Negative electrode active material for secondary battery and method for producing same

Abstract

Provided is a negative electrode active material for secondary batteries, comprising a composite material that includes: an active phase which reacts with Li; and an amorphous material phase, wherein the active phase is dispersed in the amorphous material phase, and the amorphous material phase includes Si, O, and C.

Description

[Technical Field]

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

[0002] Non-aqueous electrolyte secondary batteries, especially lithium-ion secondary batteries, have high voltage and high energy density, and are therefore expected to be used in small consumer applications, power storage devices, and electric vehicles. As higher energy densities are required for batteries, silicon (Si)-containing materials, which alloy with lithium, are expected to be used as negative electrode active materials with high theoretical capacity densities.

[0003] Patent Document 1 proposes a negative electrode active material for lithium secondary batteries in the form of a carbon-metal composite or mixture, which includes a carbon-based active material with a first ceramic coating layer and a metal-based active material, or a metal-based active material with a first ceramic coating layer and a carbon-based active material. The metal-based active material is a metal selected from the group consisting of silicon, tin, aluminum, vanadium, magnesium, antimony, or an alloy formed from a combination of one or more of these, or a compound selected from the group consisting of an oxide, nitrate, or carbide of the metal, or a combination thereof.

[0004] Patent Document 2 proposes a method for producing a porous amorphous material, which comprises impregnating a porous resin with one or more organosilicon compounds selected from crosslinkable silanes and siloxanes, forming a crosslinked product of the organosilicon compounds within the porous resin, and then heating and reacting the crosslinked product in a non-oxidizing gas at a temperature of 650 to 1350°C to obtain an amorphous material that contains silicon, carbon, and oxygen as constituent elements and has oxidation resistance.

[0005] Patent Document 3 proposes a lithium secondary battery having a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator, in which the positive electrode has a positive electrode mixture layer containing a lithium-containing composite oxide as a positive electrode active material on one or both sides of a current collector, and the negative electrode has a negative electrode mixture layer on one or both sides of a current collector, the negative electrode containing a composite of a material containing Si and O as constituent elements (wherein the atomic ratio x of O to Si is 0.5≦x≦1.5) and a carbon material, and a graphite carbon material as a negative electrode active material, in which the ratio P / N of the mass P of the positive electrode active material to the mass N of the negative electrode active material is 1.0 to 3.6, and the non-aqueous electrolyte contains 1 to 10 mass % of a halogen-substituted cyclic carbonate and 1 to 10 mass % of vinylene carbonate. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-18788 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-115548 [Patent Document 3] Japanese Patent Application Laid-Open No. 2011-233245 Summary of the Invention [Problem to be solved by the invention]

[0007] The metal-based active material including the first ceramic coating layer of Patent Document 1 expands and contracts significantly during charging and discharging, making it difficult to put into practical use. The porous amorphous material obtained by the manufacturing method of Patent Document 2 has a low capacity, making it difficult to achieve a sufficiently high capacity. The material containing Si and O as constituent elements of Patent Document 3 has a large irreversible capacity, making it difficult to control the capacity.

[0008] Meanwhile, materials made by dispersing silicon in lithium silicate and sintering the resulting material have been investigated. However, the electrolyte seeps into the grain boundaries that form during sintering, causing side reactions that decompose the electrolyte and lead to material degradation. [Means for solving the problem]

[0009] In view of the above, one aspect of the present invention relates to a negative electrode active material for a secondary battery, comprising a composite material including an active phase that reacts with Li and an amorphous material phase, wherein the active phase is dispersed in the amorphous material phase, and the amorphous material phase contains Si, O, and C.

[0010] Another aspect of the present invention relates to a method for producing a negative electrode active material for a secondary battery, comprising the steps of obtaining a mixture containing a first polymer, which is an organosilicon polymer containing Si, O, and C, and a material that constitutes an active phase that reacts with Li, and firing the mixture to produce a composite material that contains the active phase and an amorphous material phase containing Si, O, and C, wherein the active phase is dispersed in the amorphous material phase.

[0011] Yet another aspect of the present invention relates to a secondary battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte solution, wherein the negative electrode contains the above-described negative electrode active material for secondary batteries. [Effects of the Invention]

[0012] It is possible to provide a negative electrode active material for a secondary battery that has a high capacity and is resistant to deterioration. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present application, will be better understood from the following detailed description taken in conjunction with the drawings. [Brief explanation of the drawings]

[0013] [Figure 1A] 1 is a partially cutaway plan view schematically illustrating the structure of a secondary battery according to an embodiment. [Figure 1B] 1B is a cross-sectional view of the secondary battery shown in FIG. 1A taken along line XX'. [Figure 2] 1 is a cross-sectional TEM image of a composite material according to an embodiment of the present disclosure. [Figure 3] FIG. 3 is a diagram showing a TEM-EELS spectrum of the composite material shown in FIG. 2. [Figure 4] FIG. 3 is a diagram showing a C1s spectrum in an XPS analysis of the composite material shown in FIG. 2. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the present disclosure will be described using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values, materials, etc. may be exemplified, but other numerical values, materials, etc. may be applied as long as the effects of the present disclosure are obtained. Constituent elements other than those characteristic of the present disclosure may be those of known secondary batteries and their negative electrodes. In this specification, when a "range between numerical value A and numerical value B" is mentioned, the range includes numerical value A and numerical value B. When multiple materials are exemplified, one may be selected from the materials and used alone, or two or more may be used in combination.

[0015] The negative electrode active material for a secondary battery according to an embodiment of the present disclosure includes a composite material including an active phase and an amorphous material phase. The form of the composite material is not particularly limited, and it may be supplied in a bulk state, a powder containing a plurality of particles, or the like.

[0016] <Composite materials> The active phase is formed of a material that exhibits capacity through a Faraday reaction with lithium (Li). The active phase is dispersed in an amorphous material phase. The active phase repeatedly expands and contracts with a significant volume change during charging and discharging. In other words, the active phase is a material that expands more than the amorphous material phase when the negative electrode active material absorbs lithium. The active phase may contain a metal element that alloys with lithium.

[0017] The amorphous material phase is made of a material that can generate a smaller capacity per unit mass (mAh / g) than the active phase. The amorphous material phase undergoes little or no volume change during charging and discharging. The amorphous material phase relieves stress caused by the expansion and contraction of the active phase and suppresses side reactions between the active phase and the electrolyte.

[0018] The composite material may have an island-sea structure. In the island-sea structure, the active phase constitutes the island portions, and the amorphous material phase constitutes a continuous sea portion or matrix. When the composite material is a powder, a plurality of island portions are dispersed in the sea portion within a single particle of the composite material. In some of the island portions, at least a portion of the surface of the island portion may be exposed and not covered by the sea portion. From the viewpoint of alleviating stress and enhancing the effect of suppressing side reactions with the electrolyte, it is desirable that, for example, 80% or more of the island portions are buried in the sea portion on a number basis. It is desirable that 80% or more of the surface of the island portions buried in the sea portion is covered by the sea portion, and they may be completely covered by the sea portion. The proportion of the island portions buried in the sea portion on a number basis is 1 μm 2 It can be determined from a cross-sectional TEM photograph of the composite material having the above field of view.

[0019] The amorphous material phase contains Si (silicon), O (oxygen), and C (carbon). The amorphous material phase forms a compound in which Si, O, and C are randomly bonded by covalent bonds. The amorphous material phase has, for example, Si-O-C bonds (covalent bonds). In other words, the amorphous material phase is not a simple mixture or complex of multiple compounds (SiO, SiC, etc.). The amorphous material phase is a phase with an unspecified crystal structure. The amorphous material phase may form a single phase without an interface between different materials, as occurs in composites of two or more materials. When the amorphous material phase contains Si, O, and C as essential components and is a phase with an unspecified crystal structure, the composite material becomes hard and also has elasticity or flexibility that relieves stress caused by the expansion and contraction of the active phase.

[0020] In the cross-section of the composite material, it is desirable that the amorphous material phase has substantially no grain boundaries. Since the amorphous material phase has a continuous structure substantially without grain boundaries, the hardness of the composite material becomes higher, the phenomenon of the electrolyte penetrating into the grain boundaries is reduced, side reactions accompanied by the decomposition of the electrolyte are suppressed, and the material is less likely to deteriorate. In this case, the composite material can be formed as primary particles. However, the amorphous material phase may have grain boundaries even if there are a few. The state where the amorphous material phase substantially has no grain boundaries can be confirmed by the cross-sectional TEM image of the composite material. When the field of view is 500 nm 2 in the cross-sectional TEM image of the composite material, if the total length of the grain boundaries in the amorphous material phase is 1 nm or less, it can be said that the amorphous material phase substantially has no grain boundaries. It is desirable to perform the same measurement on any 10 composite particles and average the measured values to obtain the length of the grain boundaries.

[0021] When the amorphous material phase is represented by the general formula (1): Li a SiO x N y C z it satisfies, for example, 0 ≦ a ≦ 2, 0.1 ≦ x ≦ 1.5, 0 ≦ y ≦ 0.5, 0 < 1 - 0.5x - 0.75y < z ≦ 6. Such a composition is a composition empirically determined from the composition of the amorphous material phase produced by the manufacturing method described later. During charge and discharge, Li can be trapped as an irreversible capacity. Also, lithium may be pre-doped. In this case, the range of a may satisfy 0.1 ≦ a ≦ 2.

[0022] In the general formula (1), the range of z indicating the atomic ratio of C to Si may be 0.1 ≦ z ≦ 6, or 0.5 ≦ z ≦ 4, or 0.5 ≦ z ≦ 3. By controlling the amount of carbon contained in the amorphous material phase within the above range, it becomes difficult to form Si - O - Li bonds, the irreversible capacity of the composite material is reduced, and the amorphous material phase itself comes to have a capacity, which is also advantageous for increasing the capacity.

[0023] The composition of the amorphous material phase (quantitation of each element in the composite material) can be determined by transmission electron microscope (TEM)-energy dispersive X-ray (EDX) analysis. In TEM-EDX analysis, the cross section of the composite material is observed using a TEM, and elemental mapping analysis is performed using EDX. The observation magnification is preferably 2000 to 20,000 times. Mapping analysis is preferably performed on a region at least 1 μm inward from the peripheral edge of the cross section of the composite material particle. In mapping analysis, the elemental content (i.e., composition) can be calculated using image analysis software. Similar measurements can be performed on 10 random particles and the measured values ​​can be averaged to determine the composition. The maximum diameter of the particles to be measured is preferably 5 μm or more. The cross section of the composite material particle can be formed, for example, by filling the composite material in a thermosetting resin, curing it, and then using a cross-section polisher (CP). Alternatively, the battery can be disassembled, the negative electrode removed, immersed in a thermosetting resin, and cured, and then formed on the cross section of the negative electrode.

[0024] Desirable measurement conditions for cross-sectional TEM-EDX analysis are shown below. Processing equipment: JEOL SM-09010 (Cross Section Polisher) Processing conditions: Accelerating voltage 6 kV Current value: 140 μA Vacuum degree: 1×10 -3 ~2×10 -3 Pa Measuring device: JEOL JEM-F200 Accelerating voltage during analysis: 200 kV

[0025] In the C1s spectrum obtained by X-ray photoelectron spectroscopy (XPS) of the composite material, several types of C1s peaks contained in the amorphous material phase are observed. For example, C1s peaks attributed to Si-C bonds, Si-O-C bonds, C-O bonds, C-C bonds, etc. are observed. The presence of such peaks indicates that the amorphous material phase contains a considerable amount of regions other than oxides. In particular, the C1s peak attributed to C-C bonds is a peak characteristic of the composite material of the present disclosure, indicating that the amorphous material phase contains a region composed of carbon. The peak attributed to C-C bonds is observed, for example, around 284.8 eV.

[0026] Here, the ratio of the peak area Sx derived from CC bonds to the total area St of all C1s peaks is, for example, 2% or more, or may be 3% or more, or 5% or more. The ratio of Sx to St is, for example, 90% or less, or may be 70% or less, or may be 20% or less. The ratio of Sx to St may be 2% or more and 90% or less, or may be 2% or more and 20% or less.

[0027] Furthermore, the ratio of the sum of the peak area Sy derived from the C-Si bond and the peak area Sz derived from the CO-Si bond to the total area St may be 50% or more, 60% or more, or 70% or more.

[0028] In XPS of composite materials, powder of the composite material is used as a sample, and analysis is performed from the sample surface along the depth direction of the composite material (10 to 100 nm) to analyze the internal state of the composite material.

[0029] Desirable XPS measurement conditions are shown below. Measurement equipment: ULVAC-PHI PHI5000 X-rays used: Monochrome Al-Kα, 25W, 15kV Vacuum degree: 5×10 -7 Pa

[0030] In the C-NMR measurement of the composite material, peaks derived from C-C bonds characteristic of the composite material according to the present disclosure are also observed. The peak area Sv between 9 ppm and 30 ppm in the spectrum obtained by the C-NMR measurement of the composite material can be, for example, between 10% and 90% of the area of ​​the entire spectrum.

[0031] Desirable 13C NMR measurement conditions are shown below. Measuring device: JNM-ECZ-600 Magnetic field strength: 14.1T MAS-rotation speed: 15kHz

[0032] As described above, the present disclosure provides a composite material with high hardness and resistance to deterioration, in which the amorphous material phase has substantially no grain boundaries. The hardness of the particles of the composite material can be evaluated by the fracture hardness of the particles. The fracture hardness of the particles can be, for example, 600 MPa or less when the particle diameter (maximum diameter) is 6 μm, and can be, for example, 500 MPa or less when the particle diameter (maximum diameter) is 12 μm. In this case, the particles are less likely to be crushed during the electrode plate manufacturing process and charge / discharge cycles, which is advantageous for suppressing deterioration of cycle characteristics. However, it is desirable that the fracture strength be, for example, 100 MPa or more when the particle diameter (maximum diameter) is 6 μm, and, for example, 50 MPa or more when the particle diameter (maximum diameter) is 12 μm. The fracture strength may be calculated as the average value of five particles.

[0033] The particles used to measure breaking strength are prepared so that the particle diameter (maximum diameter) obtained from the captured image is the specified particle diameter (6 μm or 12 μm) and has a circularity of 75% or more. The particles are compressed with an indenter while gradually increasing the load. The load at which the particle breaks is taken as the breaking strength of that particle. Breaking strength can be measured using a microcompression tester MCT-211 manufactured by Shimadzu Corporation. For example, using a flat indenter with a tip diameter of 50 μm and a displacement rate of 5 μm / sec, the breaking strength of five particles is measured and the average value is calculated. If a clear breaking point cannot be determined, the breaking strength is calculated from the test force at a compression rate of 20%.

[0034] The active phase may be composed of any material capable of electrochemically reacting reversibly with Li. Such a material may include, for example, at least one selected from the group consisting of metals and intermetallic compounds. The material constituting the active phase may also be a silicon compound such as silicon carbide, or a composite oxide such as a lithium-titanium composite oxide. The active phase may be composed of one material alone or two or more materials in combination.

[0035] When a metal is used as the material constituting the active phase, the metal may be at least one selected from the group consisting of Si, Sn, Ti, Al, and Mg. Among them, Si and Sn have high capacity, and Si is particularly preferable because it is inexpensive.

[0036] When an intermetallic compound is used as the material that constitutes the active phase, the intermetallic compound may be at least one selected from the group consisting of CrSi2, MnSi2, FeSi2, CoSi2, NiSi2, and LiNiSn.

[0037] The mass proportion of the active phase in the composite material can be appropriately controlled. It is desirable that the mass proportion of the active phase in the composite material be as high as possible from the viewpoint of obtaining a high-capacity active material. On the other hand, it is necessary for the composite material to contain a certain amount of amorphous material phase from the viewpoint of alleviating stress due to expansion and contraction of the active phase and suppressing side reactions between the active phase and the electrolyte. The mass proportion of the active phase in the composite material may be, for example, 20% by mass or more and 95% by mass or less, or 35% by mass or more and 75% by mass or less.

[0038] When the active phase is silicon, the content of the Si phase in the composite material can be measured by Si-NMR.

[0039] When the active phase is particulate, the average particle diameter of the active phase may be 1 nm or more and 1,000 nm or less. A particulate state refers to a state in which the active phase (or island portions) each have a particle shape. The particle shape is not particularly limited, but the ratio A / B of the maximum particle diameter A to the maximum width B in the direction perpendicular to the maximum diameter may be, for example, 1 to 20, 1 to 10, 1 to 5, or 1 to 3. A / B may be calculated as the average value of any 10 active phases (or island portions) having a particle shape. The smaller the average particle diameter of the active phase, the easier it is to relieve stress due to expansion and contraction of the active phase and to suppress deterioration due to cracks in the active phase itself. On the other hand, the larger the average particle diameter of the active phase, the easier it is to suppress side reactions between the active phase and the electrolyte. The average particle diameter of the active phase may be 200 nm or less, 100 nm or less, or 50 nm or less.

[0040] The average particle size of the active phase is measured using a cross-sectional image of the composite material obtained by TEM or SEM, and is calculated by averaging the maximum diameters of 100 randomly selected active phase particles.

[0041] <Negative electrode active material> The negative electrode active material may contain at least one carbon material selected from the group consisting of natural graphite, artificial graphite, hard carbon, and soft carbon, in addition to the composite material. The carbon material may also be composited with an amorphous material phase. For example, the active phase may be dispersed in a composite material of the carbon material and the amorphous material phase.

[0042] The carbon material expands and contracts less during charge and discharge than the active phase, and therefore its use in combination with the composite material facilitates improving the cycle characteristics of the battery. The content of the carbon material in the negative electrode active material may be, for example, 70% by mass or more and 99% by mass or less, or 85% by mass or more and 95% by mass or less, or 90% by mass or more and 95% by mass or less. This facilitates achieving both high capacity and better cycle characteristics.

[0043] <Method of manufacturing a negative electrode active material containing a composite material> The composite material contained in the negative electrode active material for secondary batteries can be produced, for example, by the following production method (hereinafter also referred to as "production method A") Production method A includes a first step and a second step.

[0044] (i) First step The first step is to obtain a mixture containing a first polymer and a material that constitutes an active phase that reacts with Li. The first polymer is a raw material for an amorphous material phase that contains Si, O, and C. The first polymer may be an organosilicon polymer.

[0045] Organosilicon polymers, also known as ceramic precursor polymers, can be converted into ceramics by controlling the firing conditions. Organosilicon polymers are generally soluble in organic solvents and are easy to handle. Many organosilicon polymers are thermoplastic and become liquid when heated in the subsequent second step. The organosilicon polymer may be a polymer that is liquid at room temperature (25°C to 35°C). By using a first polymer that is liquid at room temperature or when heated, an amorphous material phase with substantially no grain boundaries can be produced, resulting in a composite material with high particle fracture strength.

[0046] The first polymer may be, for example, polysiloxane, polycarbosilane, polysilazane, silicone resin, silicone oil, polyorganoborosilazanes, polymetalloxanes, polyborosiloxanes, polycarbosilazanes, etc. These may be used alone or in combination of two or more. For example, at least one selected from the group consisting of polysiloxane, polycarbosilane, polysilazane, silicone resin, and silicone oil may be used. Some structural examples of the repeating units of the first polymer are shown in the following formulas (2) to (4).

[0047] Formula (2): [ka]

[0048] The first polymer represented by formula (3) is called polycarbosilane. [ka]

[0049] The first polymer represented by formula (4): is called a polysiloxane. [ka]

[0050] These first polymers are called polysilazanes. These first polymers can produce an amorphous material phase represented by general formula (1).

[0051] In formulas (2) to (4), R1 and R2 are each independently, for example, a hydrogen atom or an organic group having 1 to 8 carbon atoms. The organic group includes hydrocarbon groups having a substituent (or functional group) and hydrocarbon groups having no substituent (or functional group). The functional group may be, but is not limited to, a hydroxyl group, a cyano group, an amino group, or the like. The hydrocarbon group may be, for example, an alkyl group, a vinyl group, an alkoxy group, an aryl group, an aryloxy group, a ketone group, a carboxyl group, an ester group, or the like. The multiple repeating units of the first polymer may have the same structure or different structures. That is, in the multiple repeating units of the first polymer, R1 and R2 may be the same or different.

[0052] Examples of alkyl groups include methyl, ethyl, propyl, isopropyl, butyl, t-butyl, pentyl, and hexyl groups. Examples of aryl groups include phenyl, benzyl, and toluyl groups. Examples of aryloxy groups include phenoxy groups. Examples of alkoxy groups include oxyalkyl groups having 1 to 8 carbon atoms. Examples of ester groups include condensation groups of alcohols having 1 to 8 carbon atoms with carboxylic acids having 1 to 8 carbon atoms. For example, R1 and R2 may each independently be a phenyl group, a methyl group, an ethyl group, or the like.

[0053] The weight average molecular weight (Mw) of the first polymer may be, for example, 1,000 or more and 100,000 or less, 1,000 or more and 10,000 or less, or 2,000 or more and 10,000 or less.

[0054] The material constituting the active phase may be particles of the aforementioned materials (hereinafter also referred to as "active particles"). The active particles may be nanoparticles. The average particle size of the active particles may be 1 nm or more and 1000 nm or less, 200 nm or less, 100 nm or less, or 50 nm or less. For example, silicon nanoparticles may be used as the active particles.

[0055] The average particle size of the active particles is measured using a cross-sectional image of the composite material obtained by TEM or SEM, and is calculated by averaging the maximum diameters of 100 randomly selected active particles.

[0056] In the mixture, the amount of material constituting the active phase per 100 parts by mass of the first polymer may be set appropriately depending on the desired proportion of the active phase of the composite material, but may be, for example, 20 parts by mass or more and 250 parts by mass or less, or 50 parts by mass or more and 200 parts by mass or less.

[0057] (ii) Second step The second step is to calcinate the mixture to produce a composite material containing an active phase and an amorphous phase containing Si, O, and C, with the active phase dispersed in the matrix of the amorphous phase. The mixture containing the first polymer and the material constituting the active phase is in a fluid slurry state at room temperature, or at least passes through a fluid state when heated for calcination. In other words, most of the surface of the material constituting the active phase is covered with the fluid first polymer. Using a mixture in such a state is desirable for obtaining a dense composite material.

[0058] The mixture can be fired, for example, at a temperature of 600°C or higher and 1000°C or lower in an inert atmosphere. The inert atmosphere may be a reduced pressure atmosphere or may be a flow of inert gas. The inert gas may be argon, nitrogen, helium, or the like. The firing time may be any time that is sufficient to sufficiently carbonize the carbon atoms contained in the first polymer. The composite material obtained after firing is a solid that does not have fluidity. By pulverizing the composite material, a powdered composite material is produced.

[0059] In the step of obtaining the mixture, a second polymer that is carbonized together with the carbon atoms in the first polymer when the mixture is fired may be included in the mixture. The second polymer may be included in the mixture as at least one raw material selected from the group consisting of natural graphite, artificial graphite, hard carbon, and soft carbon.

[0060] The second polymer is not particularly limited, but is preferably a material that has excellent compatibility with the first polymer. The second polymer may be, for example, at least one selected from the group consisting of polyvinyl resin, polyimide resin, polyacrylonitrile, acrylic resin, and polyolefin resin.

[0061] [Negative electrode] The negative electrode will be further described below. The negative electrode includes, for example, a sheet-like negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector. The negative electrode active material is contained in, for example, the negative electrode mixture layer. The negative electrode mixture layer is a negative electrode mixture formed in the form of a layer or a coating film. The negative electrode mixture includes the negative electrode active material as an essential component, and may include a binder, a conductive additive, a thickener, etc. as optional components.

[0062] The negative electrode mixture layer can be formed, for example, by applying a negative electrode slurry, in which the negative electrode mixture is dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the applied film. The dried coating film may be rolled, if necessary.

[0063] The negative electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0064] Examples of binders include resin materials, such as fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resin; polyimide and polyamideimide; acrylic resins such as polyacrylic acid, polymethyl acrylate, and ethylene-acrylic acid copolymer; vinyl resins such as polyacrylonitrile and polyvinyl acetate; polyvinylpyrrolidone; polyethersulfone; and rubber-like materials such as styrene-butadiene copolymer rubber (SBR). These may be used alone or in combination of two or more.

[0065] Examples of conductive additives include carbon black such as acetylene black, carbon nanotubes (hereinafter also referred to as CNTs), metal fibers, carbon fluoride, metal powder, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, and organic conductive materials such as phenylene derivatives. These may be used alone or in combination of two or more.

[0066] Next, an example of a secondary battery using the negative electrode active material for a secondary battery according to the present disclosure will be described. The secondary battery includes, for example, the above-described negative electrode, a positive electrode, and an electrolyte solution.

[0067] [Positive electrode] The positive electrode includes a positive electrode active material capable of electrochemically absorbing and desorbing lithium ions. The positive electrode includes, for example, a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry, in which the positive electrode mixture is dispersed in a dispersion medium, to the surface of the positive electrode current collector and drying the applied layer. The dried coating may be rolled as necessary. The positive electrode mixture includes a positive electrode active material as an essential component and may include a binder, a conductive agent, etc. as optional components.

[0068] As the positive electrode active material, for example, a lithium-containing composite oxide can be used. a CoO2, Li a NiO2, Li aMnO2, Li a Co b Ni 1-b O2, Li a Co b M 1-b O c , Li a Ni 1-b M b O c , Li a Mn2O4, Li a Mn 2-b M b O 4、 LiMPO 4、 Li2MPO4F (M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B). Here, a = 0 to 1.2, b = 0 to 0.9, and c = 2.0 to 2.3. The value a, which indicates the molar ratio of lithium, increases or decreases with charge and discharge.

[0069] The binder and conductive agent may be the same as those exemplified for the negative electrode. As the conductive agent, graphite such as natural graphite or artificial graphite may be used.

[0070] The shape and thickness of the positive electrode current collector can be selected from the shape and range corresponding to those of the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0071] [Electrolyte] The electrolyte solution contains a solvent and an electrolyte salt. The solvent may be a non-aqueous solvent, or water may be used. In the case of a lithium ion secondary battery, the electrolyte salt contains at least a lithium salt.

[0072] The concentration of the lithium salt in the electrolyte solution is preferably, for example, 0.5 mol / L or more and 2 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte solution having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0073] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0074] Examples of lithium salts include LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, and LiB 10 Cl 10 Examples of the lithium salt include lithium carboxylates of lower aliphatic groups, LiCl, LiBr, LiI, borates, imide salts, etc. The lithium salts may be used singly or in combination of two or more.

[0075] [Separator] It is usually desirable to interpose a separator between the positive electrode and the negative electrode. The separator has ion permeability and insulating properties. The separator may be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0076] An example of the structure of the secondary battery is a structure in which an electrode group in which a positive electrode and a negative electrode are wound with a separator interposed therebetween and an electrolyte solution are housed in an exterior body. A stacked electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween may also be applied. The secondary battery may have a shape such as a cylindrical shape, a prismatic shape, a coin shape, a button shape, or a sheet shape.

[0077] A secondary battery according to one embodiment of the present invention will be described below with reference to Figures 1A and 1B. Figure 1A is a partially cutaway plan view schematically showing an example of the structure of a secondary battery. Figure 1B is a cross-sectional view taken along line XX' in Figure 1A.

[0078] As shown in FIGS. 1A and 1B, the secondary battery 100 is a sheet-type battery, and includes an electrode plate group 4 and an exterior case 5 that houses the electrode plate group 4.

[0079] The electrode plate group 4 has a structure in which a positive electrode 10, a separator 30, and a negative electrode 20 are stacked in this order, with the positive electrode 10 and the negative electrode 20 facing each other with the separator 30 interposed therebetween, thereby forming the electrode plate group 4. The electrode plate group 4 is impregnated with an electrolyte (not shown).

[0080] The positive electrode 10 includes a positive electrode mixture layer 1a and a positive electrode current collector 1b. The positive electrode mixture layer 1a is formed on the surface of the positive electrode current collector 1b.

[0081] The negative electrode 20 includes a negative electrode mixture layer 2a and a negative electrode current collector 2b. The negative electrode mixture layer 2a is formed on the surface of the negative electrode current collector 2b.

[0082] A positive electrode tab lead 1c is connected to the positive electrode current collector 1b, and a negative electrode tab lead 2c is connected to the negative electrode current collector 2b. The positive electrode tab lead 1c and the negative electrode tab lead 2c each extend to the outside of the outer case 5.

[0083] The positive electrode tab lead 1c and the outer case 5, and the negative electrode tab lead 2c and the outer case 5 are insulated by insulating tab films 6, respectively.

[0084] The present invention will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0085] Comparative Example 1 Polycarbosilane, an organosilicon polymer (R1 and R2 in formula (2) are -H and -CH2-CH=CH2 groups, respectively, and is liquid at room temperature), was baked in a nitrogen atmosphere at 850°C for 3 hours until the carbon atoms contained in the polycarbosilane were carbonized, yielding amorphous material B1. The composition of amorphous material B1 was measured by TEM-EDX and found to be SiO x N y C z (x=0.32, y=0, z=1.62).

[0086] Comparative Example 2 Polysiloxane, an organosilicon polymer (R1 and R2 in formula (3) are -H and -CH=CH2 groups, respectively, and is liquid at room temperature), was baked in a nitrogen atmosphere at 850°C for 3 hours until the carbon atoms contained in the polysiloxane were carbonized, yielding amorphous material B2. The composition of amorphous material B2 was measured by TEM-EDX and found to be SiO x N y C z (x=0.82, y=0, z=1.21).

[0087] Comparative Example 3 Polysilazane, an organosilicon polymer (R1 and R2 in formula (4) are -CH3 and -CH=CH2 groups, respectively, and is liquid at room temperature), was baked in a nitrogen atmosphere at 850°C for 3 hours until the carbon atoms contained in the polysilazane were carbonized, yielding amorphous material B3. The composition of amorphous material B3 was measured by TEM-EDX and found to be SiO x N y C z (x=1.4, y=0.2, z=2.6).

[0088] Example 1 Silicon nanoparticles with an average particle size of 40 nm were prepared as the material for the active phase. 200 parts by mass of the polycarbosilane used in Comparative Example 1 was mixed with 100 parts by mass of the nanoparticles until a uniform slurry was obtained, yielding a mixture. The resulting mixture was fired under the same conditions as in Comparative Example 1, yielding Composite Material A1. The content of nanoparticles (active phase) in Composite Material A1 was approximately 45% by mass.

[0089] The fracture strength of composite material A1 was measured by the method described above, and the fracture hardness was 375 MPa when the particle diameter was 6 μm and 226 MPa when the particle diameter was 12 μm. At least a portion of the surface of composite material A1 was coated with a carbonaceous conductive film.

[0090] Example 2 The same silicon nanoparticles as those used in Example 1 were prepared, and 100 parts by mass of the nanoparticles were mixed with 220 parts by mass of the polysiloxane used in Comparative Example 2 until a uniform slurry was obtained, yielding a mixture. The resulting mixture was fired under the same conditions as in Comparative Example 2, yielding Composite Material A2. The content of nanoparticles (active phase) in Composite Material A2 was approximately 45% by mass.

[0091] When the fracture strength of the composite material A2 was measured by the method described above, the fracture hardness was 335 MPa when the particle diameter was 6 um, and 210 MPa when the particle diameter was 12 um.

[0092] As in Example 1, at least a portion of the surface of the composite material A2 was coated with a carbonaceous conductive film.

[0093] [evaluation] <tem-edx> Figure 2 shows a cross-sectional TEM image of composite material A1 near the interface with the conductive film. In the cross-sectional TEM image, a sea of ​​amorphous material phase and islands of active phase (nano-Si) dispersed in the sea can be observed. Composite material A1 has a clear sea-island structure. No grain boundaries or voids can be observed in the amorphous material phase, indicating a dense structure.

[0094] EELS analysis was performed on (1) the island portion (inside the nano-Si), (2) the boundary between the island portion and the sea portion, and (3) the sea portion in the cross-sectional TEM image above. Figure 3 shows the EELS spectrum. Figure 3 shows that oxidation of the nanoparticles has not progressed and that there are no voids at the interface.

[0095] <xps> Next, XPS analysis of composite material A1 was performed. The C1s spectrum is shown in Figure 4. In the C1s spectrum, several types of C1s peaks attributable to Si-C bonds, Si-O-C bonds, C-O bonds, C-C bonds, etc. are observed.

[0096] In composite material A1, the ratio of the peak area Sx derived from C-C bonds to the total area St of all C1s peaks was 6.8%, the ratio of the peak area Sy derived from C-Si bonds to the total area St was 80.5%, and the ratio of the peak area Sz derived from CO-Si bonds to the total area St was 8.4%.

[0097] Similarly, XPS analysis of composite material A2 revealed that the ratio of the peak area Sx to the total area St was 9.7%, the ratio of the peak area Sy derived from C-Si bonds to the total area St was 12.6%, and the ratio of the peak area Sz derived from CO-Si bonds to the total area St was 66.8%.

[0098] <Test cell capacity> Cells containing amorphous materials B1 to B3 of Comparative Examples 1 to 3 and composite materials A1 and A2 of Examples 1 and 2 as active materials were assembled.

[0099] (1) Working electrode (negative electrode) A negative electrode mixture was prepared by mixing the active material (AM) consisting of amorphous materials B1-B3 or composite materials A1-A2, the conductive additive carbon nanotubes (CNT), the binder polyacrylic acid (PAA) and styrene-butadiene copolymer rubber (SBR), and the thickener carboxymethyl cellulose (CMC). The mass ratio of the materials in the negative electrode mixture was AM:CNT:PAA:CMC:SBR = 100:0.5:5:5:5. The negative electrode mixture was molded into a 12 mm diameter disk to prepare a coin-shaped negative electrode.

[0100] (2) Opposite A lithium metal foil was attached to one side of an electrolytic copper foil (current collector) and punched out to a diameter of 15 mm to prepare a counter electrode.

[0101] (3) Preparation of electrolyte An electrolyte solution was prepared by dissolving LiPF6 at a concentration of 1 mol / L in a mixed solvent of fluoroethylene carbonate (FEC) and dimethyl carbonate (DMC) in a volume ratio of 1:4.

[0102] (4) Cell preparation The electrode assembly was constructed by arranging the negative electrode and the counter electrode facing each other with a separator in between, and then housed in a coin-shaped outer can. After the electrolyte was poured into the outer can, the outer can was sealed to complete a coin-shaped cell with a design capacity of 5 mAh.

[0103] (5) Charge / discharge test In a thermostatic chamber at 25°C, the negative electrode was charged with lithium at a constant current of 0.05C (1C is the current value required to discharge the design capacity in 1 hour) for 2 hours, followed by a 12-hour rest period. Next, the negative electrode was further charged with lithium at a constant current of 0.05C until the cell voltage reached 0.01V, after which the battery was rested for 20 minutes. Next, lithium was discharged from the negative electrode at a constant current of 0.05C until the cell voltage reached 1.5V, and the discharge capacity was calculated. The results are shown in Table 1.

[0104] [Table 1]

[0105] Table 1 shows that each composite material is useful as a negative electrode active material that exhibits high capacity. [Industrial Applicability]

[0106] The negative electrode active material for secondary batteries according to the present invention is useful in negative electrodes for secondary batteries (particularly non-aqueous electrolyte secondary batteries) used as main power sources for mobile communication devices, portable electronic devices and the like. While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention. [Explanation of symbols]

[0107] 1a: positive electrode mixture layer, 1b: positive electrode current collector, 1c: positive electrode tab lead, 2a: negative electrode mixture layer, 2b: negative electrode current collector, 2c: negative electrode tab lead, 4: electrode plate group, 5: outer case, 6: insulating tab film, 10: positive electrode, 20: negative electrode, 30: separator, 100: secondary battery< / xps>

Claims

1. a composite material including an active phase that reacts with Li and an amorphous material phase; the active phase is dispersed in the amorphous phase; The amorphous material phase is represented by the general formula: Li a SiO x N y C z ; 0≦a≦2 0.1≦x≦1.5 0≦y≦0.5 A negative electrode active material for a secondary battery, which satisfies 0<1-0.5x-0.75y<z≦6.

2. 2. The negative electrode active material for a secondary battery according to claim 1, wherein the amorphous material phase does not substantially have grain boundaries in a cross section of the composite material.

3. 3. The negative electrode active material for a secondary battery according to claim 1, wherein in a C1s spectrum obtained by X-ray photoelectron spectroscopy (XPS) measurement of the composite material, a ratio of a peak area Sx derived from a C—C bond to a total area St of all C1s peaks is 2% or more and 90% or less.

4. 4. The negative electrode active material for a secondary battery according to claim 3, wherein a ratio of the peak area Sx to the total area St is 2% or more and 20% or less.

5. 5. The negative electrode active material for a secondary battery according to claim 3, wherein a ratio of a sum of a peak area Sy derived from a C—Si bond and a peak area Sz derived from a C—O—Si bond to the total area St is 50% or more.

6. 6. The negative electrode active material for a secondary battery according to claim 1, wherein a peak area Sv of 9 ppm or more and 30 ppm or less in a spectrum obtained by C-NMR measurement of the composite material is 10% or more and 90% or less of the area of ​​the entire spectrum.

7. 7. The negative electrode active material for a secondary battery according to claim 1, wherein the composite material has a fracture hardness of 600 MPa or less when the particle diameter of the particles is 6 μm.

8. 8. The negative electrode active material for a secondary battery according to claim 7, wherein the composite material has a fracture hardness of 500 MPa or less when the particle diameter of the particles is 12 μm.

9. 9. The negative electrode active material for a secondary battery according to claim 1, wherein the material constituting the active phase includes at least one selected from the group consisting of metals and intermetallic compounds.

10. the metal is at least one selected from the group consisting of Si, Sn, Ti, Al, and Mg; The intermetallic compound is CrSi ２ , MnSi ２ , FeSi ２ , CoSi ２ , NiSi ２ 10. The negative electrode active material for a secondary battery according to claim 9, which is at least one selected from the group consisting of LiNiSn and LiNiSn.

11. 11. The negative electrode active material for a secondary battery according to claim 1, wherein a mass ratio of the active phase in the composite material is 20 mass % or more and 95 mass % or less.

12. 12. The negative electrode active material for a secondary battery according to claim 11, wherein a mass ratio of the active phase in the composite material is 35 mass % or more and 75 mass % or less.

13. the active phase is in particulate form; 13. The negative electrode active material for a secondary battery according to claim 1, wherein the active phase has an average particle size of 1 nm or more and 1000 nm or less.

14. 14. The negative electrode active material for a secondary battery according to claim 1, further comprising at least one selected from the group consisting of natural graphite, artificial graphite, hard carbon, and soft carbon.

15. obtaining a mixture containing a first polymer, which is an organosilicon polymer containing Si, O, and C, a second polymer that is carbonized together with the carbon atoms in the first polymer by calcination, and a material that constitutes an active phase that reacts with Li; and calcining the mixture to produce a composite material comprising the active phase and an amorphous phase comprising Si, O, and C, wherein the active phase is dispersed in the amorphous phase; the second polymer is at least one selected from the group consisting of polyvinyl resin, polyimide resin, polyacrylonitrile, acrylic resin, and polyolefin resin.

16. 16. The method for producing a negative electrode active material for a secondary battery according to claim 15, wherein the first polymer is at least one selected from the group consisting of polysiloxane, polycarbosilane, polysilazane, silicone resin, and silicone oil.

17. The battery comprises a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte solution; A secondary battery, wherein the negative electrode comprises the negative electrode active material for a secondary battery according to claim 1 .

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