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

Abstract

This negative electrode material for lithium ion secondary batteries contains silicon particles. The silicon particles have an average particle diameter of 1 [mu] m to 10 [mu] m. The silicon particles have an inner region and a surface region. The surface region contains amorphous silicon or silicon having a crystallite size of 200 nm or less. The inner region contains silicon having a crystallite size of more than 200 nm.

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 also 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 mainly depends on 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 required. 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. When the negative electrode active material undergoes volume expansion, for example, the negative electrode active material is broken, or the conductive path between the negative electrode active materials is cut off, or peeling occurs at the interface between the negative electrode active material layer and the current collector, or cracks occur in the SEI (Solid Electrolyte Interphase) coating film and decomposition of the electrolyte occurs. These all decrease 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] Technical Problem 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 disclosure 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 Technical Problem

[0013] In order to solve the above problems, the following means is provided.

[0014] The negative electrode material for a lithium-ion secondary battery of the first aspect contains silicon particles. The average particle diameter of the silicon particles is 1 μm or more and 10 μm or less. The silicon particles have an inner region and a surface region. The surface region contains amorphous silicon or silicon having a crystallite size of 200 nm or less. The inner region contains silicon having a crystallite size of more than 200 nm.

[0015] Effects of Invention

[0016] The lithium-ion secondary battery using the negative electrode material for a lithium-ion secondary battery of the above-described aspect has excellent cycle characteristics. BRIEF DESCRIPTION OF DRAWINGS

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

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

[0019] Hereinafter, the embodiments will be described in detail with appropriate reference to the drawings. In the following description, the drawings used in the description are sometimes shown with portions enlarged for the sake of easy understanding of the features, and sometimes the size ratio and the like of each constituent element are different from the actual ones. The materials, dimensions, and the like exemplified in the following description are examples, and the present disclosure is not limited to these, and can be appropriately changed and implemented within the scope of the gist thereof.

[0020] "Negative electrode 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 is a TEM image of a cross section of the negative electrode material of the first embodiment measured using a transmission electron microscope (TEM). The silicon particle 1 has an inner region 2 and a surface region 3.

[0023] The inner region 2 is a region located more inward than the surface region 3. The inner region 2 contains a single crystal of silicon. The inner region 2 can also be a polycrystal formed of a collection of single crystals.

[0024] In the case where the inner region 2 is a polycrystal, a grain boundary 4 is confirmed within the inner region 2. When the grain boundary 4 is present within the inner region 2, lithium ions diffuse along the grain boundary, and thus the lithium ions easily reach the inside of the silicon particle 1. By the lithium ions reaching the inside of the silicon particle 1 uniformly, stress locally concentrated due to expansion and contraction of the silicon particle 1 can be prevented, and the silicon particle 1 can be prevented from being broken at the time of charge and discharge of the lithium-ion secondary battery.

[0025] The internal region 2 contains silicon having a crystallite size of 200 nm or more. The crystallite size of the crystal constituting the internal region 2 is, for example, 200 nm or more and 2000 nm or less, preferably more than 200 nm and 1500 nm or less, and more preferably 300 nm or more and 1000 nm or less. The internal region 2 undertakes the main task of absorbing and releasing lithium ions in the silicon particle 1. The larger the crystallite size of the crystal constituting the internal region 2, the smoother the charge and discharge of lithium ions. The crystallite size can be confirmed from a TEM image.

[0026] The internal region 2 mainly contains silicon. The internal region 2 can also contain SiO x The silicon oxide described above. x satisfies, for example, 0.8≤x≤2. The internal region 2 can also contain, for example, silicon-carbon composite material (Si-C).

[0027] The surface region 3 is formed on the surface of the silicon particle 1. The surface region 3 is a region on the outside of a closed space drawn by connecting a position at a length of 75% from the geometric center of the line segment connecting the geometric center and the outer surface of the silicon particle 1. The geometric center is a point where the major axis and the minor axis intersect in the TEM cross section of the silicon particle 1.

[0028] The surface region 3 contains amorphous silicon or silicon having a crystallite size of 200 nm or less. The surface region 3 contains, for example, silicon having a crystallite size of 5 nm or more and 200 nm or less.

[0029] The crystallite size of the crystal constituting the internal region 2 is large, and the path into which lithium ions can intrude is limited. In contrast, when the surface region 3 has amorphous or small crystallite size, lithium ions can intrude into the inside of the silicon particle 1 from various directions. This is because amorphous silicon has no crystal orientation, and silicon having a small crystallite size is oriented in different directions. By covering the surface of the internal region 2 with the surface region 3, the diffusion of lithium ions within the silicon particle 1 becomes uniform, and the volume change of the silicon particle 1 can be suppressed.

[0030] The surface region 3 can also have at least one metal element selected from the group consisting of Mg, Al, Ca, Ti, V, Cr, Mo, Mn, Fe, Co, Ni, Cu, Zn. These metal elements can exist as a single body, or can exist in a form of being combined with other elements. For example, the metal element can also exist as a silicide. For example, FeSi2, FeSi, Fe3Si, CrSi2, NiSi2, MoSi2, VS2, Mg2Si, TiSi2 are examples of silicides.

[0031] These metal elements deform the crystal structure of silicon. When the crystal structure of silicon is deformed, the invasion path of lithium ions cannot be fixed in a prescribed direction, the diffusion of lithium ions within the silicon particle 1 becomes uniform, and the volume change of the silicon particle 1 locally can be suppressed. It is preferable that these metal elements exist at a position at which silicon is in contact.

[0032] The molar ratio of these metal elements in the surface region 3 is, for example, 1 mol% or more and 40 mol% or less, and is preferably 3 mol% or more and 30 mol% or less. The molar ratio is considered with the total of silicon and metal elements being 100. When a sufficient amount of metal elements exists in the surface region 3, the silicon in the surface region 3 easily becomes amorphous or polycrystalline. As a result, the diffusion of lithium ions within the silicon particle 1 becomes uniform, and the volume change of the silicon particle 1 locally can be suppressed. In addition, when the amount of metal elements in the surface region 3 is set to be equal to or less than a prescribed value, the charge and discharge capacity of the lithium-ion secondary battery becomes high. This is because the presence ratio of silicon that is active in the battery reaction in the surface region 3 becomes higher.

[0033] The thickness of the surface region 3 is, for example, 10 nm or more and 500 nm or less, and is preferably 30 nm or more and 300 nm or less. The thickness of the surface region 3 is the thickness in the radial direction of the silicon particle 1. The thickness of the surface region 3 is the average of the thicknesses of four measurement points of the cross-sectional TEM image of the silicon particle 1 obtained by a transmission electron microscope. The four measurement points are an arbitrarily selected first measurement point, a second measurement point at a position rotated by 90° from the first measurement point with the geometric center as a reference, a third measurement point at a position rotated by 180° from the first measurement point with the geometric center as a reference, and a fourth measurement point at a position rotated by 270° from the first measurement point with the geometric center as a reference.

[0034] The average particle diameter of the silicon particle is 1 μm or more and 10 μm or less, and is preferably 1 μm or more and 8 μm or less, and more preferably 1 μm or more and 7 μm or less. If the average particle diameter of the silicon particle is in the above range, the cycle characteristics are improved. When the silicon particle is too small, the contact area of the silicon particle and the electrolyte solution increases, and the risk of side reactions such as decomposition of the electrolyte solution becomes high. In addition, when the silicon particle is too large, the risk of side reactions such as decomposition of the electrolyte solution at the new surface generated due to breakage of the silicon particle due to expansion and contraction becomes high.

[0035] In the case where the silicon particle can be obtained in a particle state, the median particle diameter (D50) can be obtained as the average 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, for example, the average of the particle diameters of 50,000 particles is obtained.

[0036] In a case where the silicon particles are present in the electrode and separation of the silicon particles is difficult, the average particle diameter can be obtained using at least 100 silicon particles identified in the cross-sectional image. The average particle diameter measured using the particle size distribution measuring device and the average particle diameter obtained from the cross-sectional image do not greatly deviate and are approximately consistent.

[0037] First, a threshold value of contrast is set, and the silicon particles (negative electrode material) are extracted from the image. Then, the diameters of at least 100 extracted silicon particles are each obtained. The frequency of the obtained diameters of the respective silicon particles is graphed, and the mode is set as the average particle diameter. In a case where the shape of the silicon particles is amorphous, the diameter of the long axis is used for the calculation of the average particle diameter.

[0038] The average circularity of the silicon particles is 0.80 or more and 0.99 or less, preferably 0.910 or more and 0.988 or less, and more preferably 0.920 or more and 0.985 or less.

[0039] The average circularity of the silicon particles is obtained using the circularities of at least 100 or more silicon particles. The circularity is obtained by dividing the circumference length of a circle having the same area as the silicon particle to be measured by the surrounding length of the silicon particle to be measured.

[0040] As with the average particle diameter, in a case where the silicon particles can be obtained in a particle state, the average circularity can be obtained using the particle size distribution measuring device. In a case where the particle size distribution measuring device is used, for example, the mode of the circularities of 50,000 particles is obtained. In addition, in a case where separation of the silicon particles is difficult, the average circularity can be obtained using the cross-sectional image. In a case where the cross-sectional image is used, for example, the respective circularities of 100 particles are obtained. Specifically, the silicon particles are extracted from the image, and the surrounding length and the area of each silicon particle are obtained. In addition, the circumference length of a circle having the same area as the obtained area of each silicon particle is calculated, and the circularity of each silicon particle is calculated. Then, the mode of the circularities of the silicon particles is set as the average circularity. The average circularity measured using the particle size distribution measuring device and the average circularity obtained from the cross-sectional image do not greatly deviate and are approximately consistent.

[0041] The average aspect ratio of the silicon particles is 0.60 or more and 0.99 or less, preferably 0.65 or more and 0.98 or less, and more preferably 0.80 or more and 0.97 or less.

[0042] The average aspect ratio of the silicon particles is obtained using the aspect ratios of at least 100 or more silicon particles. The aspect ratio is obtained by dividing the short axis length of the silicon particle to be measured by the long axis length.

[0043] The average aspect ratio can be obtained using a particle size distribution measuring device in the case where the silicon particles are obtained in a state of particles, and can be obtained using a cross-sectional image in the case where the silicon particles are difficult to separate. In the case of using the particle size distribution measuring device, for example, the mode of the aspect ratio of 50,000 particles is obtained, and in the case of using the cross-sectional image, for example, the mode of the aspect ratio of 100 particles is obtained. The average aspect ratio measured using the particle size distribution measuring device and the average aspect ratio obtained from the cross-sectional image do not greatly deviate and are substantially consistent.

[0044] Here, the silicon particles present in the electrode after the charge and discharge of the lithium ion secondary battery and the untreated silicon particles that can be obtained in a state of particles do not necessarily have consistent average particle diameters, average aspect ratios, and average circularities. However, if the average particle diameters, average aspect ratios, and average circularities obtained by any of the methods satisfy the above ranges, the cycle characteristics of the lithium ion secondary battery are improved in the subsequent charge and discharge.

[0045] The negative electrode material of the first embodiment can be produced by performing a core production process and a surface region production process.

[0046] In the core production process, the core of the silicon particle 1 corresponding to the inner region 2 is produced. The core can be produced, for example, by melting silicon and then solidifying again. If the silicon is once melted, the silicon can be made spherical by the surface tension. The melting of the silicon can be performed, for example, using an atomization method or a thermal plasma method. The shape of the core (for example, the average particle diameter, the average circularity, and the average aspect ratio) varies depending on the manufacturing conditions. The manufacturing conditions somewhat fluctuate in each manufacturing device, and thus it is preferable to perform optimization of the manufacturing conditions by preliminary research and then determine the actual manufacturing conditions.

[0047] The production conditions of the core are, for example, as follows. In the case of using the thermal plasma method, as a raw material, silicon having an average particle diameter of 1 μm or more and 8 μm or less is melted. The average particle diameter of the raw material is one of the parameters that affect the particle diameter of the core. In addition, as the parameters that affect the shape of the core, there are a melting temperature, a melting time, a cooling temperature, a cooling rate, and the like.

[0048] The melting temperature of the core is, for example, set to 1,200°C or more and 12,000°C or less. The melting time of the core is, for example, set to 1 s or more and 300 s or less. The cooling temperature at the time of solidifying the core is, for example, set to 15°C or more and 800°C or less. The cooling rate at the time of solidifying the core is, for example, set to 5°C / s or more and 10,000°C / s or less. When the cooling rate is fast, the degree of crystallization of the particles decreases, the diffusion of lithium at the time of charge and discharge becomes uniform, and the cycle characteristics of the lithium ion secondary battery are improved. The cooling rate is preferably 1,000°C / s or more. In addition, the atmosphere at the time of melting and cooling the silicon is preferably an inert atmosphere such as Ar or nitrogen.

[0049] In addition, in a case where a nozzle is used to introduce molten silicon into the cooling space, the diameter, shape, length of the nozzle, and the flow rate of the molten silicon to the nozzle are designed. These also affect the shape of the nucleus. The diameter, shape, length of the nozzle, and the flow rate of the molten silicon to the nozzle are studied in advance, and conditions suitable for the device are determined.

[0050] Next, in the surface region production process, amorphous silicon or silicon having a crystallite size of 200 nm or less is attached to the surface of the nucleus. In the surface region production process, a metal element can also be attached to the surface of the nucleus. After the fine silicon is attached to the surface of the nucleus, a thermal plasma treatment is performed. In the thermal plasma treatment, heat is applied instantaneously, and the fine silicon adheres to the surface of the nucleus. By changing the amount of fine silicon attached, the thickness of the surface region 3 can be adjusted. In addition, by changing the treatment conditions such as thermal plasma, the crystallite size of the silicon in the surface region 3 can be changed. In addition, by attaching a metal element to the surface of the nucleus, the crystallinity of the silicon in the surface region 3 can be changed.

[0051] 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 prescribed silicon particle 1. The silicon particle 1 has fine silicon on the surface. The fine silicon makes the diffusion of lithium ions in the silicon particle 1 uniform, and suppresses the local volume change of the silicon particle 1. In addition, the silicon having a large crystallite size in the inner region 2 of the silicon particle 1 lowers the reaction potential at the time of discharge of the lithium ion secondary battery, and increases the discharge capacity of the lithium ion secondary battery.

[0052] "Lithium Ion Secondary Battery"

[0053] Figure 2 is a schematic view of the lithium ion secondary battery of the first embodiment. Figure 2 The lithium ion secondary battery 100 shown has a power generating element 40, an exterior body 50, and an electrolyte (for example, a nonaqueous electrolyte). The exterior body 50 covers around the power generating element 40. The power generating element 40 is connected to the outside by a pair of terminals 60, 62 connected to the power generating element 40. The nonaqueous electrolyte is housed inside the exterior body 50. InIn the lithium ion secondary battery 100 shown, a case where one power generating element 40 is inside the exterior body 50 is exemplified, but a plurality of power generating elements 40 can also be laminated. Figure 2

[0054] (Power Generating Element)

[0055] 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 of these, or a roll of a structure obtained by rolling these.

[0056] <Positive Electrode>

[0057] 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 face of the positive electrode current collector 22.

[0058] [Positive electrode current collector]

[0059] 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.

[0060] [Positive electrode active material layer]

[0061] 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.

[0062] The positive electrode active material contains an electrode active material capable of reversibly performing adsorption and release of lithium ions, detachment and insertion (intercalation) of lithium ions, or doping and dedoping of lithium ions and counter anions.

[0063] 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 manganite (LiMn2O4), and a compound of the general formula: LiNi x Co y Mn z M a O2 (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, and Cr), lithium vanadium compound (LiV2O5), olivine-type LiMPO4 (where M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr or VO), 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, or polyparaphenylene.

[0064] 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's 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 in combination with a plurality of materials. In the case where the positive electrode active material is a material that does not contain lithium, for example, discharging is first performed. By discharging, lithium is intercalated into the positive electrode active material. In addition to this, the material that does not contain lithium can also be chemically or electrochemically pre-doped with lithium.

[0065] 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.

[0066] The content rate of the conductive assistant in the positive electrode active material layer 24 is not particularly limited. For example, the content rate 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.

[0067] The binder in the positive electrode active material layer 24 binds the positive electrode active materials to each other. The binder can use a publicly known material. The binder is preferably a binder that is not dissolved in an electrolyte, has oxidation resistance, and has adhesiveness. 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) to which maleic anhydride is grafted, a mixture thereof. The binder used for the positive electrode active material layer is particularly preferably PVDF.

[0068] The content rate of the binder in the positive electrode active material layer 24 is not particularly limited. For example, the content rate of the binder with respect to the total mass of the positive electrode active material, the conductive assistant, 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. When the content rate of the binder is small, the adhesion strength of the positive electrode 20 becomes weak. When the content rate of the binder is high, the binder is electrochemically inert and does not contribute to the discharge capacity, and thus the energy density of the lithium ion secondary battery 100 becomes low.

[0069] <NEGATIVE ELECTRODE>

[0070] 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 face of the negative electrode current collector 32.

[0071] [Negative electrode current collector]

[0072] The negative electrode current collector 32 is, for example, a conductive sheet. The same material as the positive electrode current collector 22 can be used for the negative electrode current collector 32.

[0073] [Negative electrode active material layer]

[0074] 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, as necessary, a conductive aid, a dispersion stabilizer, and the like. The negative electrode active material is 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.

[0075] The same materials as the positive electrode 20 can be used for the conductive aid and the binder. The binder in the negative electrode 30 can be, in addition to the materials described above for the positive electrode 20, for example, cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamide-imide resin, acrylic resin, and the like. The cellulose can be, for example, carboxymethyl cellulose (CMC).

[0076] <Separator>

[0077] 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 is spread in the plane of the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

[0078] The separator 10 has, for example, a porous structure that is electrically insulating. The separator 10 is, for example, a single layer of a polyolefin film, a laminate. The separator 10 can also be an extended film of a mixture of polyethylene or polypropylene, or the like. The separator 10 can also be a fiber nonwoven fabric made of a constituent material selected from at least one of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene, and polypropylene. The separator 10 can be, for example, a solid electrolyte. The solid electrolyte is, for example, a polymer solid electrolyte, an oxide-based solid electrolyte, a sulfide-based solid electrolyte. The separator 10 can also be an inorganic coating separator. The inorganic coating separator is a film on which a mixture of a resin such as PVDF or CMC and an inorganic substance such as alumina or silica is coated on the surface of the film described above. The inorganic coating separator is excellent in heat resistance, and inhibits the deposition of transition metals eluted from the positive electrode onto the surface of the negative electrode.

[0079] <Electrolyte solution>

[0080] The electrolyte is enclosed within the outer casing 50 and impregnates the power generating element 40. The electrolyte is not limited to a liquid electrolyte; a solid electrolyte may also be used. For example, a non-aqueous electrolyte comprises a non-aqueous solvent and an electrolytic salt. The electrolytic salt is dissolved in the non-aqueous solvent.

[0081] The solvent is not particularly limited as long as it is a solvent commonly used in lithium ion secondary batteries. The solvent, for example, contains any one of a cyclic carbonate compound, a chain carbonate compound, a cyclic ester compound, and a chain ester compound. The solvent may also be mixed in any proportion and contain these. Examples of cyclic carbonate compounds include ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate, and vinylene carbonate. Examples of chain carbonate compounds include diethyl carbonate (DEC) and ethyl methyl carbonate (EMC). Examples of cyclic ester compounds include gamma-butyrolactone and the like. Examples of chain ester compounds include propyl propionate, ethyl propionate, and ethyl acetate.

[0082] 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 degree of dissociation of the electrolytic salt in a carbonate solvent at room temperature is preferably 10% or greater.

[0083] The electrolyte is preferably an electrolyte in which LiPF6 is dissolved 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.

[0084] <Exterior body>

[0085] The outer casing 50 seals the power generation element 40 and the non-aqueous electrolyte therein, thereby preventing the non-aqueous electrolyte from leaking to the outside and preventing moisture from entering the lithium ion secondary battery 100 from the outside.

[0086] For example, Figure 1 As shown, the exterior body 50 includes a metal foil 52 and a resin layer 54 laminated on each surface of the metal foil 52. The exterior body 50 is a metal laminate film in which the metal foil 52 is coated from both sides with a polymer film (resin layer 54).

[0087] As the metal foil 52, for example, an aluminum foil can be used. The resin layer 54 can utilize a high molecular film of polypropylene or the like. The material constituting the resin layer 54 can also be different between the inside and the outside. For example, as the material of the outside, a high molecular having a high melting point, such as polyethylene terephthalate (PET), polyamide (PA), or the like, can be used, and as the material of the high molecular film of the inside, polyethylene (PE), polypropylene (PP), or the like can be used.

[0088] < Terminals >

[0089] 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 undertake electrical connection of 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. In order to prevent short circuiting, the terminals 60, 62 are preferably protected by an insulating tape.

[0090]

[0091] The lithium ion secondary battery 100 is produced by preparing the negative electrode 30, the positive electrode 20, the separator 10, the electrolyte, and the exterior body 50, respectively, and assembling them. Hereinafter, an example of the manufacturing method of the lithium ion secondary battery 100 will be described.

[0092] The negative electrode 30 is produced, for example, by sequentially performing a slurry production step, an electrode coating step, a drying step, and a calendering step.

[0093] The slurry production step is a step of producing a slurry by mixing a negative electrode active material, a binder, a conductive aid, and a solvent. The negative electrode active material uses the negative electrode material described above. When a dispersing stabilizer is added to the slurry, agglomeration of the negative electrode active material can be suppressed.

[0094] The slurry production step is a step of producing 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 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% in terms of mass ratio. The mass ratio of these is adjusted so that the total becomes 100 wt%. The container used when producing the slurry is preferably a metal container such as SUS. In the case where the solvent is a polar solvent such as N-methyl-2-pyrrolidone, the electrostatic capacity of the oxidized coating film on the surface of the silicon particles becomes large. The polar solvent prevents the repulsive force of the conductive aid from the silicon particles. By suppressing these repulsive forces, it is possible to prevent a decrease in the capacity of the lithium ion secondary battery.

[0095] ​The negative electrode active material can also be a material in which the active material particles and the conductive material are mixed while applying a cutting force to one side. If the cutting force is applied and the mixing is performed to such a degree that the active material particles are not deteriorated, 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 by the degree of mixing. In addition, the negative electrode active material after production can be sieved to make the particle diameters uniform.

[0096] 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. For example, a slit die coating method, a doctor blade method can be used as the method of coating the slurry. The slurry is coated, for example, at room temperature.

[0097] The drying step is a step of removing the solvent from the slurry. For example, the negative electrode current collector 32 on which the slurry is coated is dried in an atmosphere of 80°C or higher and 350°C or lower.

[0098] 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 using, for example, a roll press device or the like.

[0099] The positive electrode 20 can be produced in the same order as the negative electrode 30. The separator 10 and the exterior body 50 can be used as commercially available products.

[0100] 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 wound body, the positive electrode 20, the negative electrode 30, and the separator 10 are wound with one end side thereof as an axis.

[0101] Finally, the power generating element 40 is enclosed in the exterior body 50. A non-aqueous electrolyte is injected into the exterior body 50. By performing pressure reduction, heating, or the like after the non-aqueous electrolyte is injected, the non-aqueous electrolyte is impregnated in the power generating element 40. The exterior body 50 is sealed by applying heat or the like, and thus the lithium ion secondary battery 100 is obtained. Alternatively, the power generating element 40 can be impregnated in the electrolyte without injecting the electrolyte into the exterior body 50. It is preferable to stand for 24 hours after the electrolyte is injected into the power generating element.

[0102] 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 having a prescribed shape.

[0103] The embodiments of the present application have been described in detail with reference to the drawings, but the structures of the respective embodiments and combinations thereof are examples, and the addition, omission, substitution, and other changes of the structures can be made within the scope of the gist of the present application.

[0104] Example

[0105] "Example 1"

[0106] An anode slurry was applied to one side of an aluminum foil having a thickness of 15 μm. The anode slurry was prepared by mixing an anode active material, a conductive aid, a binder, and a solvent.

[0107] The anode active material used Li x CoO2. The conductive aid used acetylene black. The binder used polyvinylidene fluoride (PVDF). The solvent used N-methyl-2-pyrrolidone. The anode slurry was prepared by mixing 97 parts by mass of the anode 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 anode active material in the anode active material layer after drying was set to 25 mg / cm 2 . The solvent was removed from the anode slurry in a drying oven, and the anode active material layer was prepared. The anode active material layer was pressed by rolling to prepare the anode.

[0108] Next, an anode active material to be added to the anode slurry was prepared. First, as the core, a silicon particle having an average particle diameter of 5.1 μm, an average circularity of 0.949, and an average aspect ratio of 0.90 was used. The average particle diameter, the average circularity, and the average aspect ratio were obtained by measuring 50,000 particles using a particle size distribution meter manufactured by Malvern Panalytical, Inc. Next, silicon having a crystallite size of 21 nm was attached to the surface of the core. The attachment of the silicon having a small crystallite size to the surface of the core was performed using a thermal plasma method.

[0109] The cross section of the prepared anode active material was confirmed by TEM. The anode active material had an inner region and a surface region. The average particle diameter, the average circularity, and the average aspect ratio of the anode active material were substantially the same as those of the core. The thickness of the surface region was 459 nm.

[0110] Next, an anode slurry was prepared using the anode active material. The conductive aid used carbon black. The binder used a polyimide resin. The solvent used N-methyl-2-pyrrolidone. The anode slurry was prepared by mixing 90 parts by mass of the anode active material, 5 parts by mass of the conductive aid, and 5 parts by mass of the binder with N-methyl-2-pyrrolidone.

[0111] Then, the anode slurry was applied to one side of a copper foil having a thickness of 10 μm and was dried. The loading amount of the anode active material in the anode active material layer after drying was set to 2.5 mg / cm 2 . The anode active material layer was pressed by rolling and was fired at 300°C or higher for 5 hours in a nitrogen atmosphere.

[0112] Next, an electrolyte was prepared. The solvent of the electrolyte 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. LiPF6was used as the electrolyte salt. The concentration of LiPF6was set to 1 mol / L.

[0113] (Production of lithium ion secondary battery for evaluation)

[0114] The prepared negative electrode and positive electrode were stacked with the positive electrode active material layer and the negative electrode active material layer facing each other with a separator (porous polyethylene sheet) interposed therebetween, to obtain a laminate. The laminate was inserted into an outer packaging body of an aluminum laminated film, and the periphery was heat-sealed except for one portion, to form a seal portion. Furthermore, after the electrolyte described above was injected into the outer packaging body, the remaining one portion was sealed by heat-sealing while reducing the pressure using a vacuum sealing machine, to produce a lithium ion secondary battery. The produced lithium ion secondary battery was left to stand for 24 hours.

[0115] (Determination of capacity retention rate after 300 cycles)

[0116] The cycle characteristics of the lithium ion secondary battery were determined. The cycle characteristics were determined using a secondary battery charge-discharge testing device (manufactured by Hokuto Denko Corporation).

[0117] Charging was performed by constant current charging at a charge rate of 1C (a current value at which charging ends in 1 hour when constant current charging is performed at 25°C) until the battery voltage became 4.2 V, and discharging was performed by constant current discharging at a discharge rate of 1.0C until the battery voltage became 2.5 V. The discharge capacity after the charging and discharging ended was detected, and the battery capacity Q1 before the cycle test was determined. The battery capacity Q1 was 3684 mAh / g.

[0118] The battery from which the battery capacity Q1 was determined above was again used in the secondary battery charge-discharge testing device, and charging was performed by constant current charging at a charge rate of 1C until the battery voltage became 4.2 V, and discharging was performed by constant current discharging at a discharge rate of 1C until the battery voltage became 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 300 cycles of charging and discharging ended was detected, and the battery capacity Q2 after 300 cycles was determined. The capacity retention rate E after 300 cycles was determined from the battery capacities Q1, Q2 determined above. The capacity retention rate E was determined by E = Q2 / Q1 x 100. The capacity retention rate of Example 1 was 82%.

[0119] “Examples 2 to 4”

[0120] In Examples 2 to 4, the average particle diameter of the negative electrode active material was changed, unlike in Example 1. The average particle diameter of the negative electrode active material was adjusted by changing the size of the core at the time of producing the negative electrode active material. The other conditions were the same as in Example 1, and the capacity retention rate after 300 cycles was obtained.

[0121] "Examples 5, 6"

[0122] In Examples 5 and 6, the crystallite size of the internal region of the negative electrode active material was changed, unlike in Example 1. The crystallite size of the internal region of the negative electrode active material was changed by changing the heat treatment conditions at the time of producing the core. The other conditions were the same as in Example 1, and the capacity retention rate after 300 cycles was obtained.

[0123] "Examples 7 to 9"

[0124] In Examples 7 to 9, the state of the crystal of the surface region of the negative electrode active material was changed, unlike in Example 1. The crystallite size of the surface region of the negative electrode active material was changed by changing the conditions of the heat plasma treatment after the fine silicon was attached to the core. In Example 9, amorphous silicon was provided. The other conditions were the same as in Example 1, and the capacity retention rate after 300 cycles was obtained.

[0125] "Examples 10 to 22"

[0126] In Examples 10 to 22, a metal element was added to the surface region of the negative electrode active material, unlike in Example 1. The metal element was attached together with the silicon having a small crystallite size that was attached to the surface of the core. The other conditions were the same as in Example 1, and the capacity retention rate after 300 cycles was obtained.

[0127] "Examples 23 to 26"

[0128] In Examples 23 to 26, the molar ratio of the metal element in the surface region of the negative electrode active material was changed, unlike in Example 11. The other conditions were the same as in Example 11, and the capacity retention rate after 300 cycles was obtained.

[0129] "Examples 27 to 30"

[0130] In Examples 27 to 30, the thickness of the layer containing the silicon having a small crystallite size formed on the surface of the negative electrode active material was changed, unlike in Example 1. The thickness of the layer was changed by changing the amount of the silicon having a small crystallite size attached to the core at the time of producing the negative electrode active material. The other conditions were the same as in Example 1, and the capacity retention rate after 300 cycles was obtained.

[0131] "Examples 31 to 34"

[0132] In Examples 31 to 34, the average circularity and the average aspect ratio of the negative electrode active material were changed, unlike in Example 1. The ratio can be adjusted by changing the heat treatment conditions at the time of producing the core. The other conditions were the same as in Example 1, and the capacity retention rate after 300 cycles was obtained.

[0133] "Comparative Example 1"

[0134] In Comparative Example 1, the silicon having a small crystallite size was not adhered to the surface after the core was produced, unlike in Example 1. That is, the negative electrode active material of Comparative Example 1 was composed only of the core. The other conditions were the same as in Example 1, and the capacity retention rate after 300 cycles was obtained.

[0135] "Comparative Example 2"

[0136] In Comparative Example 2, the crystallite size of the surface region of the negative electrode active material was changed, unlike in Example 1. The other conditions were the same as in Example 1, and the capacity retention rate after 300 cycles was obtained.

[0137] "Comparative Example 3"

[0138] In Comparative Example 3, the crystallite size of the internal region of the negative electrode active material was changed, unlike in Example 1. The other conditions were the same as in Example 1, and the capacity retention rate after 300 cycles was obtained.

[0139] The results of Examples 1 to 34 and Comparative Examples 1 to 3 are summarized in the following table. In the negative electrode active materials of Examples 2 to 34 and Comparative Examples 1 to 3, in addition to the intentionally changed parameters, there were cases in which various parameters deviated from those of the negative electrode active material of Example 1 due to the influence of differences in the manufacturing conditions.

[0140]

[0141]

[0142] The capacity retention rates of Examples 1 to 34 were higher than those of Comparative Examples 1 to 3, and the cycle characteristics were excellent.

[0143] Explanation of Symbols

[0144] 1 silicon particle

[0145] 2 internal region

[0146] 3 surface region

[0147] 10 separator

[0148] 20 positive electrode

[0149] 22 positive electrode current collector

[0150] 24 positive electrode active material layer

[0151] 30 negative electrode

[0152] 32 negative electrode current collector

[0153] 34 negative electrode active material layer

[0154] 40 power generating element

[0155] 50 outer body

[0156] 52 metal foil

[0157] 54 resin layer

[0158] 60, 62 terminal

[0159] 100 lithium ion secondary battery

Claims

1. A negative electrode material for a lithium ion secondary battery, wherein, silicon particles are contained, the average particle diameter of the silicon particles is 1 μm or more and 10 μm or less, the silicon particles have an inner region and a surface region, the surface region contains amorphous silicon or silicon having a crystallite size of 200 nm or less, the inner region contains silicon having a crystallite size of more than 200 nm.

2. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein, the surface region has at least one metal element selected from the group consisting of Mg, Al, Ca, Ti, V, Cr, Mo, Mn, Fe, Co, Ni, Cu, and Zn.

3. The negative electrode material for a lithium ion secondary battery according to claim 2, wherein, in the surface region, the molar ratio of the metal element is 1 mol% or more and 40 mol% or less.

4. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein, the average circularity of the silicon particles is 0.920 or more and 0.985 or less, the average aspect ratio of the silicon particles is 0.80 or more and 0.97 or less.

5. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein, the average particle diameter of the silicon particles is 1 μm or more and 7 μm or less.

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

7. A lithium ion secondary battery, wherein, the negative electrode for a lithium ion secondary battery according to claim 6, a positive electrode, and an electrolyte are contained.

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