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, a silicon carbide layer, and a coating layer. The composite particles include amorphous carbonaceous particles and amorphous silicon particles having an average primary particle diameter of from 1 nm to 50 nm (inclusive). And the silicon carbide layer is positioned between the composite particles and the coating layer. The silicon carbide layer has a film thickness of 1 nm or more and 100 nm or less. The coating layer contains a compound of magnesium or fluorine.

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, a silicon carbide layer, and a coating layer. The composite particles comprise amorphous carbonaceous particles and amorphous silicon particles having an average primary particle size of 1 nm to 50 nm. The silicon carbide layer is located between the composite particles and the coating layer. The silicon carbide layer has a thickness of 1 nm to 100 nm. The coating layer comprises a magnesium or fluorine compound.

[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 view 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 drawings used in the following description may show enlarged portions of the features, and the dimensional ratios of the 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 contents. 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 a negative electrode material 1 according to the first embodiment. The negative electrode material 1 includes composite particles 2 , a silicon carbide layer 3 , and a coating layer 4 .

[0023] The composite particles 2 are secondary particles formed by a plurality of particles being aggregated. The average secondary particle size of the composite particles 2 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 2 is within the above range, the cycle characteristics are improved. If the composite particles 2 are too small, it is difficult to secure sufficient strength and conductivity after the formation of the negative electrode active material layer, 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 presence of them in the negative electrode active material layer increases, the capacity of the lithium ion secondary battery becomes small. In addition, if the composite particles 2 are too large, the composite particles 2 are likely to be 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 the case where the composite particles 2 can be obtained in the 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 the case of using the particle size distribution measuring device, the average of the particle diameters of, for example, 50000 particles can be obtained.

[0026] In the case where the composite particles 2 are located in the electrode and the separation of the composite particles 2 is difficult, the average secondary particle diameter can be obtained using at least 100 composite particles 2 that can be confirmed in the 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, the threshold value of the contrast is set, and the composite particles 2 are extracted from the image. Then, the diameters of at least 100 composite particles 2 extracted are respectively obtained. The frequency of the diameters of the respective composite particles 2 obtained is graphed, and the mode value is set as the average secondary particle diameter. In the case where the shape of the composite particles 2 is amorphous, the diameter of the long axis is used for the calculation of the average secondary particle diameter.

[0028] The composite particles 2 contain carbonaceous particles and silicon particles. The weight ratio of the carbonaceous particles to the silicon particles in the composite particles 2 is preferably 0.5 or more and 5 or less, obtained by dividing the weight of the carbonaceous particles by the weight of the silicon particles. The weight ratio of the carbonaceous particles can be measured by a high-frequency induction heating combustion-infrared absorption method or the like, and the weight of the silicon particles can be measured by an ICP (inductively coupled plasma) emission spectrometry method or the like.

[0029] 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 to this, the crystal direction of the carbonaceous particles and the silicon particles, which are amorphous, is not fixed to a prescribed direction, and the direction through which lithium ions intrude is not limited. When the carbonaceous particles and the silicon particles that constitute the composite particles 2 are amorphous, the diffusion of lithium ions in the negative electrode material 1 becomes uniform, and the local volume change of the negative electrode material 1 can be suppressed.

[0030] The silicon particles can be elemental silicon, or silicon oxide (SiO x : x satisfies, for example, 0.8 ≤ x ≤ 2), or a silicon alloy represented by SiXn n : x satisfies, for example, 0.8 ≤ x ≤ 2), or a silicon alloy represented by SiXn

[0031] 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 negative electrode material 1. The cross-sectional image can be measured using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). For example, a scanning electron microscope JSM-7600 (manufactured by JEOL Ltd.) can be used to observe the negative electrode material 1 at a magnification of 100,000 times, and the average primary particle diameter can be calculated by image processing of the captured image.

[0032] 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 captured image, and remove particles for which the entire particle is not captured at the end of the observation field of view. Furthermore, for each particle, the maximum length (diameter of the circumscribed circle of the particle) is calculated, and the particle diameter is converted from the maximum length. Such calculation is performed for 200 particles, the cumulative particle size distribution is obtained, and the average primary particle diameter is calculated.

[0033] When the average primary particle diameter of the silicon particles is within the above range, an increase in the film resistance of the electrolyte accompanying a side reaction due to contact of the silicon particles with the electrolyte can be suppressed. Furthermore, when the average primary particle diameter of the silicon particles is within the above range, breakage of the silicon particles due to expansion and contraction during charge and discharge can be suppressed.

[0034] 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, or the like. The carbonaceous particles can also be two or more kinds.

[0035] The pitch-based can be coal-based pitch-based, petroleum-based pitch-based, and synthetic pitch-based, and examples can include 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.

[0036] 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, polyether sulfone, polyether ether ketone, polyvinyl chloride, or the like.

[0037] 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

[0038] The silicon carbide layer 3 is positioned between the composite particle 2 and the coating layer 4. The silicon carbide layer 3 coats at least a part of the surface of the composite particle 2. The silicon carbide layer 3 can also coat the entire surface of the composite particle 2.

[0039] The silicon carbide layer 3 suppresses the direct contact of the composite particle 2 with the electrolyte solution to cause an irreversible reaction. The irreversible reaction is a reaction in which the electrolyte solution is reduced and decomposed, and if the irreversible reaction occurs, the amount of lithium ions in the electrolyte solution decreases, and the cycle characteristics of the lithium ion secondary battery decrease. In addition, the silicon carbide layer 3 increases the strength of the composite particle 2, and suppresses the breakage of the negative electrode material 1.

[0040] The film thickness of the silicon carbide layer 3 is, for example, 1 nm or more and 100 nm or less, and is preferably 3 nm or more and 20 nm or less. The film thickness of the silicon carbide layer 3 is, for example, an average value of the film thickness of the silicon carbide layer 3 formed in 100 composite particles 2 obtained from a cross-sectional image. When the film thickness of the silicon carbide layer 3 is within the above range, the irreversible reaction can be suppressed, and the strength of the negative electrode material 1 can be sufficiently increased. In addition, when the film thickness of the silicon carbide layer 3 is around 100 nm, the silicon carbide layer 3 does not significantly hinder the conduction of lithium ions.

[0041] The coating layer 4 coats the composite particle 2 and the silicon carbide layer 3. The coating layer 4 coats at least a part of the composite particle 2 or the silicon carbide layer 3. The coating layer 4 can also completely coat the composite particle 2 and the silicon carbide layer 3.

[0042] The coating layer 4 contains a compound of magnesium or fluorine. The compound of magnesium or fluorine is, for example, one or more selected from lithium fluoride (for example, LiF), magnesium oxide (for example, Mg2O), magnesium phosphate (for example, Mg3(PO4)2), and magnesium fluoride (for example, Mg2F).

[0043] The coating layer 4 suppresses the occurrence of irreversible reactions of the composite particles 2 with the electrolyte solution. In addition, the compound of magnesium or fluorine reacts with the electrolyte solution to form a high-quality SEI coating film.

[0044] The film thickness of the coating layer 4 is, for example, 10 nm or more and 300 nm or less, preferably 50 nm or more and 200 nm or less, and more preferably 80 nm or more and 150 nm or less. When the film thickness of the coating layer 4 is within this range, a high-quality SEI coating film can be formed, and the irreversible reaction of the electrolyte solution can be suppressed. In addition, when the film thickness of the coating layer 4 is around 300 nm, the coating layer 4 does not significantly hinder the conduction of lithium ions. The film thickness of the coating layer 4 is, for example, an average value of the film thickness of the coating layer 4 formed on 100 composite particles 2 obtained from a cross-sectional image.

[0045] The total film thickness of the silicon carbide layer 3 and the coating layer 4 is preferably, for example, 10 nm or more and 500 nm or less, and more preferably 20 nm or more and 300 nm or less.

[0046] The surface of the coating layer 4 can be further coated with carbon or carbon nanotubes. Carbon or carbon nanotubes support the electrical conductivity between the negative electrode materials 1. If the surface of the coating layer 4 is coated with carbon or carbon nanotubes, the conductive network between the negative electrode materials 1 is easily maintained even if the volume of the negative electrode materials 1 changes.

[0047] The negative electrode material of the first embodiment can be produced by performing a core production process and a coating layer production process.

[0048] In the core production process, the composite particles 2 are produced. The composite particles can be produced by mixing silicon particles and a carbon source in an organic solvent. The weight ratio of the silicon particles to the carbon particles in the composite particles 2 can be adjusted by the mixing ratio of the silicon particles to the carbon source.

[0049] The carbon source is graphite, graphene, pitch-based, resin-based, or the like. The pitch-based and resin-based can use the above-described pitch-based and resin-based. The carbon source is preferably at least one selected from the group consisting of graphite, graphene, novolac-type phenol aldehyde resin, resol-type phenol aldehyde resin, coal-based pitch, and petroleum-based pitch. Two or more kinds of carbon sources can be used. The carbon source has an influence on the powder resistivity of the negative electrode active material.

[0050] The organic solvent is methanol, ethanol, tetrahydrofuran, or the like. A dispersant can be added to the organic solvent. By adding the dispersant, the carbon particles uniformly cover the silicon particles at the time of compounding. Such composite particles 2 have excellent electronic conductivity and are less likely to undergo side reactions with the electrolyte solution at the time of charge and discharge.

[0051] 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.

[0052] Next, heat treatment is applied to the dried powder. By the heat treatment process, the resin or the resin composition as the carbon source is incompletely combusted and carbonized, thereby forming carbonaceous particles. Thus, the silicon particles are compounded with the carbonaceous particles.

[0053] 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 can decrease. 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.

[0054] 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.

[0055] Next, in the coating layer production process, a compound containing Mg or F is attached to the surface of the composite particles 2. The compound containing Mg or F is, for example, lithium fluoride (for example, LiF), magnesium oxide (for example, Mg2O), magnesium phosphate (for example, Mg3(PO4)2), or magnesium fluoride (for example, Mg2F).

[0056] After the compound containing Mg or F is attached to the surface of the composite particles 2, heat plasma treatment is performed. In the heat plasma treatment, the compound is instantaneously heated. The silicon and the carbon on the surface of the composite particles 2 are caused to react by the heat, and a silicon carbide layer 3 is formed. The compound containing Mg or F diffuses on the surface of the composite particles 2, and a coating layer 4 is formed.

[0057] 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 negative electrode material 1. The surface of the negative electrode material 1 is covered with the silicon carbide layer 3 and the coating layer 4, and the irreversible reaction of the composite particles 2 with the electrolyte solution can be suppressed. Lithium ions are used in the irreversible reaction of the electrolyte solution, and become a cause of a decrease in the cycle characteristics of the lithium ion secondary battery. In addition, the negative electrode material 1 is less likely to be damaged because the surface is covered with the silicon carbide layer 3 and the coating layer 4. If the negative electrode material 1 is damaged, a new surface is exposed, and the irreversible reaction of the electrolyte solution occurs.

[0058] [lithium ion secondary battery]

[0059] 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.

[0060] (Power generating element)

[0061] 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.

[0062] (Positive electrode)

[0063] 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.

[0064] [Positive electrode current collector]

[0065] 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.

[0066] [Positive electrode active material layer]

[0067] 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 necessary, a conductive aid, a binder.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] <NEGATIVE ELECTRODE>

[0076] 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.

[0077] [Negative electrode current collector]

[0078] The negative electrode current collector 32 is, for example, a conductive sheet. The negative electrode current collector 32 can use the same material as the positive electrode current collector 22.

[0079] [Negative electrode active material layer]

[0080] 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.

[0081] 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).

[0082] <Separator>

[0083] 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.

[0084] 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.

[0085]

[0086] The electrolyte solution is enclosed in the package 50, and permeates the power generating element 40. The electrolyte solution is not limited to a liquid electrolyte, and can also be a solid electrolyte. The nonaqueous electrolyte solution has, for example, a nonaqueous solvent and an electrolytic salt. The electrolytic salt is dissolved in the nonaqueous solvent.

[0087] 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.

[0088] ​The electrolytic salt is, for example, a lithium salt. Examples of the electrolyte include LiPF6, LiClO4, LiBF4, LiCF3SO3, LiCF3CF2SO3, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(CF3CF2CO)2, LiBOB, and LiN(FSO2)2. A single lithium salt may be used alone, or two or more may be used in combination. From the perspective of ionization, the electrolyte preferably contains LiPF6. The electrolytic salt preferably has a dissociation rate of 10% or more in a carbonate solvent at room temperature.

[0089] The electrolyte is preferably an electrolyte obtained by dissolving LiPF6 in a carbonate solvent. The concentration of LiPF6 is, for example, 1 mol / L. When the polyimide resin contains a large amount of aromatics, the polyimide resin sometimes exhibits charging behavior similar to that of soft carbon. When the electrolyte is a carbonate electrolyte solvent containing a cyclic carbonate, lithium and polyimide can be uniformly reacted. In this case, the cyclic carbonate is preferably ethylene carbonate, fluoroethylene carbonate, or vinylene carbonate.

[0090] <Package>

[0091] The package 50 seals the power generation element 40 and the non-aqueous electrolyte therein, thereby preventing the non-aqueous electrolyte from leaking out and preventing moisture from entering the lithium ion secondary battery 100 from outside.

[0092] For example, Figure 1 As shown, the package 50 includes a metal foil 52 and a resin layer 54 laminated on each surface 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.

[0093] As the metal foil 52, aluminum foil can be used, for example. A polymer film such as polypropylene can be used as the resin layer 54. The materials constituting the resin layer 54 can also be different on the inner and outer sides. For example, the outer side material can be a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide (PA), while the inner polymer film material can be polyethylene (PE) or polypropylene (PP).

[0094] <Terminal>

[0095] 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.

[0096] [Method for manufacturing lithium-ion secondary battery]

[0097] 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.

[0098] 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.

[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 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.

[0100] 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.

[0101] 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.

[0102] 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.

[0103] 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.

[0104] The calendering step is performed as necessary. The calendering step is a step of adjusting the density of the negative electrode active material layer 34 by applying pressure to the negative electrode active material layer 34. The calendering step is performed by, for example, a roll press device or the like.

[0105] The positive electrode 20 can be produced in the same steps as the negative electrode 30. The separator 10 and the packaging body 50 can be commercially available products.

[0106] Next, the positive electrode 20 and the negative electrode 30 are stacked with the separator 10 interposed therebetween, and the power generating element 40 is produced. In the case where the power generating element 40 is a jelly-roll, the positive electrode 20, the negative electrode 30, and the separator 10 are wound with one end side thereof as the axis.

[0107] Finally, the power generating element 40 is enclosed in the packaging body 50. A non-aqueous electrolyte solution is injected into the packaging body 50. The non-aqueous electrolyte solution is allowed to permeate into the power generating element 40 by performing pressure reduction, heating, or the like after the injection of the non-aqueous electrolyte solution. The packaging body 50 is sealed by heating or the like, and the lithium ion secondary battery 100 is obtained. Note that the electrolyte solution can not be injected into the packaging body 50, and the power generating element 40 can be immersed in the electrolyte solution. It is preferable to stand for 24 hours after the injection into the power generating element.

[0108] The lithium ion secondary battery 100 of the first embodiment is excellent in cycle characteristics because the negative electrode active material contains the negative electrode material of the prescribed shape.

[0109] The embodiments of the present application have been described in detail with reference to the drawings, but the configurations and combinations thereof in the embodiments are merely examples, and the addition, omission, substitution, and other modifications can be made within the scope of the gist of the present application.

[0110] Example

[0111] [Example 1]

[0112] The positive electrode slurry was applied to one surface of an aluminum foil having a thickness of 15 μm. The positive electrode slurry was produced by mixing a positive electrode active material, a conductive aid, a binder, and a solvent.

[0113] The positive electrode active material used was Li x CoO2. The conductive aid used was acetylene black. The binder used was polyvinylidene fluoride (PVDF). The solvent used was N-methyl-2-pyrrolidone. The positive electrode slurry was produced by mixing 97 parts by mass of the positive electrode active material, 1 part by mass of the conductive aid, 2 parts by mass of the binder, and 70 parts by mass of the solvent. The loading amount of the positive electrode active material in the positive electrode active material layer after drying 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.

[0114] 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 2 nm were mixed with a carbon source and fired to thereby form composite particles. The average secondary particle diameter of the composite particles was 8.6 μm.

[0115] Next, MgF2was attached to the surface of the composite particles and heat plasma treatment was performed. The negative electrode active material after formation was confirmed by TEM, and it was confirmed that a silicon carbide layer and a coating layer were formed. The film thickness of the silicon carbide layer was 5 nm, and the film thickness of the coating layer was 152 nm.

[0116] Next, a negative electrode slurry was prepared using the negative electrode active material. A carbon black was used as a conductive aid. A polyimide resin was used as a binder. N-methyl-2-pyrrolidone was used as a solvent. A 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.

[0117] Then, the negative electrode slurry was applied to one surface of a copper foil having a thickness of 10 μm and was 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.

[0118] 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. LiPF6was used as an electrolyte salt. The concentration of LiPF6was set to 1 mol / L.

[0119] (Production of lithium ion secondary battery for evaluation)

[0120] The negative electrode and the positive electrode that were produced were stacked with the positive electrode active material layer and the negative electrode active material layer facing each other with a separator (a 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 sealing was performed on all but 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 using a vacuum sealing machine while reducing the pressure and using heat sealing, and a lithium ion secondary battery was produced. The lithium ion secondary battery after production was left to stand for 24 hours.

[0121] (Measurement of capacity retention rate after 300 cycles)

[0122] The cycle characteristics of the lithium ion secondary battery were measured. The cycle characteristics were measured using a secondary battery charge-discharge testing device (manufactured by Hokuto Denko Corporation).

[0123] 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 charging and discharging was measured, and the battery capacity Q1 before the cycle test was calculated. The battery capacity Q1 was 3712 mAh / g.

[0124] The battery, of which the battery capacity Q1 was calculated as described above, was subjected to constant current charging at a charge rate of 1 C using the secondary battery charge-discharge testing device 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 charging and discharging was counted as one cycle, and 300 cycles of charging and discharging were performed. Then, the discharge capacity after the end of the 300 cycles of charging and discharging was measured, and the battery capacity Q2 after 300 cycles was calculated. The capacity retention rate E after 300 cycles was calculated from the battery capacities Q1, Q2 calculated as described above. The capacity retention rate E was calculated according to E = Q2 / Q1 x 100. The capacity retention rate of Example 1 was 93%.

[0125] (Rate characteristics)

[0126] In addition, the rate characteristics of the lithium ion secondary battery were calculated. The rate characteristics were measured using the secondary battery charge-discharge testing device. The rate characteristics were evaluated in terms of the rate characteristics (%) by setting the voltage range to 4.2 V to 2.5 V, and setting 1 C = 1000 mAh per the designed capacity of the entire battery. The rate characteristics are the proportion of the discharge capacity when the CCCV charging (constant current constant voltage charging, termination current value 0.05 C) is performed at a current value of 0.2 C and the discharging is performed at 2 C, with respect to the discharge capacity when the CCCV charging (constant current constant voltage charging, termination current value 0.05 C) is performed at a current value of 0.2 C and the discharging is performed at a current value of 0.2 C, and are represented by the following formula (1).

[0127] (2C capacity retention rate (%)) = (2C discharge capacity) / (0.2C discharge capacity) x 100 (1)

[0128] [Example 2, Example 3]

[0129] 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 retention rate after 300 cycles was calculated.

[0130] [Examples 4 to 7]

[0131] Examples 4 to 7 differ from Example 1 in that the film thickness of the silicon carbide layer was changed. The film thickness of the silicon carbide layer was adjusted by changing the treatment conditions of the thermal plasma treatment. The other conditions were the same as in Example 1, and the capacity retention rate after 300 cycles was obtained.

[0132] [Examples 8, 9]

[0133] Examples 8, 9 differ from Example 1 in that the film thickness of the coating layer was changed. The film thickness of the coating layer was adjusted by changing the amount of MgF2attached. The other conditions were the same as in Example 1, and the capacity retention rate after 300 cycles was obtained.

[0134] [Examples 10, 11]

[0135] Examples 10, 11 differ from Example 1 in that the average secondary particle diameter of the composite particles was changed. The average secondary particle diameter of the composite 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 retention rate after 300 cycles was obtained.

[0136] [Examples 12 to 14]

[0137] Examples 12 to 14 differ from Example 1 in that the compound attached to the surface of the composite particles was changed from MgF2to another compound, and the thermal plasma treatment was performed.

[0138] In Example 12, LiF was used instead of MgF2.

[0139] In Example 13, Mg2O was used instead of MgF2.

[0140] In Example 14, Mg3(PO4)2was used instead of MgF2.

[0141] The other conditions were the same as in Example 1, and the capacity retention rate after 300 cycles was obtained.

[0142] [Example 15]

[0143] Example 15 differs from Example 1 in that carbon was attached to the surface of the coating layer. 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 retention rate after 300 cycles was obtained.

[0144] [Comparative Example 1]

[0145] Comparative Example 1 differs from Example 1 in that the MgF2is not attached to the surface after the composite particles are produced. That is, the negative electrode active material of Comparative Example 1 is composed only of the composite particles. The other conditions are the same as in Example 1, and the capacity retention rate after 300 cycles is obtained.

[0146] [Comparative Example 2, Comparative Example 3]

[0147] Comparative Examples 2 and 3 differ from Example 1 in that the average primary particle diameter of the silicon particles that constitute 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.

[0148] [Comparative Example 4, Comparative Example 5]

[0149] Comparative Examples 4 and 5 differ from Example 1 in that the film thickness of the silicon carbide layer is changed. The film thickness of the silicon carbide layer is adjusted by changing the treatment conditions of the thermal plasma treatment. The other conditions are the same as in Example 1, and the capacity retention rate after 300 cycles is obtained.

[0150] The results of Examples 1 to 15 and Comparative Examples 1 to 5 are summarized in the following table. The negative electrode active materials of Examples 2 to 15 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.

[0151] [Table 1]

[0152]

[0153] Examples 1 to 15 have higher capacity retention rates and superior cycle characteristics compared to Comparative Examples 1 to 4. Comparative Example 5 has a decrease in rate characteristics due to the thicker silicon carbide layer.

[0154] Explanation of symbols:

[0155] 1…negative electrode material

[0156] 2…composite particle

[0157] 3…silicon carbide layer

[0158] 4…coating layer

[0159] 10…separator

[0160] 20…positive electrode

[0161] 22…positive electrode current collector

[0162] 24…positive electrode active material layer

[0163] 30…negative electrode

[0164] 32…negative electrode current collector

[0165] 34... negative active material layer

[0166] 40... power generating element

[0167] 50... package

[0168] 52... metal foil

[0169] 54... resin layer

[0170] 60, 62... terminal

[0171] 100... lithium ion secondary battery

Claims

1. A negative electrode material for a lithium ion secondary battery, wherein a composite particle, a silicon carbide layer, and a coating layer are provided, the composite particle includes an amorphous carbon particle and an amorphous silicon particle having an average primary particle diameter of 1 nm or more and 50 nm or less, the silicon carbide layer is located between the composite particle and the coating layer, a film thickness of the silicon carbide layer is 1 nm or more and 100 nm or less, and the coating layer includes a compound of magnesium or fluorine.

2. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein a total film thickness of the silicon carbide layer and the coating layer is 10 nm or more and 500 nm or less.

3. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein a film thickness of the silicon carbide layer is 3 nm or more and 20 nm or less.

4. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein a thickness of the coating layer is 10 nm or more and 300 nm or less.

5. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein an average secondary particle diameter of the composite particle is 1 μm or more and 10 μm or less.

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

7. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the compound of magnesium or fluorine is any one or more selected from the group consisting of lithium fluoride, magnesium oxide, magnesium phosphate, and magnesium fluoride.

8. 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.

9. A lithium ion secondary battery, wherein a lithium ion secondary battery including the negative electrode for a lithium ion secondary battery according to claim 8, a positive electrode, and an electrolyte is provided.

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