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 contains silicon particles. The silicon particles have an average particle size of 0.1 μm to 10 μm, inclusive. The silicon particles have an internal region and a surface region. The surface region includes a silicide. The thickness of the surface region is 10 nm to 500 nm, inclusive.

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 silicon particles. The silicon particles have an average particle diameter of 0.1 μm or more and 10 μm or less. The silicon particles have an internal region and a surface region. The surface region includes silicide. The thickness of the surface region is 10 nm or more and 500 nm or less.

[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 photograph of a negative electrode material for a lithium ion secondary battery according to Embodiment 1. FIG. 2 is a schematic diagram of a lithium ion secondary battery according to Embodiment 1.

[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 transmission electron microscope (TEM) image of a cross section of the negative electrode material according to the first embodiment. A silicon particle 1 has an inner region 2 and a surface region 3.

[0016] The internal region 2 is a region located inside the surface region 3. The internal region 2 contains silicon single crystals. The internal region 2 may also be a polycrystalline body formed by an aggregation of single crystals.

[0017] When the internal region 2 is polycrystalline, grain boundaries 4 are found within the internal region 2. If grain boundaries 4 are present within the internal region 2, lithium ions diffuse along the grain boundaries, making it easier for the lithium ions to reach the interior of the silicon particles 1. By allowing the lithium ions to reach the interior of the silicon particles 1 uniformly, it is possible to prevent stress caused by the expansion and contraction of the silicon particles 1 from concentrating locally, and it is possible to prevent damage to the silicon particles 1 during charging and discharging of the lithium ion secondary battery.

[0018] The internal region 2 contains silicon with a crystallite size of 200 nm or more. The crystallite size of the crystals 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 preferably 300 nm or more and 1000 nm or less. In the silicon particle 1, the internal region 2 is mainly responsible for absorbing and releasing lithium ions. The larger the crystallite size of the crystals constituting the internal region 2, the smoother the charging and discharging of lithium ions. The crystallite size can be confirmed from a TEM image.

[0019] The inner region 2 mainly contains silicon. The inner region 2 mainly contains SiO ｘ where x satisfies, for example, 0.8≦x≦2. The inner region 2 may include, for example, a silicon carbon composite material (Si—C).

[0020] In addition to the above materials, the internal region 2 may also contain silicide. The silicide may be, for example, FeSi ２ , FeSi, Fe ３ Si, CrSi ２ , NiSi ２ , MoSi ２ , V.S. ２ , Mg ２ Si, TiSi ２ The compound is at least one selected from the group consisting of:

[0021] The silicide contained in the internal region 2 undergoes smaller volumetric changes than silicon during charge and discharge. When the silicide is contained in the internal region 2, the silicide reduces the volumetric changes of silicon during charge and discharge, preventing damage to the silicon particles 1. Damage to the silicon particles 1 is one of the causes of deterioration in the cycle characteristics of lithium-ion secondary batteries.

[0022] When the internal region 2 contains silicide, the abundance ratio of silicon in the internal region 2 is preferably 5 to 99 times that of silicide. That is, the abundance ratio of silicon to silicide in the internal region 2 is preferably within a range of 5:1 to 99:1. The total abundance ratio of silicon to silicide is set to 100. The abundance ratio is a mass ratio.

[0023] The abundance ratio of silicon to silicide is determined by first processing the particles with an FIB to expose a cross section, then creating an elemental map using EDS analysis, EELS analysis, etc., and measuring the area of ​​each element other than silicon contained in the silicon and silicide. Taking the particle diameter into consideration, the volume ratio is calculated from the area ratio, and then the mass ratio is calculated from the molecular weights of silicon and silicide. Furthermore, the crystal structure of the silicide is identified using electron beam diffraction, and the composition formula of the compound is identified in combination with the elements obtained from the elemental map. The abundance ratio of silicon to silicide is determined using at least 100 silicon particles confirmed in the cross-sectional image.

[0024] The surface region 3 is formed on the surface of the silicon particle 1. The surface region 3 may cover the entire surface of the internal region 2, or may cover only a portion of it. The surface region 3 contains crystals having a smaller crystallite size than the internal region 2. The surface region 3 is an aggregate of crystals having a smaller crystallite size than the internal region 2. The crystallite size of the crystals contained in the surface region 3 is, for example, 5 nm or more and 200 nm or less. The surface region 3 is located within a region of the silicon particle 1 outside a closed space drawn by connecting positions at 75% of the length from the geometric center of a line segment connecting the geometric center of the silicon particle 1 to the outer surface.

[0025] The crystals constituting the internal region 2 have large crystallite sizes, limiting the paths through which lithium ions can penetrate. In contrast, the crystals contained in the surface region 3 have small crystallite sizes and are oriented in various directions, so the direction through which lithium ions can penetrate is not limited. Covering the surface of the internal region 2 with the surface region 3 allows lithium ions to diffuse uniformly, thereby suppressing local volume changes in the silicon particle 1.

[0026] The surface region 3 includes a silicide, for example, FeSi ２ , FeSi, Fe ３ Si, CrSi ２ , NiSi ２ , MoSi ２ , V.S. ２ , Mg ２ Si, TiSi ２ Preferably, the metal oxide is at least one selected from the group consisting of FeSi ２ , FeSi, Fe ３ The silicide is at least one selected from the group consisting of Si. The crystallite size of the silicide is, for example, 5 nm to 200 nm.

[0027] Silicide has better conductivity than silicon. When the surface region 3 contains silicide, electrons generated within the silicon particle 1 can be quickly transported to the outside of the silicon particle 1. In addition, the fine silicide allows lithium ions to diffuse uniformly and suppresses local volume changes in the silicon particle 1.

[0028] The surface region 3 may contain silicon in addition to silicide. When the surface region 3 contains silicide, the abundance ratio of silicon in the surface region 3 is preferably 0.01 to 5 times that of silicide. That is, the abundance ratio of silicon to silicide in the surface region 3 is preferably within a range of 1:100 to 5:1. The total abundance ratio of silicon to silicide is 100. The abundance ratio is a mass ratio.

[0029] The thickness of the surface region 3 is, for example, 10 nm or more and 500 nm or less, and 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 thicknesses at four measurement points on a cross-sectional TEM image of the silicon particle 1 obtained with a transmission electron microscope. The four measurement points are an arbitrarily selected first measurement point, a second measurement point located at a position rotated 90° from the first measurement point centered on the center, a third measurement point located at a position rotated 180° from the first measurement point centered on the center, and a fourth measurement point located at a position rotated 270° from the first measurement point centered on the center.

[0030] The average particle size of the silicon particles is 0.1 μm or more and 10 μm or less, preferably 0.5 μm or more and 8 μm or less, and more preferably 1 μm or more and 7 μm or less. If the average particle size of the silicon particles is within the above range, the cycle characteristics are improved. If the silicon particles are too small, the contact area between the silicon particles and the electrolyte increases, increasing the risk of side reactions such as decomposition of the electrolyte. If the silicon particles are too large, the risk of side reactions such as decomposition of the electrolyte increases on the new surfaces created by damage to the silicon particles due to expansion and contraction.

[0031] When silicon particles are available in particle form, the median diameter (D50) can be determined as the average particle diameter using a particle size distribution analyzer (e.g., manufactured by Malvern Panalytical). When using a particle size distribution analyzer, the average particle diameter of 50,000 particles is determined, for example.

[0032] When silicon particles are present inside the electrode and are difficult to separate, the average particle size can be determined using at least 100 silicon particles identified in the cross-sectional image. The average particle size measured using a particle size distribution analyzer and the average particle size determined from the cross-sectional image generally coincide with each other without significant deviation.

[0033] First, silicon particles (negative electrode material) are extracted from the image by setting a contrast threshold and binarizing the image. Then, the diameter of at least 100 extracted silicon particles is determined. The frequency of each determined silicon particle diameter is graphed, and the most frequent value is used as the average particle diameter. If the silicon particles are irregular in shape, the diameter of the major axis is used to calculate the average particle diameter.

[0034] 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.

[0035] The average circularity of silicon particles is determined using the circularities of at least 100 silicon particles. The circularity is determined by dividing the circumferential length of a circle having the same area as the silicon particle to be measured by the perimeter of the silicon particle to be measured.

[0036] Like the average particle diameter, the average circularity can be determined using a particle size distribution measuring device when silicon particles are available in particle form. When using a particle size distribution measuring device, for example, the mode of circularity of 50,000 particles is determined. Furthermore, when it is difficult to separate silicon particles, the average circularity can be determined using a cross-sectional image. When using a cross-sectional image, for example, the circularity of each of 100 particles is determined. Specifically, silicon particles are extracted from the image, and the perimeter and area of ​​each silicon particle are determined. Furthermore, the circumferential length of a circle with the same area as each of the determined silicon particles is calculated, and the circularity of each silicon particle is calculated. The mode of circularity of the silicon particles is then taken as the average circularity. The average circularity measured using a particle size distribution measuring device and the average circularity determined from the cross-sectional image do not deviate significantly and generally coincide.

[0037] 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.

[0038] The average aspect ratio of silicon particles is determined using the aspect ratios of at least 100 silicon particles, by dividing the minor axis length of the silicon particles to be measured by the major axis length.

[0039] Like the average particle diameter, the average aspect ratio can be determined using a particle size distribution analyzer when silicon particles are available in particle form, or using cross-sectional images when separation of silicon particles is difficult. When using a particle size distribution analyzer, the mode of the aspect ratios of, for example, 50,000 particles is determined, and when using cross-sectional images, the mode of the aspect ratios of, for example, 100 particles is determined. The average aspect ratio measured using a particle size distribution analyzer and the average aspect ratio determined from cross-sectional images do not deviate significantly and generally coincide.

[0040] Here, the silicon particles present in the electrode after charging and discharging the lithium-ion secondary battery do not necessarily have the same average particle size, average aspect ratio, and average circularity as untreated silicon particles available in particle form. However, if the average particle size, average aspect ratio, and average circularity determined by either method fall within the above ranges, the cycle characteristics of the lithium-ion secondary battery will be improved during subsequent charging and discharging.

[0041] The negative electrode material according to the first embodiment can be produced by carrying out a core production step and a surface region production step.

[0042] In the core preparation process, the core of the silicon particle 1 corresponding to the internal region 2 is prepared. The core can be prepared, for example, by melting silicon and then solidifying it again. Once the silicon is melted, it can be made spherical by surface tension. For example, atomization or thermal plasma can be used to melt the silicon. The shape of the core (e.g., average particle diameter, average circularity, and average aspect ratio) varies depending on the manufacturing conditions. Since the manufacturing conditions vary somewhat depending on the manufacturing equipment, it is preferable to optimize the manufacturing conditions in advance and then determine the actual manufacturing conditions.

[0043] The core fabrication conditions are, for example, as follows: When using the thermal plasma method, silicon with an average particle size of 1 μm or more and 8 μm or less is melted as the raw material. The average particle size of the raw material is one of the parameters that affect the particle size of the core. Parameters that affect the shape of the core include the melting temperature, melting time, cooling temperature, and cooling rate.

[0044] The melting temperature of the core is, for example, 1200°C or higher and 12000°C or lower. The melting time of the core is, for example, 1 s or higher and 300 s or lower. The cooling temperature when solidifying the core is, for example, 15°C or higher and 800°C or lower. The cooling rate when solidifying the core is, for example, 5°C / s or higher and 10000°C / s or lower. A fast cooling rate reduces the crystallinity of the particles, making it easier for lithium to diffuse uniformly during charge and discharge, improving the cycle characteristics of the lithium ion secondary battery. A cooling rate of 1000°C / s or higher is preferable. Furthermore, the atmosphere when melting and cooling the silicon is preferably an inert atmosphere such as Ar or nitrogen.

[0045] Furthermore, when using a nozzle to introduce molten silicon into the cooling space, the diameter, shape, and length of the nozzle, as well as the flow rate of molten silicon into the nozzle, must be designed. These also affect the shape of the core. The diameter, shape, and length of the nozzle, as well as the flow rate of molten silicon into the nozzle, must be studied in advance and the conditions determined to suit the equipment.

[0046] Next, in the surface region preparation step, silicide is attached to the surface of the core. In the surface region preparation step, a dissimilar metal may be attached to the surface of the core, followed by thermal plasma treatment. In the thermal plasma treatment, heat is instantaneously applied to the dissimilar metal, forming a silicide containing the dissimilar metal on the surface of the core. In the surface region preparation step, the thickness of the surface region 3 can be adjusted by changing the amount of silicide or dissimilar metal attached to the core. Furthermore, the crystallite size of the silicide can be changed by changing the treatment conditions, such as the thermal plasma treatment.

[0047] 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 predetermined silicon particles 1. The silicon particles 1 have fine silicide on their surfaces. The silicide aids in electron conduction. The silicide also makes the diffusion of lithium ions in the silicon particles 1 uniform, suppressing local volume changes in the silicon particles 1. Furthermore, the silicide suppresses excessive growth of the SEI film caused by a reaction between the electrolyte and the negative electrode material.

[0048] "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.

[0049] (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.

[0050] <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.

[0051] [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.

[0052] [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.

[0053] 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.

[0054] 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 LiMPO ４ (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.

[0055] The positive electrode active material may be a lithium-free material, such as FeF ３Examples 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] <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.

[0061] [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.

[0062] [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.

[0063] 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).

[0064] <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.

[0065] 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.

[0066] <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.

[0067] 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.

[0068] The electrolyte salt is, for example, a lithium salt. ６ , LiClO ４ , LiBF４ , 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.

[0069] 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.

[0070] <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.

[0071] 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).

[0072] 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.

[0073] <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.

[0074] "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.

[0075] 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.

[0076] 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.

[0077] The slurry preparation process is a process of mixing a negative electrode active material, a binder, a conductive additive, and a solvent to prepare a slurry. Examples of the solvent include water and N-methyl-2-pyrrolidone. The mass ratios of the negative electrode active material, conductive material, and binder are preferably 70 wt% to 100 wt%: 0 wt% to 10 wt%: 0 wt% to 20 wt%. These mass ratios are adjusted so that the total is 100 wt%. The container used for preparing the slurry is preferably made of a metal such as stainless steel. When a polar solvent such as N-methyl-2-pyrrolidone is used as the solvent, the capacitance of the oxide film on the surface of the silicon particles increases. The polar solvent prevents repulsion between the conductive additive and the silicon particles. Suppressing this repulsion can prevent a decrease in the capacity of the lithium-ion secondary battery.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] The positive electrode active material is Li ｘ CoO ２ The 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.

[0089] Next, a negative electrode active material to be added to the negative electrode slurry was prepared. First, silicon particles with an average particle diameter of 5.2 μm, an average circularity of 0.949, and an average aspect ratio of 0.90 were used as cores. The average particle diameter, average circularity, and average aspect ratio were determined by measuring 50,000 particles using a particle size distribution analyzer manufactured by Malvern Panalytical. Next, FeSi ２ FeSi ２ The crystallite size was set to 20 nm.

[0090] The cross section of the prepared negative electrode active material was observed by TEM. The negative electrode active material had an inner region and a surface region. The average particle size, average circularity, and average aspect ratio of the negative electrode active material were almost the same as those of the core. The thickness of the surface region was 120 nm.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] (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.

[0095] (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).

[0096] 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 3382 mAh / g.

[0097] The battery capacity Q １ The 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 91%.

[0098] "Examples 2 and 3" Examples 2 and 3 differ from Example 1 in that the average particle diameter of the negative electrode active material was changed. The average particle diameter of the negative electrode active material was adjusted by changing the core size when producing the negative electrode active material. In the negative electrode active materials of Examples 2 and 3, variations in parameters other than the average particle diameter also occurred due to the influence of variations in manufacturing conditions. The other conditions were the same as in Example 1, and the capacity retention rate after 300 cycles was determined.

[0099] Examples 4 to 7 Examples 4 to 7 differ from Example 1 in that the thickness of the surface region of the negative electrode active material was changed. The thickness of the surface region of the negative electrode active material was changed by changing the amount of silicide attached to the core when producing the negative electrode active material. In the negative electrode active materials of Examples 4 to 7, variations in the manufacturing conditions affected parameters other than the thickness of the surface region, and the capacity retention rate after 300 cycles was determined. The other conditions were the same as in Example 1.

[0100] Examples 8 to 16 Examples 8 to 16 differ from Example 1 in that the type of silicide constituting the surface region of the negative electrode active material was changed. The type of silicide in the surface region was changed by changing the type of silicide attached to the core when producing the negative electrode active material. In the negative electrode active materials of Examples 8 to 16, variations in the manufacturing conditions affected parameters other than the type of silicide in the surface region, and the capacity retention rate after 300 cycles was determined. The other conditions were the same as in Example 1.

[0101] Examples 17 to 20 differ from Example 1 in that the crystallite size of the silicide constituting the surface region of the negative electrode active material was changed. The crystallite size of the silicide constituting the surface region of the negative electrode active material was changed by changing the crystallite size of the silicide attached to the core when producing the negative electrode active material. In the negative electrode active materials of Examples 17 to 20, variations in the manufacturing conditions affected parameters other than the silicide crystallite size, and the capacity retention rate after 300 cycles was determined under the same conditions as in Example 1.

[0102] Examples 21 to 24 Examples 21 to 24 differ from Example 1 in that the ratio of silicon to silicide in the internal region of the negative electrode active material was changed. This ratio could be adjusted by changing the heat treatment conditions when adhering silicide to the core surface. In the negative electrode active materials of Examples 21 to 24, variations in the manufacturing conditions affected parameters other than the ratio of silicon to silicide in the internal region. The other conditions were the same as in Example 1, and the capacity retention rate after 300 cycles was determined.

[0103] Examples 25 to 28 Examples 25 to 28 differ from Example 1 in that the ratio of silicon to silicide in the surface region of the negative electrode active material was changed. This ratio could be adjusted by changing the mixture ratio of silicide and silicon attached to the core surface. In the negative electrode active materials of Examples 25 to 28, variations in the manufacturing conditions affected parameters other than the ratio of silicon to silicide in the surface region, and the capacity retention rate after 300 cycles was determined. The other conditions were the same as in Example 1.

[0104] Examples 29 to 32 Examples 29 to 32 differ from Example 1 in that the average circularity and average aspect ratio of the negative electrode active material were changed. This ratio can be adjusted by changing the heat treatment conditions when producing the core. In the negative electrode active materials of Examples 29 to 32, variations in parameters other than shape also occurred due to the influence of variations in manufacturing conditions. The other conditions were the same as in Example 1, and the capacity retention rate after 300 cycles was determined.

[0105] Comparative Example 1 Comparative Example 1 differs from Example 1 in that after the core was produced, silicide was not attached to the surface. That is, the negative electrode active material of Comparative Example 1 consisted only of the core. In the negative electrode active material of Comparative Example 1, variations in the manufacturing conditions caused variations in various parameters. The other conditions were the same as in Example 1, and the capacity retention rate after 300 cycles was determined.

[0106] The results of Examples 1 to 32 and Comparative Example 1 are summarized in the table below.

[0107]

[0108] In comparison with Comparative Example 1, Examples 1 to 32 had a higher capacity retention rate and better cycle characteristics.

[0109] REFERENCE SIGNS LIST 1 silicon particle 2 internal region 3 surface region 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 silicon particles, the silicon particles having an average particle diameter of 0.1 μm or more and 10 μm or less, the silicon particles having an internal region and a surface region, the surface region including silicide, and the thickness of the surface region being 10 nm or more and 500 nm or less.

2. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the thickness of the surface region is 30 nm or more and 300 nm or less.

3. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the internal region has a grain boundary.

4. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the internal region contains silicide, and the amount of silicon in the internal region is 5 to 99 times the amount of the silicide.

5. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the surface region contains silicon, and the amount of silicon in the surface region is 0.01 to 5 times that of the silicide.

6. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the crystallite size of the silicide is 5 nm or more and 200 nm or less.

7. The negative electrode material for lithium ion secondary batteries according to claim 1, wherein the silicon particles have an average circularity of 0.920 or more and 0.985 or less, and an average aspect ratio of 0.80 or more and 0.97 or less.

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.

Images