Negative electrode material for lithium ion secondary battery, negative electrode for lithium ion secondary battery, and lithium ion secondary battery

Abstract

The present invention relates to a negative electrode material for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery. This negative electrode material for lithium ion secondary batteries is provided with composite particles. The composite particles have a plurality of carbonaceous particles and a plurality of silicon particles. The plurality of carbonaceous particles and the plurality of silicon particles are each amorphous. The average primary particle diameter of the plurality of silicon particles is from 1 nm to 50 nm (inclusive). In the composite particles, the molar ratio of the silicon particles is from 15 mol% to 40 mol% (inclusive), and the molar ratio of the carbonaceous particles is from 50 mol% to 80 mol% (inclusive). Each of the plurality of silicon particles is bonded.

Description

TECHNICAL FIELD

[0001] The present application relates to a negative electrode material for a lithium-ion secondary battery, a negative electrode for a lithium-ion secondary battery, and a lithium-ion secondary battery. BACKGROUND

[0002] Lithium-ion secondary batteries are widely used as power sources for mobile devices such as mobile phones and notebook computers, and for hybrid electric vehicles.

[0003] The capacity of a lithium-ion secondary battery is mainly determined by the active material of the electrode. Graphite is generally used as the negative electrode active material, but a negative electrode active material with higher capacity is being sought. Therefore, silicon (Si) having a theoretical capacity much larger than that of graphite (372 mAh / g) has attracted attention.

[0004] A negative electrode active material containing silicon undergoes a large volume expansion upon charging. The volume expansion of the negative electrode active material is a cause of a decrease in the cycle characteristics of the battery. If the negative electrode active material undergoes volume expansion, for example, the negative electrode active material is broken, the conductive path between the negative electrode active materials is cut off, the interface between the negative electrode active material layer and the current collector is peeled off, or cracks are generated in the SEI (Solid Electrolyte Interphase) coating film, and decomposition of the electrolyte solution occurs. These cause a decrease in the cycle characteristics of the battery.

[0005] For example, in Patent Literature 1, it is described that the cycle characteristics are improved by specifying the aspect ratio of silicon particles and the inclination angle of the silicon particles with respect to the current collector.

[0006] PRIOR ART DOCUMENTS

[0007] PATENT LITERATURE

[0008] Patent Literature 1: Japanese Patent Application Publication No. 2019-149333 SUMMARY

[0009] PROBLEMS TO BE SOLVED BY THE INVENTION

[0010] The cycle characteristics are an important parameter, and it is desired that the cycle characteristics can be improved by a method other than the method described in Patent Literature 1.

[0011] The present application was made in view of the above problems, and an object thereof is to provide a lithium-ion secondary battery having excellent cycle characteristics.

[0012] MEANS FOR SOLVING THE PROBLEMS

[0013] In order to solve the above problems, the following solutions are provided.

[0014] The negative electrode material for a lithium-ion secondary battery of the first embodiment comprises composite particles. The composite particles include a plurality of carbonaceous particles and a plurality of silicon particles. The plurality of carbonaceous particles and the plurality of silicon particles are each amorphous. The average primary particle size of the plurality of silicon particles is not less than 1 nm and not more than 50 nm. In the composite particles, the molar ratio of the silicon particles is not less than 15 mol% and not more than 40 mol%, and the molar ratio of the carbonaceous particles is not less than 50 mol% and not more than 80 mol%. The plurality of silicon particles are each bonded.

[0015] Effects of the Invention

[0016] A lithium ion secondary battery using the negative electrode material for a lithium ion secondary battery according to the above-described embodiment has excellent cycle characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a cross-sectional photograph of the negative electrode material for a lithium ion secondary battery according to the first embodiment.

[0018] Figure 2 It is a schematic diagram of the lithium ion secondary battery of the first embodiment. DETAILED DESCRIPTION

[0019] Below, with appropriate reference to the attached Figure 1 The embodiments will be described in detail. To facilitate understanding of the features, the following drawings may show enlarged portions of the features, and the dimensional ratios of the various components may differ from the actual ones. The materials, dimensions, etc. illustrated in the following description are merely examples, and the present invention is not limited to these. Appropriate modifications may be made within the scope of the present invention.

[0020] [Anode material]

[0021] The negative electrode material of the first embodiment is used for a lithium ion secondary battery and contains silicon particles. The negative electrode material of the first embodiment functions as, for example, a negative electrode active material.

[0022] Figure 1 This is a cross-sectional view of the negative electrode material according to the first embodiment. The negative electrode material includes composite particles 1 .

[0023] The composite particles 1 are secondary particles formed by aggregating a plurality of particles. The average secondary particle size of the composite particles 1 is, for example, 1 μm to 10 μm, preferably 2 μm to 8 μm, and more preferably 3 μm to 7 μm.

[0024] When the average secondary particle diameter of the composite particles 1 is within the above range, the cycle characteristics are improved. If the composite particles 1 are too small, it is difficult to secure sufficient strength and conductivity when the negative electrode active material layer is formed, and the amount of the binder and the conductive aid to be used increases. Since the binder and the conductive aid are not active materials that perform charge and discharge, if the ratio of their presence in the negative electrode active material layer increases, the capacity of the lithium ion secondary battery becomes smaller. In addition, if the composite particles 1 are too large, the composite particles 1 are broken due to expansion and contraction, and the risk of side reactions such as decomposition of the electrolyte at the newly generated surface increases.

[0025] In a case where the composite particles 1 can be obtained in a state of particles, the median particle diameter (D50) can be obtained as the average secondary particle diameter using a particle size distribution measuring device (for example, manufactured by Malvern Panalytical). In a case where the particle size distribution measuring device is used, the average of the particle diameters of, for example, 50,000 particles can be obtained.

[0026] In a case where the composite particles 1 are located in the electrode and it is difficult to separate the composite particles 1, the average secondary particle diameter can be obtained using at least 100 composite particles 1 that can be confirmed in a cross-sectional image. The average secondary particle diameter measured using the particle size distribution measuring device and the average secondary particle diameter obtained from the cross-sectional image do not greatly deviate, and are generally consistent.

[0027] First, a threshold value of contrast is set, and the composite particles 1 are extracted from the image. Then, the diameters of at least 100 composite particles 1 extracted are obtained respectively. The frequency of the diameters of the respective composite particles 1 obtained is graphed, and the mode value is set as the average secondary particle diameter. In a case where the shape of the composite particles 1 is amorphous, the diameter of the long axis is used for the calculation of the average secondary particle diameter.

[0028] The composite particles 1 include a plurality of carbonaceous particles and a plurality of silicon particles. The composite particles 1 include, in addition to the carbonaceous particles and the silicon particles, for example, hydrogen, oxygen, nitrogen, and the like.

[0029] In Figure 1 In the image of the transmission electron microscope (TEM) illustrated in FIG. 1, the portion that appears white is the silicon particle, and the portion that appears black is the carbonaceous particle. As illustrated in FIG. 2, each of the plurality of silicon particles is combined with an adjacent silicon particle. The silicon particles are combined with the adjacent silicon particles to be connected in a network shape. Figure 1

[0030] ​The molar ratio of the silicon particles in the composite particles 1 is, for example, 15 mol% or more and 40 mol% or less, and is preferably 20 mol% or more and 30 mol% or less. The weight of the silicon in the composite particles 1 can be measured by ICP (inductively coupled plasma) emission spectrometry or the like. Alternatively, the weight of the silicon in the composite particles 1 can be calculated based on the proportion of elements other than silicon (e.g., oxygen, nitrogen, hydrogen, carbon) obtained using oxygen, nitrogen, hydrogen analysis (ONH analysis) and carbon-sulfur analysis (C-S analysis). The molar ratio of the silicon particles can be calculated based on the weight of the silicon.

[0031] The molar ratio of the carbonaceous particles in the composite particles 1 is, for example, 50 mol% or more and 80 mol% or less, and is preferably 60 mol% or more and 70 mol% or less. The weight ratio of the carbonaceous particles can be measured by high-frequency induction heating combustion-infrared absorption method or the like. The molar ratio of the carbonaceous particles can be calculated based on the weight of the carbonaceous particles.

[0032] By controlling the molar ratio of the silicon particles and the carbonaceous particles in the composite particles 1, it is possible to improve the cycle characteristics while ensuring the capacity of the lithium ion secondary battery. The silicon particles are the most contributing part to the charge and discharge, and the molar ratio of the silicon particles has an influence on the capacity of the lithium ion secondary battery. The carbonaceous particles function as a buffer when the silicon particles are accumulated. By having a prescribed amount of the carbonaceous particles in the composite particles 1, it is possible to prevent the breakage of the composite particles 1 and improve the cycle characteristics of the lithium ion secondary battery.

[0033] The molar ratio of the components other than the silicon particles and the carbonaceous particles (e.g., hydrogen, oxygen, nitrogen) in the composite particles 1 is preferably, for example, 15% or less.

[0034] The carbonaceous particles and the silicon particles are each amorphous. Lithium ions intrude along the orientation direction of the crystal. Therefore, in the crystal, the path through which lithium ions can intrude is limited. In contrast, in the amorphous carbonaceous particles and silicon particles, the crystal orientation is not fixed to a prescribed direction, and the direction in which lithium ions intrude is not limited. When the carbonaceous particles and the silicon particles that constitute the composite particles 1 are amorphous, the diffusion of lithium ions in the composite particles 1 becomes uniform, and it is possible to suppress the local volume change of the composite particles 1.

[0035] The silicon particles can be elemental silicon, or can be silicon oxide (SiO x : x satisfies, for example, 0.8 ≤ x ≤ 2), or can be a silicon alloy represented by Si n X is a cation. X is, for example, Ba, Mg, Al, Zn, Sn, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, Y, Zr, Nb, Mo, W, Au, Ti, Na, K, or the like. n satisfies 0 ≤ n ≤ 0.5.

[0036] The average primary particle diameter of the silicon particles is, for example, 1 nm or more and 50 nm or less, and is preferably 3 nm or more and 30 nm or less. The average primary particle diameter of the silicon particles is obtained from a cross-sectional image of the composite particle 1. The cross-sectional image can be measured using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). For example, the composite particle 1 can be observed at a magnification of 100,000 times using a scanning electron microscope JSM-7600 (manufactured by JEOL Ltd.), and the average primary particle diameter can be measured by image processing of the photographed image. The average primary particle diameter of the silicon particles obtained by image processing is substantially the same as the particle diameter of the silicon particles prepared before manufacturing.

[0037] The average primary particle diameter can be obtained using image processing software HALCON (registered trademark, manufactured by MVTec Software GmbH). The software can recognize the particles in the photographed image and remove the particles for which the entire particle is not photographed at the end of the observation field. When measuring the average primary particle diameter, the connecting portion between the particles is removed. Specifically, as shown in FIG. 6, the portion connecting the particles also appears white, and therefore, only the particles are extracted by removing this portion. For example, the portion having a width of 1 nm or less in the portion that can be confirmed as white in the image is removed. Then, for each of the extracted particles, the shortest width (the shortest diameter of the circumscribed circle of the particle) is measured, and the particle diameter is converted from the shortest width. Such measurement is performed for 200 particles, the cumulative particle size distribution is obtained, and the average primary particle diameter is obtained by calculation. Figure 1

[0038] When the average primary particle diameter of the silicon particles is within the above range, the increase in the film resistance of the electrolyte accompanying the side reaction due to the contact of the silicon particles with the electrolyte can be suppressed. In addition, when the average primary particle diameter of the silicon particles is within the above range, the breakage of the silicon particles due to expansion and contraction at the time of charge and discharge can be suppressed.

[0039] The carbonaceous particles are compounded with the silicon particles. The carbonaceous particles are, for example, graphite, graphene, carbides generated by baking pitch-based, carbides generated by baking resin-based, and the like. The carbonaceous particles can also be two or more kinds.

[0040] The pitch-based can be coal-based pitch-based, petroleum-based pitch-based, synthetic pitch-based, and for example, coal tar, tar light oil, tar middle oil, tar heavy oil, naphthalene oil, anthracene oil, coal tar pitch, pitch oil, mesophase pitch, oxygen-crosslinked petroleum pitch, heavy oil, coke, low-molecular heavy oil, derivatives of these, and the like.

[0041] ​The resin is, for example, a thermoplastic resin such as polyvinyl alcohol, a phenol resin, an epoxy resin, a melamine resin, a urea resin, an aniline resin, a cyanate ester resin, a furan resin, a ketone resin, an unsaturated polyester resin, a polyurethane resin, a modified product of these, or the like. The phenol resin is, for example, a novolak type phenol resin, a resol type phenol resin, or the like. The epoxy resin is, for example, a bisphenol type epoxy resin, a novolak type epoxy resin, or the like. The resin is, for example, polyethylene, polystyrene, acrylonitrile-styrene (AS) resin, acrylonitrile-butadiene-styrene (ABS) resin, polypropylene, polyethylene terephthalate, polycarbonate, polyacetal, polyphenylene ether, polybutylene terephthalate, polyphenylene sulfide, polysulfone, polyethersulfone, polyether ether ketone, polyvinyl chloride, or the like.

[0042] The carbonaceous particle preferably contains graphite or graphene. Graphite or graphene has a high electron conductivity of sp 2 The electron conductivity of the sp

[0043] The surface of the composite particle 1 can be further coated with carbon or carbon nanotubes. The carbon or carbon nanotubes contribute to the electrical conductivity between the composite particles 1. When the surface of the composite particle 1 is coated with carbon or carbon nanotubes, the electrical conduction network between the composite particles 1 is easily maintained even if the volume of the composite particle 1 changes.

[0044] The negative electrode material of the first embodiment can be produced by performing a core production step and a coating layer production step.

[0045] In the core production step, the composite particle 1 is produced. The composite particle 1 can be produced by mixing the silicon particle and the carbon source in an organic solvent. The molar ratio of the silicon particle to the carbonaceous particle in the composite particle 1 can be adjusted by the mixing ratio of the silicon particle to the carbon source.

[0046] The carbon source is graphite, graphene, pitch, resin, or the like. The pitch and the resin can be the above-described pitch and resin. The carbon source is preferably at least one selected from the group consisting of graphite, graphene, novolak type phenol resin, resol type phenol resin, coal-based pitch, and petroleum-based pitch. Two or more kinds of carbon sources can be used. The carbon source has an effect on the powder resistivity of the negative electrode active material.

[0047] The organic solvent is methanol, ethanol, tetrahydrofuran, or the like. A dispersant can be added to the organic solvent. By adding the dispersant, the carbonaceous particle uniformly covers the silicon particle at the time of compounding. Such a composite particle 1 has excellent electron conductivity and is less likely to undergo a side reaction with an electrolyte at the time of charge and discharge.

[0048] Next, the mixed mixture is dried. By drying, the organic solvent is removed from the mixture and a powder is obtained. The drying method is not particularly limited and is, for example, a spray drying method.

[0049] Next, heat treatment is applied to the dried powder. By the heat treatment process, the resin or resin composition as a carbon source is incompletely combusted and carbonized, thereby forming carbonaceous particles. Thus, the silicon particles are compounded with the carbonaceous particles.

[0050] The heat treatment is preferably performed on the mixture at a heat treatment temperature of 350 to 1200°C. If the heat treatment temperature is low, the carbon source cannot be sufficiently carbonized, and can become a cause of capturing lithium at the time of charge and discharge. If lithium is captured, the initial efficiency of the lithium ion secondary battery decreases. If the heat treatment temperature is high, the silicon particles and the carbonaceous particles react, and silicon carbide is excessively generated. Silicon carbide has a small contribution to charge and discharge among silicon compounds, and becomes a cause of a decrease in the conductivity of lithium ions and a decrease in the discharge capacity of the lithium ion secondary battery.

[0051] In addition, the heat treatment time is preferably 1 hour or more and 72 hours or less. The heat treatment atmosphere is preferably a reducing atmosphere such as a nitrogen atmosphere or an argon atmosphere.

[0052] Next, the compounded particles are irradiated with laser light or an electron beam, and heat fusion treatment of the silicon particles and the carbonaceous particles is performed. By performing the heat fusion treatment, a network structure in which the silicon particles are connected to each other is formed.

[0053] In the coating layer production process, carbon or a carbon nanotube is attached to the surface of the composite particle 1. The coating layer production process can not be performed. The surface of the composite particle 1 is coated with carbon or a carbon nanotube by chemical vapor deposition (CVD) or an electrostatic adsorption method.

[0054] The lithium ion secondary battery of the negative electrode material of the first embodiment has excellent cycle characteristics. This is because the negative electrode material has the specific composite particle 1. The composite particle 1 has the silicon particles and the carbonaceous particles at a specific molar ratio, and thus it is possible to improve the cycle characteristics while ensuring the capacity of the lithium ion secondary battery.

[0055] In addition, since the silicon particles that constitute the composite particle 1 are connected to each other, the particle strength of the composite particle 1 is high, and breakage is unlikely to occur. If the composite particle 1 is broken, a new surface is exposed, and an irreversible reaction of the electrolyte occurs. The irreversible reaction of the electrolyte uses lithium ions, and becomes a cause of a decrease in the cycle characteristics of the lithium ion secondary battery.

[0056] In addition, since the silicon particles are extended in a network shape, the reaction area with lithium ions is expanded, and the charge and discharge reaction of the lithium ion secondary battery becomes smooth.

[0057] [Lithium ion secondary battery]

[0058] Figure 2 is a schematic view of the lithium ion secondary battery of the first embodiment. Figure 2The illustrated lithium-ion secondary battery 100 has a power generating element 40, a package 50, and an electrolyte (for example, a nonaqueous electrolyte). The package 50 covers the periphery of the power generating element 40. The power generating element 40 is connected to the outside through a pair of terminals 60, 62 connected to the power generating element 40. The nonaqueous electrolyte is housed in the package 50. In Figure 2 The case where the package 50 has one power generating element 40 is exemplified in the above, but the power generating element 40 can also be laminated plural.

[0059] (Power generating element)

[0060] The power generating element 40 has a separator 10, a positive electrode 20, and a negative electrode 30. The power generating element 40 can be a laminate in which these are laminated, or a jelly-roll in which a structure in which these are laminated is wound.

[0061] (Positive electrode)

[0062] The positive electrode 20 has, for example, a positive electrode current collector 22 and a positive electrode active material layer 24. The positive electrode active material layer 24 is in contact with at least one surface of the positive electrode current collector 22.

[0063] [Positive electrode current collector]

[0064] The positive electrode current collector 22 is, for example, a conductive sheet. The positive electrode current collector 22 is, for example, a metal thin plate of aluminum, copper, nickel, titanium, stainless steel, or the like. Aluminum, which is light in weight, is suitable for the positive electrode current collector 22. The average thickness of the positive electrode current collector 22 is, for example, 10 μm or more and 30 μm or less.

[0065] [Positive electrode active material layer]

[0066] The positive electrode active material layer 24 contains, for example, a positive electrode active material. The positive electrode active material layer 24 can also contain, as needed, a conductive aid, a binder.

[0067] The positive electrode active material contains an electrode active material capable of reversibly performing adsorption and release of lithium ions, deintercalation and intercalation of lithium ions, or doping and dedoping of lithium ions and counter anions.

[0068] The positive electrode active material is, for example, a composite metal oxide. The composite metal oxide is, for example, lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium manganate (LiMnO2), lithium manganate spinel (LiMn2O4), and a compound represented by the general formula LiNi x Co y Mn z M aO2 (in the general formula, x + y + z + a = 1, 0 ≤ x < 1, 0 ≤ y < 1, 0 ≤ z < 1, 0 ≤ a < 1, M is an element selected from one or more of Al, Mg, Nb, Ti, Cu, Zn, Cr), a lithium vanadium compound (LiV2O5), an olivine-type LiMPO4 (where M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr or VO), a lithium titanate (Li4Ti5O 12 ), LiNi x Co y Al z O2 (0.9 < x + y + z < 1.1). The positive electrode active material can also be an organic substance. For example, the positive electrode active material can also be polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene.

[0069] The positive electrode active material can also be a material that does not contain lithium. The material that does not contain lithium is, for example, FeF3, a conjugated polymer containing an organic conductive substance, a Chevrel phase compound, a transition metal chalcogenide, a vanadium oxide, a niobium oxide, or the like. The material that does not contain lithium can be used alone or a plurality of them can be used in combination. In the case where the positive electrode active material is a material that does not contain lithium, for example, discharging is performed first. Lithium is intercalated into the positive electrode active material by discharging. Furthermore, in the case where the positive electrode active material is a material that does not contain lithium, lithium can also be pre-doped chemically or electrochemically.

[0070] The conductive assistant improves the electron conductivity between the positive electrode active materials. The conductive assistant is, for example, a carbon powder, a carbon nanotube, a carbon material, a metal fine powder, a mixture of a carbon material and a metal fine powder, a conductive oxide. The carbon powder is, for example, carbon black, acetylene black, ketjen black, or the like. The metal fine powder is, for example, a powder of copper, nickel, stainless steel, iron, or the like.

[0071] The content ratio of the conductive assistant in the positive electrode active material layer 24 is not particularly limited. For example, the content ratio of the conductive assistant with respect to the total mass of the positive electrode active material, the conductive assistant, and the binder is 0.5 mass% or more and 20 mass% or less, and is preferably 1 mass% or more and 5 mass% or less.

[0072] The binder in the positive electrode active material layer 24 causes the positive electrode active materials to bind to each other. The binder can use a publicly known binder. The binder is preferably insoluble in the electrolyte, has oxidation resistance, and has adhesion. The binder is, for example, a fluorine resin. The binder is, for example, polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), polyamide (PA), polyimide (PI), polyamide-imide (PAI), polybenzimidazole (PBI), polyether sulfone (PES), polyacrylic acid and a copolymer thereof, a metal ion crosslinker of polyacrylic acid and a copolymer thereof, polypropylene (PP) or polyethylene (PE) grafted with maleic anhydride, and a mixture of these. The binder used for the positive electrode active material layer is particularly preferably PVDF.

[0073] The content of the binder in the positive electrode active material layer 24 is not particularly limited. For example, the content of the binder with respect to the total mass of the positive electrode active material, the conductive aid, and the binder is 1 mass% or more and 15 mass% or less, and is preferably 1.5 mass% or more and 5 mass% or less. If the content of the binder is small, the adhesion strength of the positive electrode 20 is weakened. If the content of the binder is high, the discharge capacity is not improved because the binder is electrochemically inert, and thus the energy density of the lithium ion secondary battery 100 is reduced.

[0074] <NEGATIVE ELECTRODE>

[0075] The negative electrode 30 has, for example, a negative electrode current collector 32 and a negative electrode active material layer 34. The negative electrode active material layer 34 is formed on at least one surface of the negative electrode current collector 32.

[0076] [Negative electrode current collector]

[0077] The negative electrode current collector 32 is, for example, an electrically conductive sheet. The negative electrode current collector 32 can use the same material as the positive electrode current collector 22.

[0078] [Negative electrode active material layer]

[0079] The negative electrode active material layer 34 contains a negative electrode active material and a binder. The negative electrode active material layer can also contain a conductive aid, a dispersion stabilizer, and the like as needed. The negative electrode active material uses the negative electrode material described above. By using the negative electrode material described above as the negative electrode active material, the cycle characteristics of the lithium ion secondary battery 100 are improved.

[0080] The conductive aid and the binder can use the same conductive aid and binder as the positive electrode 20. The binder in the negative electrode 30 can be, for example, cellulose, styrene-butadiene rubber, ethylene-propylene rubber, a polyimide resin, a polyamide-imide resin, an acrylic resin, and the like in addition to the binders exemplified for the positive electrode 20. The cellulose can be, for example, carboxymethyl cellulose (CMC).

[0081] <Separator>

[0082] The separator 10 is sandwiched by the positive electrode 20 and the negative electrode 30. The separator 10 separates the positive electrode 20 and the negative electrode 30, and prevents short-circuiting of the positive electrode 20 and the negative electrode 30. The separator 10 extends in-plane along the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

[0083] The separator 10 has, for example, a porous structure of electrical insulation. The separator 10 is a single layer body or a laminate body of, for example, a polyolefin film. The separator 10 can also be a stretched film of a mixture of polyethylene or polypropylene, or the like. The separator 10 can also be a fiber nonwoven fabric composed of a constituent material selected from at least one of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The separator 10 can also be, for example, a solid electrolyte. The solid electrolyte is, for example, a polymer solid electrolyte, an oxide-based solid electrolyte, or a sulfide-based solid electrolyte. The separator 10 can also be an inorganic-coated separator. The inorganic-coated separator is formed by coating a mixture of a resin such as PVDF or CMC and an inorganic substance such as alumina or silica on the surface of the above-described film. The inorganic-coated separator is excellent in heat resistance, and can suppress the deposition of transition metals eluted from the positive electrode onto the surface of the negative electrode.

[0084] < Electrolyte >

[0085] The electrolyte is enclosed in the package 50, and permeates the power generating element 40. The electrolyte is not limited to a liquid electrolyte, and can also be a solid electrolyte. The nonaqueous electrolyte has, for example, a nonaqueous solvent and an electrolytic salt. The electrolytic salt is dissolved in the nonaqueous solvent.

[0086] The solvent is not particularly limited as long as it is a solvent generally used for lithium ion secondary batteries. The solvent can contain, for example, any one of a cyclic carbonate compound, a chain carbonate compound, a cyclic ester compound, or a chain ester compound. The solvent can contain a mixture of the above-described compounds in an arbitrary ratio. The cyclic carbonate compound is, for example, ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate, vinylene carbonate, or the like. The chain carbonate compound is, for example, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), or the like. The cyclic ester compound is, for example, γ-butyrolactone, or the like. The chain ester compound is, for example, propyl propionate, ethyl propionate, ethyl acetate, or the like.

[0087] The electrolytic salt is, for example, a lithium salt. The electrolyte is, for example, LiPF6, LiClO4, LiBF4, LiCF3SO3, LiCF3CF2SO3, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(CF3CF2CO)2, LiBOB, LiN(FSO2)2, or the like. The lithium salt can be used singly or in combination of two or more. From the viewpoint of ionization degree, the electrolyte preferably contains LiPF6. The dissociation rate of the electrolytic salt in the carbonate solvent at room temperature is preferably 10% or more.

[0088] The electrolytic solution is preferably, for example, an electrolytic solution in which LiPF6is dissolved in a carbonate solvent. The concentration of LiPF6is, for example, 1 mol / L. In the case where the polyimide resin contains a large amount of aromatic groups, the polyimide resin sometimes exhibits a charge behavior like soft carbon. In the case where the electrolytic solution is a carbonate electrolytic solution solvent containing a cyclic carbonate, lithium and the polyimide can be uniformly reacted. In this case, the cyclic carbonate is preferably vinylene carbonate, fluoro-vinylene carbonate, or vinylene carbonate.

[0089] <Package>

[0090] The package 50 seals the power generating element 40 and the nonaqueous electrolytic solution inside thereof. The package 50 suppresses leakage of the nonaqueous electrolytic solution to the outside and intrusion of moisture or the like from the outside into the inside of the lithium-ion secondary battery 100, and the like.

[0091] For example, as shown in FIG. 1, the package 50 has a metal foil 52 and resin layers 54 laminated on each side of the metal foil 52. The package 50 is a metal laminate film in which the metal foil 52 is coated with a polymer film (resin layer 54) from both sides. Figure 1

[0092] As the metal foil 52, an aluminum foil can be used, for example. The resin layer 54 can use a polymer film such as polypropylene. The material constituting the resin layer 54 can also be different between the inside and the outside. For example, as the material on the outside, a high-melting polymer such as polyethylene terephthalate (PET), polyamide (PA), or the like can be used, and as the material of the polymer film on the inside, polyethylene (PE), polypropylene (PP), or the like can be used.

[0093] <Terminal>

[0094] ​The terminals 62, 60 are connected to the positive electrode 20 and the negative electrode 30, respectively. The terminal 62 connected to the positive electrode 20 is a positive electrode terminal, and the terminal 60 connected to the negative electrode 30 is a negative electrode terminal. The terminals 60, 62 serve to electrically connect the power generating element to the outside. The terminals 60, 62 are formed of an electrically conductive material such as aluminum, nickel, copper, or the like. The connection method can be welding or screwing. To prevent short-circuiting, the terminals 60, 62 are preferably protected by an insulating tape.

[0095] [Method for manufacturing lithium-ion secondary battery]

[0096] The lithium-ion secondary battery 100 is manufactured by preparing the negative electrode 30, the positive electrode 20, the separator 10, the electrolyte, and the packaging body 50, respectively, and assembling them. An example of a method for manufacturing the lithium-ion secondary battery 100 will be described below.

[0097] The negative electrode 30 is manufactured by, for example, sequentially performing a slurry preparation step, an electrode coating step, a drying step, and a calendering step.

[0098] The slurry preparation step is a step of preparing a slurry by mixing a negative electrode active material, a binder, a conductive aid, and a solvent. The negative electrode active material is the negative electrode material described above. If a dispersion stabilizer is added to the slurry, the aggregation of the negative electrode active material can be suppressed.

[0099] The slurry preparation step is a step of preparing a slurry by mixing a negative electrode active material, a binder, a conductive aid, and a solvent. The solvent is, for example, water, N-methyl-2-pyrrolidone, or the like. The constituent ratio of the negative electrode active material, the conductive material, and the binder is preferably 70 wt% to 100 wt% : 0 wt% to 10 wt% : 0 wt% to 20 wt% in terms of mass ratio. The mass ratio is adjusted in such a way that the total is 100 wt%. The container used in the slurry preparation is preferably a metal container such as SUS.

[0100] The negative electrode active material can be compounded by mixing while applying a shearing force to the active material particles and the conductive material. When the active material particles are mixed while applying a shearing force without modification, the surface of the active material particles is coated with the conductive material. In addition, the particle diameter of the negative electrode active material can be adjusted according to the degree of mixing. In addition, the negative electrode active material after preparation can be sieved to make the particle diameters uniform.

[0101] The electrode coating step is a step of coating the slurry on the surface of the negative electrode current collector 32. The method of coating the slurry is not particularly limited. As the method of coating the slurry, for example, a slit die coating method or a doctor blade method can be used. The slurry is coated at, for example, room temperature.

[0102] The drying step is a step of removing the solvent from the slurry. The negative electrode current collector 32 on which the slurry is coated is dried at, for example, an atmosphere of 80°C or higher and 350°C or lower.

[0103] The rolling step is performed as needed. The rolling step is a step of applying pressure to the negative electrode active material layer 34 to adjust the density of the negative electrode active material layer 34. The rolling step is performed using, for example, a roll press device.

[0104] The positive electrode 20 can be produced by the same steps as those for the negative electrode 30. The separator 10 and the package 50 can be commercially available products.

[0105] Next, the positive electrode 20 and negative electrode 30 are stacked with the separator 10 positioned between them to produce the power generating element 40. When the power generating element 40 is a wound body, the positive electrode 20, negative electrode 30, and separator 10 are wound around one end thereof.

[0106] Finally, the power-generating element 40 is enclosed in the package 50. A non-aqueous electrolyte is injected into the package 50. After the non-aqueous electrolyte is injected, the non-aqueous electrolyte is allowed to penetrate into the power-generating element 40 by reducing pressure, heating, etc. The package 50 is sealed by heating, etc., to obtain the lithium-ion secondary battery 100. It should be noted that the power-generating element 40 can also be immersed in the electrolyte instead of injecting the electrolyte into the package 50. It is preferred to allow the electrolyte to stand for 24 hours after injection into the power-generating element.

[0107] The lithium ion secondary battery 100 according to the first embodiment has excellent cycle characteristics because the negative electrode active material includes a negative electrode material having a predetermined shape.

[0108] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the various configurations and combinations thereof in the various embodiments are merely examples, and additions, omissions, substitutions, and other changes to the configurations may be made without departing from the spirit of the present invention.

[0109] Example

[0110] [Example 1]

[0111] The positive electrode slurry was applied on one surface of an aluminum foil having a thickness of 15 μm. The positive electrode slurry was prepared by mixing a positive electrode active material, a conductive additive, a binder, and a solvent.

[0112] The positive electrode active material uses Li x CoO2. Acetylene black was used as a conductive aid. Polyvinylidene fluoride (PVDF) was used as a binder. N-methyl-2-pyrrolidone was used as a solvent. 97 parts by mass of a positive electrode active material, 1 part by mass of a conductive aid, 2 parts by mass of a binder, and 70 parts by mass of a solvent were mixed to prepare a positive electrode slurry. The positive electrode active material loading in the dried positive electrode active material layer was 25 mg / cm 2The solvent was removed from the positive electrode slurry in a drying furnace to form a positive electrode active material layer. The positive electrode active material layer was pressed using a roll press to form a positive electrode.

[0113] Next, a negative electrode active material to be added to the negative electrode slurry was prepared. First, silicon particles having an average primary particle diameter of 5 nm were mixed with a carbon source and fired to thereby form composite particles. After the composite particles were formed, heat welding treatment using a laser was performed to link the silicon particles to each other. The linking of the silicon particles in the composite particles was confirmed from a SEM image. The average secondary particle diameter of the composite particles was 8.6 μm.

[0114] Next, a negative electrode slurry was prepared using the negative electrode active material. As the conductive aid, carbon black was used. As the binder, a polyimide resin was used. As the solvent, N-methyl-2-pyrrolidone was used. The negative electrode slurry was prepared by mixing 90 parts by mass of the negative electrode active material, 5 parts by mass of the conductive aid, and 5 parts by mass of the binder in N-methyl-2-pyrrolidone.

[0115] Then, the negative electrode slurry was applied to one surface of a copper foil having a thickness of 10 μm and dried. The loading amount of the negative electrode active material in the negative electrode active material layer after drying was 2.5 mg / cm 2 After the negative electrode active material layer was pressed using a roll press, firing was performed at a temperature of 300°C or higher for 5 hours in a nitrogen atmosphere.

[0116] Next, an electrolyte solution was prepared. The solvent of the electrolyte solution was set to fluoroethylene carbonate (FEC) : ethylene carbonate (EC) : diethyl carbonate (DEC) = 10 vol% : 20 vol% : 70 vol%. In addition, an output improving additive, a gas suppressing additive, a cycle characteristic improving additive, a safety performance improving additive, and the like were added to the electrolyte solution. As the electrolyte salt, LiPF6was used. The concentration of LiPF6was set to 1 mol / L.

[0117] (Production of lithium ion secondary battery for evaluation)

[0118] The negative electrode and the positive electrode thus produced were stacked with the positive electrode active material layer and the negative electrode active material layer facing each other with a separator (porous polyethylene sheet) interposed therebetween to thereby obtain a laminate. The laminate was inserted into a packaging body of an aluminum laminated film, and heat-sealed except for one portion of the periphery to thereby form a closed portion. Then, after the electrolyte solution was finally injected into the packaging body, the remaining one portion was sealed with heat-sealing while reducing the pressure using a vacuum sealing machine to thereby produce a lithium ion secondary battery. The lithium ion secondary battery thus produced was left to stand for 24 hours.

[0119] (Measurement of capacity retention rate after 300 cycles)

[0120] The cycle characteristics of the lithium ion secondary battery were measured. The cycle characteristics were measured using a secondary battery charge-discharge tester (manufactured by Hokuto Denko Corporation).

[0121] The battery was subjected to constant current charging at a charge rate of 1 C (a current value at which charging is completed within 1 hour when constant current charging is performed at 25°C) until the battery voltage reached 4.2 V, and was subjected to constant current discharging at a discharge rate of 1.0 C until the battery voltage reached 2.5 V. The discharge capacity after the end of the charge-discharge was measured, and the battery capacity Ql before the cycle test was calculated. The battery capacity Ql was 2099 mAh / g.

[0122] The battery, of which the battery capacity Ql was calculated as described above, was subjected to constant current charging at a charge rate of 1 C using a secondary battery charge-discharge tester until the battery voltage reached 4.2 V, and was subjected to constant current discharging at a discharge rate of 1 C until the battery voltage reached 2.5 V. The above charge-discharge was counted as one cycle, and 300 cycles of charge-discharge were performed. Then, the discharge capacity after the end of the 300 cycles of charge-discharge was measured, and the battery capacity Q2 after the 300 cycles was calculated. The capacity maintenance rate E after the 300 cycles was calculated from the battery capacities Ql, Q2 calculated as described above. The capacity maintenance rate E was calculated according to E = Q2 / Ql x 100. The capacity maintenance rate of Example 1 was 93%.

[0123] [Example 2, Example 3]

[0124] Example 2 and Example 3 differ from Example 1 in that the average primary particle diameter of the silicon particles constituting the composite particles was changed. The other conditions were the same as in Example 1, and the capacity maintenance rate after the 300 cycles was calculated.

[0125] [Examples 4 to 6]

[0126] Examples 4 to 6 differ from Example 1 in that the molar ratio of the silicon particles to the carbonaceous particles constituting the composite particles was changed. The molar ratio of the silicon particles to the carbonaceous particles was changed by adjusting the amounts of the silicon particles and the carbon source at the time of producing the composite particles. The other conditions were the same as in Example 1, and the capacity maintenance rate after the 300 cycles was calculated.

[0127] [Example 7]

[0128] Example 7 differs from Example 1 in that carbon was attached to the surface of the composite particles. The attachment of carbon was performed by chemical vapor deposition (CVD) using acetylene gas. The other conditions were the same as in Example 1, and the capacity maintenance rate after the 300 cycles was calculated.

[0129] [Comparative Example 1]

[0130] Comparative Example 1 differs from Example 1 in that no heat fusion treatment by laser irradiation is performed after the compounding treatment. In the composite particles of Comparative Example 1, no linking between the silicon particles is confirmed. The other conditions are the same as in Example 1, and the capacity retention rate after 300 cycles is obtained.

[0131] [Comparative Example 2, Comparative Example 3]

[0132] Comparative Examples 2 and 3 differ from Example 1 in that the molar ratio of the silicon particles and the carbonaceous particles constituting the composite particles is changed. The molar ratio of the silicon particles and the carbonaceous particles is changed by adjusting the amounts of the silicon particles and the carbon source at the time of producing the composite particles. The other conditions are the same as in Example 1, and the capacity retention rate after 300 cycles is obtained.

[0133] [Comparative Example 4, Comparative Example 5]

[0134] Comparative Examples 4 and 5 differ from Example 1 in that the average primary particle diameter of the silicon particles constituting the composite particles is changed. The other conditions are the same as in Example 1, and the capacity retention rate after 300 cycles is obtained.

[0135] The results of Examples 1 to 7 and Comparative Examples 1 to 5 are summarized in the following table. The negative electrode active materials of Examples 2 to 7 and Comparative Examples 1 to 5 are affected by fluctuations in the manufacturing conditions in addition to the intentionally changed parameters, and sometimes deviate from the various parameters of the negative electrode active material of Example 1.

[0136] [Table 1]

[0137]

[0138] Examples 1 to 7 have higher capacity retention rates and superior cycle characteristics compared to Comparative Examples 1 to 5.

[0139] Explanation of symbols:

[0140] 1... composite particles

[0141] 10... separator

[0142] 20... positive electrode

[0143] 22... positive electrode current collector

[0144] 24... positive electrode active material layer

[0145] 30... negative electrode

[0146] 32... negative electrode current collector

[0147] 34... negative electrode active material layer

[0148] 40... power generating element

[0149] 50... package

[0150] 52... metal foil

[0151] 54... resin layer

[0152] 60, 62... terminal

[0153] 100... lithium ion secondary battery

Claims

1. A negative electrode material for a lithium ion secondary battery, wherein, a composite particle is provided, the composite particle has a plurality of carbonaceous particles and a plurality of silicon particles, the plurality of carbonaceous particles and the plurality of silicon particles are each amorphous, an average primary particle diameter of the plurality of silicon particles is 1 nm or more and 50 nm or less, in the composite particle, a molar ratio of the silicon particles is 15 mol% or more and 40 mol% or less, and a molar ratio of the carbonaceous particles is 50 mol% or more and 80 mol% or less, each of the plurality of silicon particles is bonded.

2. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein, a surface of the composite particle is coated with carbon or a carbon nanotube.

3. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein, in the composite particle, a molar ratio of the silicon particles is 20 mol% or more and 30 mol% or less, and a molar ratio of the carbonaceous particles is 60 mol% or more and 70 mol% or less.

4. A negative electrode for a lithium-ion secondary battery, wherein, a lithium ion secondary battery including the negative electrode material for a lithium ion secondary battery according to claim 1 is provided.

5. A lithium ion secondary battery, wherein, a lithium ion secondary battery including the negative electrode for a lithium ion secondary battery according to claim 4, a positive electrode, and an electrolyte is provided.

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