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

Abstract

This negative electrode material for lithium-ion secondary batteries comprises 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 size of 1 nm to 50 nm inclusive. The silicon carbide layer is between the composite particles and the coating layer. The film thickness of the silicon carbide layer is 1 nm to 100 nm inclusive. The coating layer includes a magnesium or fluorine compound.

Description

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

[0001] 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.

[0002] Lithium-ion secondary batteries are also widely used as a power source for mobile devices such as mobile phones and laptop computers, as well as hybrid cars.

[0003] The capacity of lithium-ion secondary batteries depends primarily on the active material of the electrodes. Graphite is generally used as the negative electrode active material, but there is a demand for negative electrode active materials with higher capacities. Silicon (Si), which has a theoretical capacity far greater than that of graphite (372 mAh / g), has therefore attracted attention.

[0004] A negative electrode active material containing silicon undergoes significant volume expansion during charging. The volume expansion of the negative electrode active material causes a deterioration in the cycle characteristics of the battery. When the negative electrode active material expands in volume, for example, the negative electrode active material may be damaged, the conductive path between the negative electrode active material may be cut, peeling may occur at the interface between the negative electrode active material layer and the current collector, cracks may occur in the SEI (Solid Electrolyte Interphase) coating, and decomposition of the electrolyte may occur. These problems deteriorate the cycle characteristics of the battery.

[0005] For example, Patent Document 1 describes that cycle characteristics are improved by specifying the aspect ratio of silicon particles and the inclination angle of the silicon particles relative to the current collector.

[0006] Japanese Patent Application Laid-Open No. 2019-149333

[0007] The cycle characteristics are an important parameter, and it is desired that the cycle characteristics can be improved by methods other than the method described in Patent Document 1.

[0008] The present disclosure has been made in view of the above problems, and has an object to provide a lithium ion secondary battery with excellent cycle characteristics.

[0009] In order to solve the above problems, the following means are provided.

[0010] A negative electrode material for a lithium ion secondary battery according to a first aspect includes 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 1 nm or more and 50 nm or less. The silicon carbide layer is located between the composite particles and the coating layer. The silicon carbide layer has a thickness of 1 nm or more and 100 nm or less. The coating layer includes a compound of magnesium or fluorine.

[0011] A lithium ion secondary battery using the negative electrode material for a lithium ion secondary battery according to the above embodiment has excellent cycle characteristics.

[0012] 1 is a cross-sectional view of a negative electrode material for a lithium ion secondary battery according to a first embodiment; 2 is a schematic diagram of a lithium ion secondary battery according to a first embodiment;

[0013] Hereinafter, the embodiments will be described in detail with reference to the drawings as appropriate. The drawings used in the following description may show characteristic portions enlarged for convenience in order to make the features easier to understand, and the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present disclosure is not limited thereto. Appropriate changes can be made within the scope of the present disclosure.

[0014] "Anode Material" The anode material according to the first embodiment is used in a lithium-ion secondary battery and contains silicon particles. The anode material according to the first embodiment functions as, for example, an anode active material.

[0015] 1 is a cross-sectional view of an anode material 1 according to a first embodiment. The anode material 1 has composite particles 2, a silicon carbide layer 3, and a coating layer 4.

[0016] The composite particles 2 are secondary particles formed by aggregation of a plurality of particles. The average secondary particle diameter of the composite particles 2 is, for example, 1 μm or more and 10 μm or less, preferably 2 μm or more and 8 μm or less, and more preferably 3 μm or more and 7 μm or less.

[0017] If 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 becomes difficult to ensure sufficient strength and conductivity when the negative electrode active material layer is formed, and the amounts of binder and conductive additive used increase. Since the binder and conductive additive are not active materials that perform charging and discharging, increasing their abundance ratio in the negative electrode active material layer reduces the capacity of the lithium ion secondary battery. Furthermore, if the composite particles 2 are too large, the composite particles 2 may be damaged by expansion and contraction, increasing the risk of side reactions such as decomposition of the electrolyte on the new surfaces created by the damage.

[0018] When the composite particles 2 are available in the form of particles, the median diameter (D50) can be determined as the average secondary particle diameter using a particle size distribution analyzer (for example, manufactured by Malvern Panalytical). When using a particle size distribution analyzer, for example, the average particle diameter of 50,000 particles is determined.

[0019] When the composite particles 2 are present within the electrode and it is difficult to separate the composite particles 2, the average secondary particle diameter can be determined using at least 100 composite particles 2 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 determined from the cross-sectional image do not deviate significantly and generally coincide.

[0020] First, a contrast threshold is set, and composite particles 2 are extracted from the image. Then, the diameters of at least 100 extracted composite particles 2 are determined. The frequency of the diameters of each of the determined composite particles 2 is graphed, and the most frequent value is taken as the average secondary particle diameter. When the shape of the composite particles 2 is irregular, the diameter of the major axis is used to calculate the average secondary particle diameter.

[0021] The composite particle 2 contains carbonaceous particles and silicon particles. The weight ratio of the carbonaceous particles to the silicon particles in the composite particle 2, obtained by dividing the weight of the carbonaceous particles by the weight of the silicon particles, is preferably 0.5 or more and 5 or less. 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) optical emission spectroscopy or the like.

[0022] The carbonaceous particles and silicon particles are both amorphous. Lithium ions penetrate along the orientation direction of the crystals. Therefore, the crystals have limited paths through which lithium ions can penetrate. In contrast, the crystal orientation of amorphous carbonaceous particles and silicon particles is not fixed to a specific direction, and the direction in which lithium ions penetrate is not limited. When the carbonaceous particles and silicon particles constituting the composite particle 2 are amorphous, lithium ions diffuse uniformly in the negative electrode material 1, and local volume changes in the negative electrode material 1 can be suppressed.

[0023] The silicon particles may be silicon alone or silicon oxide (SiO ｘ : x satisfies, for example, 0.8≦x≦2. ｎ The alloy may be a silicon alloy represented by Si. 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, etc. n satisfies 0≦n≦0.5.

[0024] The average primary particle diameter of the silicon particles is, for example, 1 nm or more and 50 nm or less, and preferably 3 nm or more and 30 nm or less. The average primary particle diameter of the silicon particles is determined from a cross-sectional image of the negative electrode material 1. The cross-sectional image can be measured with a scanning electron microscope (SEM) or a transmission electron microscope (TEM). For example, the average primary particle diameter can be measured by observing the negative electrode material 1 at a magnification of 100,000 times using a scanning electron microscope JSM-7600 (manufactured by JEOL Ltd.) and performing image processing on the captured image.

[0025] The average primary particle diameter is determined using image processing software HALCON (registered trademark, manufactured by MVTec Software GmbH). This software recognizes particles in the captured image and removes particles that are not entirely captured at the edge of the observation field. Then, the maximum length (diameter of the circumscribed circle of the particle) of each particle is measured, and the particle diameter is calculated from the maximum length. This measurement is performed on 200 particles to determine the number-based cumulative particle size distribution, from which the average primary particle diameter can be calculated.

[0026] When the average primary particle size of the silicon particles is within the above range, an increase in the coating resistance of the electrolyte due to side reactions caused by contact between the silicon particles and the electrolyte can be suppressed, and when the average primary particle size of the silicon particles is within the above range, damage to the silicon particles due to expansion and contraction during charge and discharge can be suppressed.

[0027] The carbonaceous particles are composited with silicon particles. Examples of the carbonaceous particles include graphite, graphene, carbides formed after firing pitches, carbides formed after firing resins, etc. Two or more types of carbonaceous particles may be used.

[0028] The pitches may be coal-based pitches, petroleum-based pitches, or synthetic pitches, such as coal tar, light tar oil, medium tar oil, heavy tar oil, naphthalene oil, anthracene oil, coal tar pitch, pitch oil, mesophase pitch, oxygen-crosslinked petroleum pitch, heavy oil, coke, low-molecular-weight heavy oil, and derivatives thereof.

[0029] Examples of resins include thermoplastic resins such as polyvinyl alcohol, phenolic resins, epoxy resins, melamine resins, urea resins, aniline resins, cyanate resins, furan resins, ketone resins, unsaturated polyester resins, urethane resins, and modified versions thereof. Examples of phenolic resins include novolac phenolic resins and resol phenolic resins. Examples of epoxy resins include bisphenol epoxy resins and novolac epoxy resins. Examples of resins include polyethylene, polystyrene, acrylonitrile-styrene (AS) resin, acrylonitrile-butadiene-styrene (ABS) resin, polypropylene, polyethylene terephthalate, polycarbonate, polyacetal, polyphenylene ether, polybutylene terephthalate, polyphenylene sulfide, polysulfone, polyethersulfone, polyetheretherketone, and polyvinyl chloride.

[0030] The carbonaceous particles preferably contain graphite or graphene. ２ The bond has high electronic conductivity and ensures a uniform supply of lithium ions to the silicon particles.

[0031] The silicon carbide layer 3 is located between the composite particle 2 and the coating layer 4. The silicon carbide layer 3 covers at least a part of the surface of the composite particle 2. The silicon carbide layer 3 may cover the entire surface of the composite particle 2.

[0032] The silicon carbide layer 3 prevents direct contact between the composite particles 2 and the electrolyte solution and the occurrence of an irreversible reaction. The irreversible reaction is a reaction in which the electrolyte solution is reduced and decomposed. If an irreversible reaction occurs, the amount of lithium ions in the electrolyte solution decreases, and the cycle characteristics of the lithium-ion secondary battery deteriorate. The silicon carbide layer 3 also increases the strength of the composite particles 2 and prevents damage to the negative electrode material 1.

[0033] The film thickness of the silicon carbide layer 3 is, for example, 1 nm or more and 100 nm or less, and preferably 3 nm or more and 20 nm or less. The film thickness of the silicon carbide layer 3 is, for example, the average film thickness of the silicon carbide layer 3 formed on 100 composite particles 2 obtained from a cross-sectional image. If the film thickness of the silicon carbide layer 3 is within the above range, the strength of the negative electrode material 1 can be sufficiently increased while suppressing irreversible reactions. Furthermore, if the film thickness of the silicon carbide layer 3 is about 100 nm, the silicon carbide layer 3 does not significantly inhibit the conduction of lithium ions.

[0034] The coating layer 4 covers the composite particle 2 and the silicon carbide layer 3. The coating layer 4 covers at least a portion of the composite particle 2 or the silicon carbide layer 3. The coating layer 4 may completely cover the composite particle 2 and the silicon carbide layer 3.

[0035] The coating layer 4 contains a compound of magnesium or fluorine. The compound of magnesium or fluorine may be, for example, lithium fluoride (e.g., LiF), magnesium oxide (e.g., Mg ２ O), magnesium phosphate (e.g., Mg ３ (P.O. ４ ) ２ ), magnesium fluoride (e.g., Mg ２ F).

[0036] The coating layer 4 prevents the composite particles 2 from coming into direct contact with the electrolyte solution and causing an irreversible reaction. The magnesium or fluorine compound reacts with the electrolyte solution to form a high-quality SEI film.

[0037] The 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. If the thickness of the coating layer 4 is within this range, a high-quality SEI film can be formed and irreversible reactions of the electrolyte can be suppressed. Furthermore, if the thickness of the coating layer 4 is about 300 nm, the coating layer 4 does not significantly inhibit the conduction of lithium ions. The thickness of the coating layer 4 is, for example, the average thickness of the coating layer 4 formed on 100 composite particles 2 obtained from a cross-sectional image.

[0038] The total thickness of the silicon carbide layer 3 and the coating layer 4 is, for example, preferably 10 nm or more and 500 nm or less, and more preferably 20 nm or more and 300 nm or less.

[0039] The surface of the coating layer 4 may be further coated with carbon or carbon nanotubes. The carbon or carbon nanotubes support the conductivity between the negative electrode materials 1. When the surface of the coating layer 4 is coated with carbon or carbon nanotubes, the conductive network between the negative electrode materials 1 is more easily maintained even if the volume of the negative electrode materials 1 changes.

[0040] The negative electrode material according to the first embodiment can be produced by carrying out a core production step and a coating layer production step.

[0041] In the core preparation step, composite particles 2 are prepared. The composite particles can be prepared by mixing silicon particles and a carbon source in an organic solvent. The weight ratio of silicon particles to carbonaceous particles in composite particles 2 can be adjusted by adjusting the mixing ratio of the silicon particles to the carbon source.

[0042] The carbon source may be graphite, graphene, pitches, resins, or the like. The pitches and resins described above can be used. The carbon source is preferably at least one selected from the group consisting of graphite, graphene, novolac phenolic resin, resol phenolic resin, coal-based pitch, and petroleum-based pitch. Two or more carbon sources may be used. The carbon source affects the powder resistivity of the negative electrode active material.

[0043] The organic solvent may be methanol, ethanol, tetrahydrofuran, or the like. A dispersant may be added to the organic solvent. By adding the dispersant, the carbonaceous particles uniformly cover the silicon particles during the composite formation. Such composite particles 2 have excellent electronic conductivity and are less likely to undergo side reactions with the electrolyte during charge and discharge.

[0044] The mixture is then dried. The organic solvent is removed from the mixture by drying, and a powder is obtained. The drying method is not particularly limited, but may be, for example, a spray drying method.

[0045] The dried powder is then heat-treated. The resin or resin composition serving as the carbon source is incompletely burned and carbonized to form carbonaceous particles, resulting in the formation of composites of silicon particles and carbonaceous particles.

[0046] The heat treatment is preferably carried out at a temperature of 350 to 1200°C. If the heat treatment temperature is low, the carbon source is not sufficiently carbonized, which may cause lithium to be trapped during charging and discharging. If lithium is trapped, the initial efficiency of the lithium-ion secondary battery decreases. If the heat treatment temperature is high, silicon particles and carbonaceous particles react with each other, resulting in the production of excessive silicon carbide. Among silicon compounds, silicon carbide has a small contribution to charging and discharging, which reduces the conductivity of lithium ions and causes a decrease in the discharge capacity of the lithium-ion secondary battery.

[0047] The heat treatment time is preferably 1 hour or more and 72 hours or less, and the heat treatment atmosphere is preferably a reducing atmosphere such as a nitrogen atmosphere or an argon atmosphere.

[0048] Next, in the coating layer preparation step, a compound containing Mg or F is attached to the surface of the composite particle 2. The compound containing Mg or F is, for example, lithium fluoride (e.g., LiF), magnesium oxide (e.g., Mg ２ O), magnesium phosphate (e.g., Mg ３ (P.O. ４ ) ２ ), magnesium fluoride (e.g., Mg ２ F).

[0049] After the compound containing Mg or F is attached to the surface of the composite particle 2, a thermal plasma treatment is performed. In the thermal plasma treatment, heat is instantaneously applied to the compound. This heat causes a reaction between silicon and carbon on the surface of the composite particle 2, forming a silicon carbide layer 3. The compound containing Mg or F spreads over the surface of the composite particle 2 and becomes a coating layer 4.

[0050] The negative electrode material according to the first embodiment has excellent cycle characteristics for lithium-ion secondary batteries. This is because the negative electrode material has a specified negative electrode material 1. The surface of the negative electrode material 1 is covered with a silicon carbide layer 3 and a coating layer 4, which can suppress irreversible reactions between the composite particles 2 and the electrolyte. Lithium ions are used in the irreversible reactions of the electrolyte, which can cause a decrease in the cycle characteristics of the lithium-ion secondary battery. Furthermore, the negative electrode material 1 is less likely to break because its surface is covered with the silicon carbide layer 3 and the coating layer 4. When the negative electrode material 1 breaks, a new surface is exposed, causing an irreversible reaction of the electrolyte.

[0051] "Lithium-ion secondary battery" Fig. 2 is a schematic diagram of a lithium-ion secondary battery according to the first embodiment. The lithium-ion secondary battery 100 shown in Fig. 2 includes a power generating element 40, an exterior body 50, and an electrolyte (e.g., a non-aqueous electrolyte solution). The exterior body 50 covers the periphery of the power generating element 40. The power generating element 40 is connected to the outside via a pair of terminals 60, 62 connected to the power generating element 40. The non-aqueous electrolyte solution is accommodated in the exterior body 50. Although Fig. 2 illustrates an example in which one power generating element 40 is provided within the exterior body 50, a plurality of power generating elements 40 may be stacked.

[0052] (Power generating element) The power generating element 40 includes a separator 10, a positive electrode 20, and a negative electrode 30. The power generating element 40 may be a laminate in which these are stacked, or a wound body in which a structure in which these are stacked is wound.

[0053] <Positive Electrode> The positive electrode 20 includes, 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.

[0054] [Positive Electrode Current Collector] The positive electrode current collector 22 is, for example, a conductive plate material. The positive electrode current collector 22 is, for example, a thin metal plate made of aluminum, copper, nickel, titanium, stainless steel, or the like. Aluminum, which is lightweight, is preferably used 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.

[0055] [Positive Electrode Active Material Layer] The positive electrode active material layer 24 contains, for example, a positive electrode active material. The positive electrode active material layer 24 may contain a conductive additive and a binder as necessary.

[0056] The positive electrode active material includes an electrode active material that can reversibly absorb and release lithium ions, desorb and insert (intercalate) lithium ions, or dope and dedope lithium ions with counter anions.

[0057] The positive electrode active material is, for example, a composite metal oxide. The composite metal oxide is, for example, lithium cobalt oxide (LiCoO ２ ), lithium nickel oxide (LiNiO ２ ), lithium manganese oxide (LiMnO ２ ), lithium manganese spinel (LiMn ２ O ４ ), and the general formula: LiNi ｘ Co ｙ Mn ｚ M ａ O ２ (wherein x+y+z+a=1, 0≦x<1, 0≦y<1, 0≦z<1, 0≦a<1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr), lithium vanadium compounds (LiV ２ O ５ ), olivine-type LiMPO４ (wherein M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr, or VO), lithium titanate (Li ４ Ti ５ O １２ ), LiNi ｘ Co ｙ Al ｚ O ２ (0.9<x+y+z<1.1). The positive electrode active material may be an organic material. For example, the positive electrode active material may be polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacene.

[0058] The positive electrode active material may be a lithium-free material, such as FeF ３ Examples of the non-lithium-containing material include conjugated polymers containing organic conductive materials, Chevrel phase compounds, transition metal chalcogenides, vanadium oxides, and niobium oxides. The non-lithium-containing material may be any one of these materials alone or in combination. When the positive electrode active material is a non-lithium-containing material, for example, discharge is first performed. Lithium is inserted into the positive electrode active material by discharging. Alternatively, a non-lithium-containing positive electrode active material may be pre-doped with lithium chemically or electrochemically.

[0059] The conductive additive enhances the electronic conductivity between the positive electrode active materials. Examples of the conductive additive include carbon powder, carbon nanotubes, carbon materials, metal powder, a mixture of carbon materials and metal powder, and conductive oxides. Examples of the carbon powder include carbon black, acetylene black, and ketjen black. Examples of the metal powder include powders of copper, nickel, stainless steel, and iron.

[0060] There are no particular limitations on the content of the conductive additive in the positive electrode active material layer 24. For example, the content of the conductive additive relative to the total mass of the positive electrode active material, the conductive additive, and the binder is 0.5 mass% or more and 20 mass% or less, and preferably 1 mass% or more and 5 mass% or less.

[0061] The binder in the positive electrode active material layer 24 binds the positive electrode active material together. Known binders can be used. The binder is preferably one that is insoluble in the electrolyte, has oxidation resistance, and has adhesive properties. The binder is, for example, a fluororesin. Examples of the binder include polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), polyamide (PA), polyimide (PI), polyamideimide (PAI), polybenzimidazole (PBI), polyethersulfone (PES), polyacrylic acid and its copolymers, metal ion crosslinked polyacrylic acid and its copolymers, maleic anhydride-grafted polypropylene (PP) or polyethylene (PE), and mixtures thereof. PVDF is particularly preferred as the binder used in the positive electrode active material layer.

[0062] The binder content in the positive electrode active material layer 24 is not particularly limited. For example, the binder content relative to the total mass of the positive electrode active material, conductive additive, and binder is 1% by mass or more and 15% by mass or less, and preferably 1.5% by mass or more and 5% by mass or less. If the binder content is low, the adhesive strength of the positive electrode 20 will be weakened. If the binder content is high, the binder will be electrochemically inactive and will not contribute to the discharge capacity, resulting in a low energy density of the lithium-ion secondary battery 100.

[0063] <Negative Electrode> The negative electrode 30 includes, 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.

[0064] [Negative Electrode Current Collector] The negative electrode current collector 32 is, for example, a conductive plate material. The negative electrode current collector 32 may be the same as the positive electrode current collector 22.

[0065] [Negative Electrode Active Material Layer] The negative electrode active material layer 34 contains a negative electrode active material and a binder. The negative electrode active material layer may contain a conductive additive, a dispersion stabilizer, and the like, as necessary. The negative electrode active material is the above-described negative electrode material. By using the above-described negative electrode material as the negative electrode active material, the cycle characteristics of the lithium-ion secondary battery 100 are improved.

[0066] The conductive additive and binder may be the same as those used in the positive electrode 20. The binder in the negative electrode 30 may be, in addition to those listed for the positive electrode 20, for example, cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamide-imide resin, acrylic resin, etc. The cellulose may be, for example, carboxymethyl cellulose (CMC).

[0067] <Separator> The separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30. The separator 10 separates the positive electrode 20 from the negative electrode 30 and prevents short-circuiting between 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.

[0068] The separator 10 has, for example, an electrically insulating porous structure. The separator 10 is, for example, a monolayer or laminate of a polyolefin film. The separator 10 may be a stretched membrane of a mixture of polyethylene, polypropylene, or the like. The separator 10 may be a fibrous nonwoven fabric made of at least one constituent material selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The separator 10 may be, for example, a solid electrolyte. The solid electrolyte may be, for example, a polymer solid electrolyte, an oxide-based solid electrolyte, or a sulfide-based solid electrolyte. The separator 10 may also be an inorganic-coated separator. The inorganic-coated separator is formed by coating the surface of the above-mentioned film with a mixture of a resin such as PVDF or CMC and an inorganic material such as alumina or silica. The inorganic-coated separator has excellent heat resistance and suppresses the deposition of transition metals eluted from the positive electrode onto the negative electrode surface.

[0069] <Electrolyte> The electrolyte is sealed in the exterior body 50 and impregnates the power generating element 40. The electrolyte is not limited to a liquid electrolyte, and may be a solid electrolyte. The non-aqueous electrolyte includes, for example, a non-aqueous solvent and an electrolytic salt. The electrolytic salt is dissolved in the non-aqueous solvent.

[0070] The solvent is not particularly limited as long as it is a solvent generally used in lithium-ion secondary batteries. Examples of the solvent include a cyclic carbonate compound, a chain carbonate compound, a cyclic ester compound, and a chain ester compound. The solvent may contain a mixture of these compounds in any ratio. Examples of the cyclic carbonate compound include ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate, and vinylene carbonate. Examples of the chain carbonate compound include diethyl carbonate (DEC) and ethyl methyl carbonate (EMC). Examples of the cyclic ester compound include γ-butyrolactone. Examples of the chain ester compound include propyl propionate, ethyl propionate, and ethyl acetate.

[0071] The electrolyte salt is, for example, a lithium salt. ６ , LiClO ４ , LiBF ４ , LiCF ３ SO ３ , LiCF ３ CF ２ SO ３ , LiC(CF ３ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ , LiN(CF ３ CF ２ SO ２ ) ２ , LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN(CF ３ CF ２ CO) ２ , LiBOB, LiN(FSO ２ ) ２ The lithium salt may be used alone or in combination of two or more. From the viewpoint of the degree of ionization, the electrolyte is preferably LiPF ６ The dissociation rate of the electrolytic salt in the carbonate solvent at room temperature is preferably 10% or more.

[0072] The electrolyte is, for example, LiPF in a carbonate solvent. ６ A solution of LiPF is preferred. ６ The concentration of is, for example, 1 mol / L. When the polyimide resin contains a large amount of aromatics, the polyimide resin may exhibit charging behavior similar to that of soft carbon. When the electrolyte is a carbonate electrolyte solvent containing a cyclic carbonate, lithium can be reacted uniformly with the polyimide. In this case, the cyclic carbonate is preferably ethylene carbonate, fluoroethylene carbonate, or vinylene carbonate.

[0073] <Exterior Body> The exterior body 50 seals the power generating element 40 and the non-aqueous electrolyte solution inside. The exterior body 50 prevents the non-aqueous electrolyte solution from leaking to the outside and prevents moisture and the like from entering the lithium-ion secondary battery 100 from the outside.

[0074] 1, the exterior body 50 has a metal foil 52 and a resin layer 54 laminated on each side of the metal foil 52. The exterior body 50 is a metal laminate film in which the metal foil 52 is coated on both sides with a polymer film (resin layer 54).

[0075] The metal foil 52 can be, for example, aluminum foil. The resin layer 54 can be a polymer film such as polypropylene. The materials constituting the inner and outer resin layers 54 can be different. For example, the outer material can be a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide (PA), and the inner polymer film can be made of polyethylene (PE), polypropylene (PP), or the like.

[0076] <Terminals> The terminals 62 and 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 and 62 electrically connect the power generating element to the outside. The terminals 60 and 62 are made of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screw fastening. It is preferable to protect the terminals 60 and 62 with insulating tape to prevent short circuits.

[0077] "Method for manufacturing lithium-ion secondary battery" The lithium-ion secondary battery 100 is fabricated by preparing and assembling the negative electrode 30, the positive electrode 20, the separator 10, the electrolyte, and the exterior body 50. An example of a method for manufacturing the lithium-ion secondary battery 100 will be described below.

[0078] The negative electrode 30 is produced by, for example, sequentially carrying out a slurry production step, an electrode application step, a drying step, and a rolling step.

[0079] The slurry preparation step involves mixing a negative electrode active material, a binder, a conductive additive, and a solvent to prepare a slurry. The negative electrode active material is the negative electrode material described above. Adding a dispersion stabilizer to the slurry can suppress aggregation of the negative electrode active material.

[0080] The slurry preparation process is a process in which a negative electrode active material, a binder, a conductive additive, and a solvent are mixed to prepare a slurry. Examples of the solvent include water and N-methyl-2-pyrrolidone. The composition 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% by mass. These mass ratios are adjusted so that the total is 100 wt%. The container used to prepare the slurry is preferably made of metal such as SUS.

[0081] The negative electrode active material may be a composite obtained by mixing active material particles and a conductive material while applying shearing force. When the active material particles are mixed under shearing force to a degree that does not alter their properties, the surfaces of the active material particles are coated with the conductive material. The particle size of the negative electrode active material can be adjusted by adjusting the degree of mixing. The negative electrode active material may also be sieved after preparation to make the particle size uniform.

[0082] The electrode coating step is a step of coating the surface of the negative electrode current collector 32 with a slurry. The method of coating the slurry is not particularly limited. For example, a slit die coating method or a doctor blade method can be used as the method of coating the slurry. The slurry is coated at room temperature, for example.

[0083] The drying step is a step of removing the solvent from the slurry. For example, the negative electrode current collector 32 coated with the slurry is dried in an atmosphere at 80° C. or higher and 350° C. or lower.

[0084] The rolling step is performed as necessary. 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.

[0085] The positive electrode 20 can be produced by the same procedure as that for the negative electrode 30. The separator 10 and the outer casing 50 can be commercially available products.

[0086] Next, the prepared positive electrode 20 and negative electrode 30 are stacked so that the separator 10 is positioned between them to prepare the power generating element 40. When the power generating element 40 is a wound body, the positive electrode 20, the negative electrode 30, and the separator 10 are wound around one end side of the electrode as an axis.

[0087] Finally, the power generation element 40 is sealed in the exterior body 50. The non-aqueous electrolyte is poured into the exterior body 50. After the non-aqueous electrolyte is poured, the pressure is reduced, heating, etc. is performed, so that the non-aqueous electrolyte is impregnated into the power generation element 40. The lithium ion secondary battery 100 is obtained by sealing the exterior body 50 by applying heat, etc. Note that the power generation element 40 may be impregnated with the electrolyte instead of pouring the electrolyte into the exterior body 50. After pouring the electrolyte into the power generation element, it is preferable to leave it to stand for 24 hours.

[0088] The lithium ion secondary battery 100 according to the first embodiment has excellent cycle characteristics because the negative electrode active material contains a negative electrode material of a predetermined shape.

[0089] The above describes the embodiments of the present invention in detail with reference to the drawings. However, each configuration and combination thereof in each embodiment is an example, and additions, omissions, substitutions, and other modifications of the configurations are possible within the scope that does not deviate from the spirit of the present invention.

[0090] Example 1 A positive electrode slurry was applied to 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.

[0091] The positive electrode active material is Li ｘ CoO ２The conductive additive was acetylene black. The binder was polyvinylidene fluoride (PVDF). The solvent was N-methyl-2-pyrrolidone. 97 parts by mass of the positive electrode active material, 1 part by mass of the conductive additive, 2 parts by mass of the binder, and 70 parts by mass of the solvent were mixed to prepare a positive electrode slurry. After drying, the amount of the positive electrode active material carried in the positive electrode active material layer was 25 mg / cm. ２ The solvent was removed from the positive electrode slurry in a drying furnace to prepare a positive electrode active material layer, which was then pressed with a roll press to prepare a positive electrode.

[0092] Next, the negative electrode active material to be added to the negative electrode slurry was prepared. First, silicon particles having an average primary particle size of 2 nm and a carbon source were mixed and sintered to produce composite particles. The average secondary particle size of the composite particles was 8.6 μm.

[0093] Next, MgF ２ The anode active material was then subjected to thermal plasma treatment. The resulting anode active material was observed with a TEM to confirm the formation of a silicon carbide layer and a coating layer. The silicon carbide layer had a thickness of 5 nm, and the coating layer had a thickness of 152 nm.

[0094] Next, a negative electrode slurry was prepared using this negative electrode active material. Carbon black was used as the conductive additive. Polyimide resin was used as the binder. N-methyl-2-pyrrolidone was used as the solvent. 90 parts by mass of the negative electrode active material, 5 parts by mass of the conductive additive, and 5 parts by mass of the binder were mixed with N-methyl-2-pyrrolidone to prepare a negative electrode slurry.

[0095] The negative electrode slurry was applied to one surface of a copper foil having a thickness of 10 μm and then dried. The amount of the negative electrode active material carried in the negative electrode active material layer after drying was 2.5 mg / cm. ２ The negative electrode active material layer was pressed with a roll press and then baked in a nitrogen atmosphere at 300° C. or higher for 5 hours.

[0096] Next, an electrolyte solution was prepared. The solvent of the electrolyte solution was fluoroethylene carbonate (FEC): ethylene carbonate (EC): diethyl carbonate (DEC) = 10 vol %: 20 vol %: 70 vol %. The electrolyte solution was also added with additives for improving output, gas suppression, cycle characteristics improvement, and safety performance improvement. The electrolyte salt was LiPF ６ LiPF was used. ６ The concentration was set to 1 mol / L.

[0097] (Preparation of Lithium-Ion Secondary Battery for Evaluation) The prepared negative electrode and positive electrode were laminated with a separator (porous polyethylene sheet) interposed between them so that the positive electrode active material layer and the negative electrode active material layer faced each other, to obtain a laminate. This laminate was inserted into an exterior body made of aluminum laminate film and heat-sealed except for one peripheral location to form a closed opening. Finally, the above-mentioned electrolyte solution was injected into the exterior body, and the remaining location was heat-sealed while reducing the pressure using a vacuum sealer, to prepare a lithium-ion secondary battery. The prepared lithium-ion secondary battery was left to stand for 24 hours.

[0098] (Measurement of Capacity Retention Rate After 300 Cycles) The cycle characteristics of the lithium ion secondary battery were measured using a secondary battery charge / discharge tester (manufactured by Hokuto Denko Corporation).

[0099] The battery was charged at a constant current charge rate of 1 C (a current value at which charging is completed in 1 hour when constant current charging is performed at 25°C) until the battery voltage reached 4.2 V, and then discharged at a constant current discharge rate of 1.0 C until the battery voltage reached 2.5 V. The discharge capacity after the end of charging and discharging was detected, and the battery capacity Q before the cycle test was １ The battery capacity Q １ was 3712 mAh / g.

[0100] The battery capacity Q １The battery for which Q was determined was again charged using a secondary battery charge / discharge tester at a constant current charge rate of 1 C until the battery voltage reached 4.2 V, and then discharged at a constant current discharge rate of 1 C until the battery voltage reached 2.5 V. The above charge / discharge cycle was counted as one cycle, and 300 charge / discharge cycles were performed. After that, the discharge capacity after 300 charge / discharge cycles was measured, and the battery capacity Q after 300 cycles was calculated. ２ The battery capacity Q calculated above was １ , Q ２ The capacity retention rate E after 300 cycles was calculated from the above. The capacity retention rate E was calculated by the following equation: E = Q ２ / Q １ × 100. The capacity retention rate of Example 1 was 93%.

[0101] (Rate Characteristics) The rate characteristics of the lithium ion secondary battery were also determined. The rate characteristics were measured using a secondary battery charge / discharge tester. The rate characteristics were evaluated as rate characteristics (%) over a voltage range of 4.2 V to 2.5 V, with 1 C = 1000 mAh per full cell design capacity. The rate characteristics are the ratio of the discharge capacity when CCCV charging (constant current / constant voltage charging, end current value 0.05 C) at a current value of 0.2 C and discharging at a current value of 0.2 C to the discharge capacity when CCCV charging (constant current / constant voltage charging, end current value 0.05 C) at a current value of 0.2 C and discharging at a current value of 2 C, and are expressed by the following formula (1): (2 C capacity retention rate (%)) = (2 C discharge capacity) / (0.2 C discharge capacity) × 100 (1)

[0102] Examples 2 and 3 Examples 2 and 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 determined.

[0103] Examples 4 to 7 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 determined.

[0104] Examples 8 and 9 Examples 8 and 9 differ from Example 1 in that the thickness of the coating layer was changed. ２ The other conditions were the same as in Example 1, and the capacity retention rate after 300 cycles was determined.

[0105] Examples 10 and 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 silicon particles and carbon source when producing the composite particles. The other conditions were the same as in Example 1, and the capacity retention rate after 300 cycles was determined.

[0106] "Examples 12 to 14" In Examples 12 to 14, the compound to be attached to the surface of the composite particles was MgF ２ Example 12 differs from Example 1 in that the compound was changed to another compound and subjected to thermal plasma treatment. ２ In Example 13, LiF was used instead of MgF. ２ Change to Mg ２ In Example 14, MgF ２ Change to Mg ３ (P.O. ４ ) ２ The other conditions were the same as in Example 1, and the capacity retention rate after 300 cycles was determined.

[0107] Example 15 Example 15 differs from Example 1 in that carbon was attached to the surface of the coating layer. The carbon was attached 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 determined.

[0108] "Comparative Example 1" In Comparative Example 1, after preparing the composite particles, MgF ２ The difference from Example 1 is that no composite particles were attached to the surface. That is, the negative electrode active material of Comparative Example 1 consisted of only composite particles. The other conditions were the same as in Example 1, and the capacity retention rate after 300 cycles was determined.

[0109] Comparative Examples 2 and 3 Comparative Examples 2 and 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 determined.

[0110] Comparative Examples 4 and 5 Comparative Examples 4 and 5 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 determined.

[0111] The results of Examples 1 to 15 and Comparative Examples 1 to 5 are summarized in the following table. In addition to the intentionally changed parameters, the negative electrode active materials of Examples 2 to 15 and Comparative Examples 1 to 5 were also affected by variations in manufacturing conditions, and some of the parameters deviated from those of the negative electrode active material of Example 1.

[0112]

[0113] Examples 1 to 15 had higher capacity retention rates and better cycle characteristics than Comparative Examples 1 to 4. Comparative Example 5 had a thick silicon carbide layer, which resulted in a decrease in rate characteristics.

[0114] REFERENCE SIGNS LIST 1 negative electrode material 2 composite particle 3 silicon carbide layer 4 coating layer 10 separator 20 positive electrode 22 positive electrode current collector 24 positive electrode active material layer 30 negative electrode 32 negative electrode current collector 34 negative electrode active material layer 40 power generating element 50 exterior body 52 metal foil 54 resin layer 60, 62 terminal 100 lithium ion secondary battery

Claims

1. A negative electrode material for a lithium ion secondary battery comprising composite particles, a silicon carbide layer, and a coating layer, wherein the composite particles contain amorphous carbonaceous particles and amorphous silicon particles having an average primary particle diameter of 1 nm or more and 50 nm or less, the silicon carbide layer is 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, and the coating layer contains a compound of magnesium or fluorine.

2. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the total 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 the silicon carbide layer has a thickness of 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 the 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 the average secondary particle diameter of the composite particles 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 the surface of the coating layer is coated with carbon or carbon nanotubes.

7. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the magnesium or fluorine compound is at least one 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, comprising the negative electrode material for a lithium ion secondary battery according to claim 1.

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

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