurry

[0087] (1) Step of manufacturing a negative electrode active material

[0088] The manufacturing step (1) of the cathode active material can be described as a ‘method for manufacturing a cathode active material’. If the method for manufacturing a cathode active material is subdivided, it includes the following steps.

[0089] (a) Reduction step: Silicon oxide (SiO₂) a , 0<a≤2) and metallic magnesium (Mg) are reacted.

[0090] (b) Acid treatment step: The material after the reaction of (a) above is acid-treated.

[0091] (c) Mechanical alloying step: For the material after the reaction of (a) above, mechanical alloying is performed until the crystallite size evaluated from the powder XRD pattern is 10 nm or less.

[0092] The method for manufacturing a negative electrode active material may further include the following steps.

[0093] (d) Coating step: The material after the reaction of (a) above is coated with a carbon-based material.

[0094] (a) Reduction step

[0095] In the reduction step, metallic magnesium (Mg) acts as a reducing agent. For example, silicon oxide (SiO₂) a It is thought that the following chemical reaction proceeds between ) and metallic magnesium (Mg).

[0096] SiO a + aMg → Si + aMgO

[0097] In the reduction step, as described above, silicon oxide (SiO₂) a It is desirable that all or as much as possible of ) is reduced to Si. SiO a If all or as much as possible of it is reduced to Si, the amount of Si phase involved in the charge-discharge reaction in the composite particles can be increased, thereby increasing the energy density of the secondary battery.

[0098] Silicon oxide used as a raw material is, for example, SiO a It can be a powder of (0 < a ≤ 2). SiO a It may have a structure in which a Si phase (Si particles) is dispersed in a microcrystalline or amorphous form within an amorphous silicon oxide matrix. Here, the Si phase and the silicon oxide matrix together are silicon oxide (SiO₂ aIt is collectively referred to as ). That is, a is the average value of the entire Si phase 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 silicon oxide of the raw material is silicon monoxide (SiO) (a=1) or silicon dioxide (SiO2) (a=2). In addition, the silicon oxide powder of the raw material is SiO having a specific value of a a It may include only, or two or more types of SiO with different values ​​of a. a It can be a mixture of powders.

[0099] Meanwhile, in the reduction step, silicon oxide (SiO₂) a There are cases where the Si phase originally contained in the composite particles undergoes crystal growth and the crystallite size increases. For example, the crystallite size of the Si phase in the composite particles immediately after the reduction step may be 10 nm or more, 50 nm or more, or 100 nm or more. However, if the crystallinity of the Si phase increases, the degree of expansion and contraction of the Si phase accompanying charging and discharging increases, which may have an adverse effect on cycle characteristics. Therefore, the crystallite size of the Si phase can be reduced again in the mechanical alloying step described later.

[0100] The average particle size 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, and more preferably 500 nm or more and 5 µm or less. If the average particle size is small, advantageously, the reaction points of the charge-discharge reaction are relatively increased, and there is a possibility that the battery capacity may increase. If the average particle size is 100 nm or more, the energy density can be increased to some extent. If the average particle size is 20 µm or less, the lifespan characteristics due to volume expansion can be suppressed.

[0101] In the reduction step, the amount 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, even 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, which serves as a charge / discharge reaction point, is included, so a certain amount of energy density can be obtained. If it is 3 or less, a sufficient reduction reaction occurs, and the battery capacity is sufficiently improved.

[0102] In addition to the above, magnesium silicide oxide can be produced as a byproduct by reacting silicon oxide with metallic magnesium (Mg) or generated magnesium oxide (MgO). Examples of magnesium silicide oxide include MgSiO3 and Mg2SiO4.

[0103] The reaction conditions are arbitrary as long as the above reaction proceeds. Since processing is simple, a method of mixing silicon oxide powder and metallic magnesium (Mg) powder and heating is preferred, but is not limited thereto; for example, these materials may be reacted in a solution. Since a reduction reaction is performed, it is preferable to carry out the reaction under an inert gas (nitrogen, argon, etc.) atmosphere or a reducing atmosphere. The reaction temperature is, for example, 500°C or higher and 1000°C or lower, preferably 600°C or higher and 900°C or lower, more preferably 650°C or higher and 850°C or lower, and even more preferably 700°C or higher and 800°C or lower. The reaction pressure is not particularly limited. The reaction time is, for example, 10 minutes or more and 3 hours or less, and preferably 30 minutes or more and 2 hours or less.

[0104] The above reduction reaction is presumed to proceed via a mechanism such as, for example, as follows. However, the following is merely a conjecture and does not bind the present invention by theory.

[0105] The silicon oxide particles of the raw material react with metallic magnesium (Mg), and the silicon oxide is reduced to Si. At the same time, magnesium oxide (MgO) or magnesium silicide oxide is produced. As a result, a composite particle is obtained in which silicon (Si), magnesium oxide (MgO), magnesium silicide oxide, and unreacted metallic magnesium (Mg) are integrated and / or aggregated. Meanwhile, if the reduction reaction is incomplete, the composite particle may contain, in addition to each of the above components, a silicon oxide matrix that remains unreduced to the Si phase. In this case, some of the Si phase may be scattered within the silicon oxide matrix.

[0106] (b) Acid treatment step

[0107] In the acid treatment step, magnesium oxide (MgO) and / or unreacted metallic magnesium (Mg) generated in the reduction step can be removed by reacting with acid. Accordingly, the decrease in energy density or initial efficiency caused by MgO or Mg can be suppressed. Specifically, magnesium oxide (MgO) and unreacted metallic magnesium (Mg) react with acid to form magnesium ions (Mg 2+ It becomes ). Magnesium ion (Mg 2+ Since ) can be removed by washing, as a result, magnesium oxide (MgO) and unreacted metallic magnesium (Mg) can be removed.

[0108] MgO + 2H + → Mg 2+ + H2O

[0109] Mg 2+ + 2H + → Mg 2+ + H2

[0110] The above acid may not contain substances capable of strongly dissolving silicon oxide (SiOx), such as hydrogen fluoride (HF).

[0111] The acid treatment step involves adding an acid to the product of the reduction step. The acid is, for example, an aqueous acid solution. The aqueous acid 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 any combination. That is, the acid treatment step may include a process 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 reaction occurs between magnesium oxide (MgO) and metallic magnesium (Mg) and hydrochloric acid.

[0112] MgO + 2HCl → MgCl2 + H2O

[0113] Mg + 2HCl → MgCl2 + H2

[0114] The concentration of the acid solution is, for example, 0.1M or more and 10M or less, preferably 0.5M or more and 5M or less. If the concentration is 0.1M or more, impurities can be sufficiently removed. If the concentration is 10M or less, sufficient manufacturing efficiency can be maintained. The amount of acid solution added per 1g of composite particle obtained in the reduction step is, for example, 1mL or more and 100mL or less, preferably 5mL or more and 80mL or less, and more preferably 10mL or more and 50mL or less. When the concentration of the acid solution is 1M, the amount of acid added per 1g of composite particle 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.

[0115] The reaction conditions are arbitrary as long as the above reaction proceeds. The reaction temperature is, for example, 10°C or higher and 90°C or lower, preferably room temperature or higher and 50°C or lower. The reaction pressure is not particularly limited. The reaction time is, for example, 1 hour or higher and 48 hours or lower, preferably 6 hours or higher and 36 hours or lower, and more preferably 12 hours or higher and 30 hours or lower.

[0116] Since magnesium silicide has low reactivity with acid, at least some of it may remain even after acid treatment; however, depending on the raw materials, acid, and reaction conditions used, there is a possibility that some or all of the magnesium silicide may decompose. To reduce the oxide phase and increase initial efficiency, it is desirable to extend the reaction time to ensure sufficient treatment.

[0117] After adding acid and causing a reaction, the acid-dissolved material can be removed by general washing. Composite particles can be obtained through washing and drying processes.

[0118] (c) Mechanical alloying step

[0119] In the mechanical alloying step, the crystallinity of at least one crystalline phase contained in the composite particles can be reduced by mechanical alloying of the composite particles. Specifically, the crystallite size of the crystalline phase inside the composite particles can be reduced before and after the mechanical alloying treatment. The crystallite size can be evaluated from the peak width of the powder XRD pattern using the Scherrer equation.

[0120] In this specification, "mechanical alloying" means mechanically stirring and mixing a target sample using milling to impart energy to the target sample. By milling, collision energy or shear energy is applied to the composite particles, and the crystallite size of the crystalline phase within the composite particles is reduced. When the crystallite size of the Si phase is reduced, the degree of expansion and contraction of the Si phase during charging and discharging can be suppressed. In addition, when the crystallite size of magnesium silicide oxide (e.g., MgSiO3 phase and / or Mg2SiO4 phase) is reduced, the grain boundary between the Si phase and the magnesium silicide oxide becomes smaller, so the occurrence of cracks within the particles accompanying the expansion and contraction of the Si phase during charging and discharging can be suppressed.

[0121] Any milling device, such as a ball mill or a stirring mill using a rotating rod, may be used as a means for milling. For example, mechanical alloying may be performed by one or more means selected from the group consisting of planetary mills, rotary mills, vibrating mills, electric mills, and stirring mills. For example, when using a ball mill, the composite particles and a suitable amount of balls are placed in a container, and rotation or vibration is applied to the container. Due to the impact energy generated by collisions between balls or collisions between balls and walls, crushing, deformation, and / or alteration of the internal structure of the composite particles (e.g., refinement of crystallites within the composite particles) may occur.

[0122] The size, shape, and material of the balls used in the ball mill are not particularly limited. The balls may be small balls (beads) corresponding to those in a so-called bead mill. Furthermore, the balls do not necessarily have to be spherical and may have any shape, such as an angular shape. While zirconia is the general material for the balls, it is not limited to this, and any material may be used.

[0123] The milling conditions can be appropriately set according to the material or characteristics of the composite particles. For example, at room temperature (e.g., 0°C or higher and 50°C or lower), the rotational speed can be 100 rpm or higher and 1000 rpm or lower, and the processing time can be 1 hour or higher and 24 hours or lower. The amount of balls can be determined such that, under given milling conditions, the crystallite size of at least one phase of the composite particles is 10 nm or less. For example, when the mass of the cathode active material is 100 parts by mass, the amount of balls may be 1000 parts by mass or higher and 10000 parts by mass or lower, 1500 parts by mass or higher and 5000 parts by mass or lower, or 2000 parts by mass or higher and 4000 parts by mass or lower.

[0124] Mechanical alloying can be performed under an inert atmosphere. When mechanical alloying is performed under an inert atmosphere, the silicon oxide reduced during the reduction step can be prevented from being oxidized again.

[0125] After mechanical alloying treatment, post-treatment of the obtained particles may be performed. Post-treatment may include one or more processes selected from grinding, sieving, chemical modification of the particles, and additive treatment. For example, the particle size of the composite particles after mechanical alloying treatment may be adjusted by grinding and / or sieving.

[0126] Although the order of the acid treatment step and the mechanical alloying step is not specifically limited, the advantages of performing mechanical alloying after acid treatment are explained. When MgO and / or Mg are removed from the composite particles through acid treatment, voids may form in that area. Such voids can cause cracks within the composite particles due to the repeated expansion and contraction of the Si phase accompanying charge-discharge reactions. However, it is believed that if mechanical alloying of the composite particles is performed after acid treatment, the composite particles may be compressed inward by the amount of the voids or fracture at the location of the voids, thereby suppressing the occurrence of cracks originating from the voids at the final electrode. In other words, the mechanical alloying step may include a process of consolidating the particles containing the voids.

[0127] (d) Coating step

[0128] Optionally, a coating step may be performed on the obtained composite particles. In the coating step, the surface of the composite particles is coated with a carbon-based material. The carbon coating treatment can be performed according to any commonly used carbon coating method, and the carbon coating method is not particularly limited. The carbon coating method may be either a dry method or a wet method. An example of the former is a method of depositing carbon onto the composite particles according to the chemical vapor deposition (CVD) method. An example of the latter is a method of mixing the carbon source and the composite particles in a solution and heating and drying them.

[0129] For example, when performing carbon deposition such as the CVD method, it is desirable to perform the process at a relatively low temperature for a long time in order to sufficiently coat the surface of the composite particles with carbon. For example, the temperature at which carbon deposition is performed is 500°C or higher and 900°C or lower, preferably 600°C or higher and 800°C or lower. For example, the time at which carbon deposition is performed is 10 minutes or more and 24 hours or less, preferably 30 minutes or more and 10 hours or less, and more preferably 1 hour or more and 2 hours or less.

[0130] Steps (b) through (d) above are not necessarily limited to the order described and can be executed in any order.

[0131] (2) Step of preparing a cathode active material slurry from a cathode active material

[0132] 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 needed. 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.

[0133] (3) Step of manufacturing a cathode from a cathode active material slurry

[0134] A cathode can be manufactured by applying a cathode active material slurry to a cathode current collector, followed by drying and rolling, so that a cathode active material layer is formed on the cathode current collector.

[0135] Alternatively, for example, the cathode may be manufactured by casting the cathode active material slurry onto another support and then laminating the film obtained by peeling off from the support onto a cathode current collector. In addition, the cathode active material layer may be formed on the cathode current collector using any other method, and the cathode active material does not necessarily have to be in the form of a slurry.

[0136] [1-2. Anode]

[0137] [1-2-1. Composition of the Anode]

[0138] In a lithium-ion secondary battery according to an embodiment, the positive electrode comprises a positive electrode current collector and a positive electrode active material layer formed on one or both sides of the positive electrode current collector. The positive electrode active material layer may be formed on the entire surface of the positive electrode current collector or only on a part thereof.

[0139] (Bipolar house whole)

[0140] The positive current collector used in the positive electrode is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, as a positive current collector, stainless steel; aluminum; nickel; titanium; calcined carbon; or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used.

[0141] The positive current collector may have a thickness of 3㎛ or more and 500㎛ or less. Fine irregularities may be formed on the surface of the positive current collector to increase adhesion with the positive active material. The positive current collector may have various forms, such as, for example, a film, sheet, foil, net, porous body, foam, or nonwoven fabric.

[0142] (Positive active material layer)

[0143] The positive active material layer can be formed, for example, by applying a positive active material slurry, in which a mixture of a positive active material, a conductive agent, and a binder is dissolved and dispersed in a solvent, onto a positive current collector, and then drying and rolling. The mixture may further include a dispersant, a filler, or any other additives as needed.

[0144] The positive active material may be included in an amount of 80 mass% or more and 99 mass% or less based on the total mass of the positive active material layer.

[0145] (Cathode active material)

[0146] As the cathode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specific examples include lithium metal composite oxides containing lithium and one or more metals such as cobalt, manganese, nickel, copper, vanadium, and aluminum. More specifically, such lithium metal composite oxides include lithium-manganese-based oxides (e.g., LiMnO2, LiMnO3, LiMn2O3, LiMn2O4, etc.); lithium-cobalt-based oxides (e.g., LiCoO2, etc.); lithium-nickel-based oxides (e.g., LiNiO2, etc.); lithium-copper-based oxides (e.g., Li2CuO2, etc.); lithium-vanadium-based oxides (e.g., LiV3O8, etc.); and lithium-nickel-manganese-based oxides (e.g., LiNi 1-z Mn z O2(0<z<1), LiMn 2-z Ni z O4 (0 < z < 2), etc.); lithium-nickel-cobalt oxides (e.g., LiNi 1-y Co y O2 (0 < y < 1), etc.); lithium-manganese-cobalt oxides (e.g., LiCo 1-z Mn z O2(0<z<1), LiMn 2-y Co y O4 (0 < y < 2), etc.); lithium-nickel-manganese-cobalt oxides (e.g., 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) oxide (e.g., Li(Ni x Co y Mn z M wExamples include )O2(M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg and Mo, and 0<x<1, 0<y<1, 0<z<1, 0<w<1, x+y+z+w=1), etc.); and compounds in which a transition metal element among these compounds is partially substituted with one or more other metal elements. The positive electrode active material layer may contain any one or two or more of these compounds. However, it is not limited to only these.

[0147] In particular, in terms of improving the capacity characteristics and stability of batteries, LiCoO2, LiMnO2, LiMn2O4, LiNiO2, lithium nickel manganese cobalt oxide (e.g., 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 (e.g., Li(Ni 0.8 Co 0.15 Al 0.05 )O2, etc. are desirable.

[0148] (Binder and Conductor)

[0149] The type and content of the binder and conductive agent used in the positive electrode active material slurry may be the same as those described for the negative electrode.

[0150] (menstruum)

[0151] The solvent used in the cathode active material slurry is not particularly limited as long as it is generally used in the manufacture of the cathode. Examples of solvents include amine-based solvents such as N,N-dimethylaminopropylamine, diethylenetriamine, and N,N-dimethylformamide (DMF); ether-based solvents such as tetrahydrofuran; ketone-based solvents such as methyl ethyl ketone; ester-based solvents such as methyl acetate; amide-based solvents such as dimethylacetamide and 1-methyl-2-pyrrolidone (NMP); and dimethyl sulfoxide (DMSO). One or more of these may be used, but are not limited to these.

[0152] The amount of solvent used is sufficient if it has a viscosity that allows for excellent thickness uniformity when applied to the anode current collector while dissolving or dispersing the anode active material, conductive material, and binder, taking into account the coating thickness or manufacturing yield of the slurry.

[0153] [1-2-2. Method of Manufacturing the Anode]

[0154] A method for manufacturing a positive electrode for a lithium-ion secondary battery according to an embodiment may include the 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 needed, and the step of obtaining a positive electrode by forming a positive electrode active material layer on a positive electrode current collector, such as by applying the positive electrode active material slurry onto a positive electrode current collector, similar to the method for manufacturing a negative electrode.

[0155] [1-3. Separator]

[0156] In a lithium-ion secondary battery according to an embodiment, the separator separates the negative electrode and the positive electrode to provide a pathway for the movement of lithium ions, and any separator commonly used in lithium-ion secondary batteries may be used without particular limitations. In particular, it is desirable that the separator has low resistance to ion movement of the electrolyte and excellent moisture retention capacity of the electrolyte. For example, a porous polymer film made from polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, or ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof, may be used as a separator. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of high-melting-point glass fiber or polyethylene terephthalate fiber, may also be used. Furthermore, to ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material may be used.

[0157] [1-4. Non-aqueous Electrolytes]

[0158] In a non-aqueous electrolyte secondary battery according to an embodiment, the non-aqueous electrolyte may be an organic liquid electrolyte, an inorganic liquid electrolyte, a solid electrolyte, etc., which can be used to manufacture a secondary battery, but is not limited to these.

[0159] The non-aqueous electrolyte may contain an organic solvent and a lithium salt, and may further contain additives as needed. Hereinafter, the liquid electrolyte is also referred to as the 'electrolyte'.

[0160] Organic solvents may be used without particular restriction as long as they can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Examples of organic solvents include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether and tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; carbonate-based 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-based solvents such as ethyl alcohol and isopropyl alcohol; and nitrile-based solvents such as R-CN (where R is a C2–C20 straight-chain, branched, or cyclic hydrocarbon group, and may include double aromatic rings or ether bonds). Examples include amide solvents such as dimethylformamide; dioxolan solvents such as 1,3-dioxolan; sulfolane solvents, etc., and one or more of these may be used, but are not limited to these.

[0161] Lithium salts can be used without particular restriction as long as they are compounds capable of providing lithium ions used in lithium-ion secondary batteries. Examples of lithium salts 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, and one or more of these may be used, but are not limited to these. The lithium salt may be contained in the electrolyte at a concentration of, for example, 0.1 mol / L or more and 2 mol / L or less. When the concentration of the lithium salt falls within the above range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance, allowing lithium ions to move effectively.

[0162] Additives may be used as needed for purposes such as improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity. Examples of additives include haloalkylene carbonate compounds such as fluoroethylene carbonate (FEC) or difluoroethylene carbonate (DFEC), pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-glycine, triamide hexaphosphate, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride, etc., and one or more of these may be used, but are not limited to these. The above additives may be contained in an amount of, for example, 0.1 mass% or more and 15 mass% or less with respect to the total mass of the electrolyte.

[0163] [2. Method for manufacturing a non-aqueous electrolyte secondary battery]

[0164] A non-aqueous electrolyte secondary battery according to an embodiment can be manufactured by interposing a separator (e.g., a separator) and an electrolyte between the negative electrode manufactured as described above and the positive electrode manufactured as described above. For example, an electrode assembly can be formed by placing a separator between the negative electrode and the positive electrode, placing this electrode assembly into a battery case such as a cylindrical battery case or a prismatic battery case, and then injecting an electrolyte to manufacture the battery. Alternatively, the electrode assembly can be stacked, impregnated with an electrolyte, and the resulting product can be placed into a battery case and sealed to manufacture the battery. However, the method of manufacturing a non-aqueous electrolyte secondary battery is not limited to the examples described above, and any other method may also be used.

[0165] The above battery case may be one commonly used in the field. The shape of the battery case may be, for example, a cylindrical shape using a can, a prismatic shape, a pouch shape, or a coin shape.

[0166] A lithium-ion secondary battery according to an embodiment can be used not only as a power source for small devices but also as a unit cell for a medium-to-large battery module comprising a plurality of battery cells. Preferred examples of such medium-to-large devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems, but are not limited thereto.

[0167] [3. Effects]

[0168] When silicon-based materials are used as negative electrode active materials to increase the capacity of lithium-ion secondary batteries, using only pure silicon (Si) results in significantly reduced cycle performance due to cracks caused by expansion during charging. Therefore, negative electrode active materials in which a small amount of silicon monoxide is mixed with graphite have been commercialized. Silicon monoxide functions as SiO₂ to mitigate the expansion of Si. xIt has a composite structure in which a Si phase is dispersed within a matrix. However, when silicon monoxide (SiO) is used, a lithium silicate phase is irreversibly formed during the first charge of the battery, resulting in a significantly lower discharge capacity relative to the charge capacity, and consequently, a very low initial efficiency of 65–70%. For this reason, it is difficult to achieve a balance with the initial efficiency of the cathode material when the amount of silicon monoxide (SiO) in the anode material is increased.

[0169] To solve the aforementioned problem, the inventors conceived of improving capacity and initial efficiency by reducing the silicate phase through the reduction of silicon oxide. However, as previously mentioned, the reduction treatment may increase the crystallinity of the Si phase, which consequently may lead to significant expansion of the electrode. Furthermore, as the crystallinity of each crystal phase increases, the grain boundaries within the negative electrode active material become more distinct, making it more difficult to absorb the expansion of the Si phase into the matrix phase. Having discovered such problems, the inventors, after careful consideration, were able to solve the problem by reducing the crystallite size of each crystal phase as described above.

[0170] According to the cathode active material of the embodiment, the amount of Si phase in the cathode active material increases due to the reduction of silicon oxide, and the silicate phase, which is the cause of irreversible capacity, decreases, thereby improving capacity and initial efficiency. Meanwhile, by having a crystallite size of 10 nm or less, the Si phase that expands and contracts during charging and discharging is dispersed within the cathode active material, thereby suppressing the expansion of the entire electrode. In addition, by reducing the crystallite size, grain boundaries are reduced or eliminated, making it difficult for the expansion of the Si phase to be limited by grain boundary surfaces. Accordingly, the occurrence of cracks during expansion can be suppressed.

[0171] According to the method for manufacturing a negative electrode active material according to an embodiment, the crystallite size of the crystalline phase within the composite particle after reduction can be reduced by mechanical alloying treatment. In particular, since there is a possibility that the crystallinity of the crystalline phase may increase when reduction treatment is performed at high temperatures, the increased crystallinity can be reduced by performing mechanical alloying after the reduction treatment. By reducing the crystallinity of the crystalline phase in this way, the aforementioned effect can be obtained in the manufactured negative electrode active material. Furthermore, by mechanical alloying treatment, the crystallinity of magnesium silicide oxide generated during reduction using Mg, as well as the Si phase, can be reduced. When the crystallinity of magnesium silicide oxide is reduced, the grain boundaries with the Si phase are reduced or eliminated, making it easier for it to function as a matrix for the Si phase, thereby mitigating the volume expansion of Si. Accordingly, the occurrence of cracks caused by the expansion and contraction of the Si phase can be suppressed, and thus the cycle characteristics of the secondary battery can be improved. In addition, since all materials constituting the composite particle are stable in water, there is an advantage that gas is not generated as in conventional methods.

[0172] Example

[0173] The present invention will be explained below by means of experimental examples. However, the present invention is not limited to the following experimental examples.

[0174] [Example 1]

[0175] (Manufacturing of cathode active material)

[0176] Silicon monoxide (SiO) powder with an average particle size of 1 μm and metallic magnesium (Mg) powder with an average particle size of 300 μm were mixed in a mass ratio of 2:1. This mixture was introduced into a plasma sintering device (Microphase product) and maintained at 850°C for 1 hour under an inert gas atmosphere to carry out the reduction reaction of the SiO powder by the Mg powder. By measuring the powder XRD pattern and elemental analysis, it was confirmed that the product contained silicon (Si), metallic magnesium (Mg), magnesium oxide (MgO), and magnesium silicide oxides (MgSiO3, Mg2SiO4).

[0177] 10 g of the above product was added to 200 mL of 2 M hydrochloric acid and stirred for 36 hours to remove the Mg phase and MgO phase. Subsequently, washing and drying at 120°C were performed. Next, 10 g of the dried powder and 200 g of zirconia balls were placed in a zirconia pot and subjected to mechanical alloying treatment under the following conditions. Afterward, the particles were removed from the pot, and the particle size was adjusted by appropriately grinding and sieving to achieve an average particle size of 8 μm, thereby obtaining a negative electrode active material.

[0178] <Conditions for Mechanical Alloying Treatment>

[0179] · Device used: Planetary ball mill P6 (FRITSCH product)

[0180] · Rotation speed: 500 rpm

[0181] · Container capacity: 250 mL

[0182] · Temperature: Room temperature

[0183] · Atmosphere: Ar atmosphere

[0184] · Processing time: 3 hours

[0185] (Manufacturing of coin cells)

[0186] The obtained cathode active material, an aqueous dispersion of carbon black (CB) and single-layer carbon nanotubes (SWCNT) as conductive materials (solid content 0.4%), styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were prepared in a mass ratio of 88.8:3.3:0.25:3.65:4.0. First, the cathode active material, CB, and CMC were mixed, and the SWCNT dispersion and water were added and kneaded. Finally, SBR was added and mixed to prepare a cathode active material slurry.

[0187] The obtained slurry was uniformly coated onto a copper foil, vacuum dried at 110°C for 10 hours, and then cut into a circular shape with a diameter of 13 mm to manufacture a cathode. A 2016-type coin cell using lithium metal was manufactured as the counter electrode to this cathode. Here, during the manufacture of the cathode, the electrode weight (i.e., the mass of the cathode composite material (from which the solvent has been removed from the cathode active material slurry) per unit area of ​​the cathode) was recorded. Meanwhile, the weight of the cathode composite material alone was calculated by subtracting the weight of the current collector, which had been measured in advance, from the total weight of the electrode.

[0188] [Example 2]

[0189] A negative electrode active material and a coin cell containing the negative electrode active material were manufactured in the same manner as in Example 1, except that CVD treatment was performed on the powder after mechanical alloying treatment. Specifically, the negative electrode active material obtained in Example 1 was subjected to CVD treatment using ethylene gas for 1 hour while rotating the core tube at 750°C to coat the particle surface of the powder with carbon. The particle size of the powder after CVD treatment was adjusted by appropriately grinding and sieving it so that the average particle size was 8 μm, thereby obtaining the negative electrode active material of Example 2. From the mass ratio before and after CVD treatment, it was confirmed that the product after treatment contained 10 mass% of carbon.

[0190] [Comparative Example 1]

[0191] A negative electrode active material and a coin cell containing the negative electrode active material were manufactured in the same manner as in Example 1, except that the plasma sintering temperature was set to 750°C and the mechanical alloying treatment was omitted.

[0192] [Comparative Example 2]

[0193] A negative electrode active material and a coin cell containing the negative electrode active material were manufactured in the same manner as in Example 1, except that the amount of zirconia balls in the mechanical alloying treatment was 100g.

[0194] [Comparative Example 3]

[0195] A negative electrode active material and a coin cell containing the negative electrode active material were manufactured in the same manner as in Example 2, except that the plasma sintering temperature was set to 750°C, the CVD treatment time was set to 1 hour, and the mechanical alloying treatment was omitted.

[0196] [Comparative Example 4]

[0197] A negative electrode active material and a coin cell containing the negative electrode active material were prepared in the same manner as in Example 2, except that the amount of zirconia balls in the mechanical alloying treatment was 100g.

[0198] [Evaluation Example 1: Measurement of X-ray Diffraction (XRD) Pattern]

[0199] Figure 1 is a diagram showing the powder XRD patterns of the cathode active materials obtained in Example 1, Example 2, and Comparative Example 1. In Comparative Example 1, in which mechanical alloying treatment was not performed, a sharp peak of Si was observed around 28.5°, and peaks of MgSiO3 and Mg2SiO4 were also confirmed. On the other hand, in Examples 1 and 2, in which mechanical alloying treatment was performed, it was confirmed that each peak of the XRD pattern broadened. Using the Scherrer equation, the crystallite size of each sample was evaluated from the linewidth of the peaks. Meanwhile, in Examples 1 and 2, no peak of Mg2SiO4 was confirmed, suggesting that Mg2SiO4 was in a completely amorphous state.

[0200] [Evaluation Example 2: Initial Characteristics of the Battery]

[0201] For the coin cells of each example and comparative example, charging and discharging were performed with a constant current of 0.2C and a cutoff voltage of 1.5V. The 'initial capacity' is defined as the value obtained by dividing the discharge capacity during this initial charging and discharging process by the mass (g) of the negative electrode active material used in each example and comparative example.

[0202] [Mathematical Formula 1]

[0203]

[0204] In addition, the charge / discharge efficiency in this initial charge / discharge process (hereinafter referred to as 'initial efficiency') is defined as follows.

[0205] [Mathematical Formula 2]

[0206]

[0207] [Evaluation Example 3: Battery Cycle Characteristics]

[0208] For the coin cells manufactured according to each example and comparative example, following the initial charge-discharge process performed in Evaluation Example 2, one more charge-discharge was performed under the same conditions, and then the same charge-discharge was repeated 48 times with a constant current of 0.5C. That is, in addition to the first and second charge-discharge processes, a total of 50 charge-discharge processes were repeated. FIG. 2 is a diagram showing the cycle characteristics of the coin cells of Example 1, Comparative Example 1, and Comparative Example 2 that were not subjected to CVD treatment. FIG. 3 is a diagram showing the cycle characteristics of the coin cells of Example 2, Comparative Example 3, and Comparative Example 4 that were subjected to CVD treatment. Specifically, FIG. 2 and FIG. 3 are graphs plotted with the number of charge-discharge cycles on the horizontal axis and the discharge capacity of the coin cells of each example and comparative example on the vertical axis.

[0209] [Evaluation Example 4: Battery Expansion Characteristics]

[0210] For the coin cells prepared according to each example and comparative example, a 51st charge was performed after 50 charge-discharge cycles of Evaluation Example 3. Subsequently, the charged coin cells were disassembled, the thickness of the negative electrode was measured, and the thickness of the current collector was subtracted from the measurement to determine the thickness (t) of the negative active material layer. The electrode weight (w) (unit: g / cm²) of the negative active material recorded at the time of manufacturing the negative electrode was divided by the thickness (t) of the negative active material layer to calculate the density d = w / t (unit: g / cm³) of the negative active material layer. Furthermore, the discharge capacity per mass (c) (unit: mAh / g) at the 50th charge-discharge cycle was multiplied by the density (d) of the negative active material layer to calculate the discharge capacity per volume C50 = c × d (unit: mAh / cm³) after 50 repeated charge-discharge cycles. 3...was calculated. The value of C50 includes factors of both the cycle characteristics of the battery and the expansion characteristics of the negative electrode active material. Specifically, as the cycle characteristics of the battery improve, the 50th discharge capacity (c) increases, and as the expansion of the negative electrode active material decreases, the density (d) of the negative electrode active material layer increases; therefore, the value of C50 increases due to either the improvement of cycle characteristics or the suppression of the expansion of the negative electrode active material. Thus, a sufficiently large value of C50 means that the balance between the cycle characteristics and the expansion characteristics of the negative electrode active material is excellent.

[0211] The results of Evaluation Examples 1 to 3 in each example and comparative example are summarized in the table below. Meanwhile, regarding the discharge capacity per volume (C50), the value is shown as a value normalized by setting the value of Example 2 to 100 for comparison.

[0212] [Table 1]

[0213]

[0214] As described above, in Examples 1 and 2, in which mechanical alloying was performed relatively vigorously using 200g of zirconia balls, the crystallite sizes of Si and MgSiO3 became 10 nm or less, and a battery with excellent initial capacity and discharge capacity C50 per volume after 50 charge-discharge cycles was obtained. In these examples, it is believed that not only the initial capacity but also the cycle characteristics were improved, and the expansion of the negative electrode active material could be suppressed. In addition, it was confirmed that Example 2, in which a carbon coating was formed by CVD treatment, exhibited even better initial efficiency, cycle characteristics, and C50 than Example 1.

[0215] Meanwhile, in Comparative Examples 1 and 3, in which mechanical alloying treatment was not performed, the crystallite size of Si exceeded 100 nm, and both the initial capacity and C50 were inferior to those of Examples 1 and 2. In addition, in Comparative Examples 2 and 4, in which mechanical alloying was performed gently using 100 g of zirconia balls, the crystallite sizes of Si were 65 nm and 72 nm, respectively. As shown in Figures 2 and 3, it was confirmed that in Comparative Examples 2 and 4, the decrease in discharge capacity was greater than in Comparative Examples 1 and 3, in which mechanical alloying treatment was not performed, and the cycle characteristics were inferior.

Claims

1. Composite particles containing Si phase and magnesium silicide oxide, and A cathode active material having a crystallite size of the above Si phase and a crystallite size evaluated from a powder X-ray diffraction pattern for at least one phase included in magnesium silicide oxide, which is 10 nm or less.

2. In Paragraph 1, A cathode active material wherein the magnesium silicide oxide comprises at least one of Mg2SiO4 and MgSiO.

3. In Paragraph 1 or 2, A negative electrode active material in which at least one of the Si phase and the magnesium silicide oxide contained in the composite particles is amorphous.

4. In Paragraph 1 or 2, A negative electrode active material in which the amount of Si phase contained in the above composite particles is 20 mass% or more and 70 mass% or less based on the total mass of the above composite particles.

5. In Paragraph 1 or 2, The above composite particles are a cathode active material that substantially does not contain MgO.

6. In Paragraph 1 or 2, The above composite particles are a cathode active material having a surface coated with a carbon-based material.

7. In Paragraph 6, A negative electrode active material, wherein the amount of the carbon-based material coating the composite particle is 5 mass% or more and 20 mass% or less based on the amount of the composite particle (excluding the carbon-based material).

8. Cathode current collector, and A cathode having a cathode active material layer formed on the cathode current collector, the layer comprising the cathode active material described in claim 1 or 2.

9. A non-aqueous electrolyte secondary battery having the negative electrode described in paragraph 8.

10. Silicon oxide (SiO₂) a A reduction step in which , 0<a≤2) reacts with metallic magnesium (Mg), An acid treatment step for acid treating the material after the above reaction, and A method for manufacturing a cathode active material, comprising a mechanical alloying step for the material after the above reaction, wherein mechanical alloying is performed until the crystallite size evaluated from the powder X-ray diffraction pattern becomes 10 nm or less.

11. In Paragraph 10, A method for manufacturing a cathode active material, wherein the above mechanical alloying is performed by one or more means selected from the group consisting of a planetary mill, a rotary mill, a vibrating mill, an electric mill, and a stirring mill.

12. In Paragraph 10 or 11, A method for manufacturing a cathode active material in which the above mechanical alloying is performed under an inert atmosphere.

13. In Paragraph 10 or 11, A method for manufacturing a cathode active material in which MgO is removed by reacting with acid through the above acid treatment step.

14. In Paragraph 10 or 11, In the above reduction step, silicon oxide (SiO₂) a A method for manufacturing a cathode active material in which at least a portion of ) is reduced to Si.

15. In Paragraph 10 or 11, A method for manufacturing a negative electrode active material, further comprising a coating step of coating the material after the above reaction with a carbon-based material.

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