Anode material for lithium-ion secondary batteries, anode for lithium-ion secondary batteries, and lithium-ion secondary batteries

Abstract

This negative electrode material for lithium ion secondary batteries includes composite particles in which amorphous carbonaceous particles and amorphous silicon particles are complexed. The average primary particle diameter of the silicon particles is 1-50 nm. The composite particles include first composite particles having a silicon content of 0.5-5 wt.%, and second composite particles having a silicon content of 60-70 wt.%.

Description

[Technical Field]

[0001] This invention relates to a negative electrode material for lithium-ion secondary batteries, a negative electrode for lithium-ion secondary batteries, and a lithium-ion secondary battery. [Background technology]

[0002] Lithium-ion rechargeable batteries are widely used as power sources for mobile devices such as cell phones and laptops, as well as hybrid cars.

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

[0004] The negative electrode active material containing silicon undergoes significant volume expansion during charging. This volume expansion of the negative electrode active material causes a decrease in the battery's cycle performance. When the negative electrode active material expands in volume, for example, the negative electrode active material may be damaged, the conductive paths between the negative electrode active material may be severed, delamination may occur at the interface between the negative electrode active material layer and the current collector, and cracks may form in the SEI (Solid Electrolyte Interphase) coating, leading to electrolyte decomposition. These factors reduce the battery's cycle performance.

[0005] For example, Patent Documents 1 to 3 describe composite particles formed by combining silicon particles and carbonaceous materials to improve the cycle characteristics of batteries. Patent Documents 1 to 3 describe methods for compounding silicon particles and carbonaceous materials, such as mechanochemical methods and mixed heating methods. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 4379971 [Patent Document 2] Patent No. 3995050 [Patent Document 3] Japanese Patent Publication No. 2008-277232 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] Using composite particles, which are a compound of silicon particles and carbonaceous materials, improves cycle characteristics. However, even when using composite particles, there are cases where the cycle characteristics do not improve sufficiently or sufficient discharge capacity cannot be obtained. For example, when compounding is performed using the mechanochemical method described in Patent Document 1, when compressive and shear forces are applied to the carbon material and silicon compound, some of the silicon compound may be converted to silicon carbide. Among silicon compounds, silicon carbide has a small contribution to charge and discharge, and the negative electrode active material may not exhibit sufficient capacity.

[0008] This disclosure has been made in view of the above-mentioned issues and aims to provide a lithium-ion secondary battery with excellent cycle characteristics. [Means for solving the problem]

[0009] To solve the above problems, the following means are provided.

[0010] (1) The negative electrode material for a lithium-ion secondary battery according to the first embodiment includes composite particles formed by compounding amorphous carbonaceous particles and amorphous silicon particles. The average primary particle diameter of the silicon particles is 1 nm or more and 50 nm or less. The composite particles include first composite particles having a silicon content of 0.5% by weight or more and 5% by weight or less, and second composite particles having a silicon content of 60% by weight or more and 70% by weight or less.

[0011] (2) The negative electrode material for lithium-ion secondary batteries according to the above embodiment may have a proportion of 3% by volume or more and 40% by volume or less of the first composite particles, and a proportion of 1% by volume or more and 20% by volume or less of the second composite particles.

[0012] (3) The negative electrode material for a lithium ion secondary battery according to the above aspect may have a proportion of the first composite particles of 6% by volume or more and 30% by volume or less, and a proportion of the second composite particles of 2% by volume or more and 15% by volume or less.

[0013] (4) The negative electrode material for a lithium ion secondary battery according to the above aspect may have an average secondary particle diameter of the composite particles of 3 μm or more and 10 μm or less.

[0014] (5) The negative electrode material for a lithium ion secondary battery according to the above aspect may have a specific surface area of 3 m 2 / g or more and 25 m 2 / g or less.

[0015] (6) The negative electrode for a lithium ion secondary battery according to the second aspect includes the negative electrode material for a lithium ion secondary battery according to the above aspect.

[0016] (7) The lithium ion secondary battery according to the third aspect includes the negative electrode for a lithium ion secondary battery according to the above aspect, a positive electrode, and an electrolyte connecting the positive electrode and the negative electrode for a lithium ion secondary battery.

Advantages of the Invention

[0017] The lithium ion secondary battery according to the above aspect has excellent cycle characteristics.

Brief Description of the Drawings

[0018] [Figure 1] It is a schematic cross-sectional view of a lithium ion secondary battery according to the first embodiment. [Figure 2] It is a view obtained by photographing a cross-section of a negative electrode active material layer according to the first embodiment with a scanning electron microscope (SEM). [Figure 3] It is a schematic view of composite particles included in a negative electrode active material layer according to the first embodiment.

Modes for Carrying Out the Invention

[0019] The embodiments will be described in detail below, with reference to the figures as appropriate. The drawings used in the following description may be enlarged for convenience to clearly illustrate the features, and the dimensional ratios of each component may differ from those of the actual components. The materials, dimensions, etc., exemplified in the following description are examples only, and this disclosure is not limited to them. It is possible to modify and implement these examples as appropriate without altering the essence of the disclosure.

[0020] "Lithium-ion rechargeable battery" Figure 1 is a schematic diagram of a lithium-ion secondary battery according to a first embodiment. The lithium-ion secondary battery 100 shown in Figure 1 comprises a power generation element 40, an outer casing 50, and an electrolyte (for example, a non-aqueous electrolyte). The outer casing 50 covers the periphery of the power generation element 40. The power generation element 40 is connected to the outside by a pair of terminals 60, 62 connected to the power generation element 40. The non-aqueous electrolyte is contained within the outer casing 50. In Figure 1, a case in which there is one power generation element 40 within the outer casing 50 is illustrated, but multiple power generation elements 40 may be stacked.

[0021] (Power generation element) The power generation element 40 comprises a separator 10, a positive electrode 20, and a negative electrode 30. The power generation element 40 may be a laminate in which these are stacked, or a wound structure in which a laminate of these is wound.

[0022] <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.

[0023] [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 such as aluminum, copper, nickel, titanium, or stainless steel. Lightweight aluminum is suitably 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.

[0024] [Cathode active material layer] The positive electrode active material layer 24 includes, for example, a positive electrode active material. The positive electrode active material layer 24 may also include a conductive additive and a binder as needed.

[0025] The positive electrode active material includes an electrode active material capable of reversibly carrying out intercalation (intercalation) of lithium ions, or doping and dedoping of lithium ions and counteranions.

[0026] The positive electrode active material is, for example, a composite metal oxide. The composite metal oxide is, for example, an oxide mainly containing a transition metal element and lithium. The transition metal element is, for example, Ti, V, Cr, Mn, Fe, Co, Ni, Mo, or W, and preferably V, Cr, Mn, Fe, Co, or Ni. The molar ratio of lithium to the transition metal element (lithium / transition metal element) is, for example, 0.3 or more and 2.2 or less.

[0027] The positive electrode active material may contain, for example, Al, Ga, In, Ge, Sn, Pb, Sb, Bi, Si, P, B, etc., in a range of 30 mol% or less relative to the transition metal element. The positive electrode active material may be, for example, Li y MO2 (where M is at least one of Co, Ni, Fe, or Mn, 0 ≤ y ≤ 1.2), or Li z Materials having a spinel structure represented as N2O4 (where N contains at least Mn; 0 ≤ z ≤ 2) are preferred.

[0028] The positive electrode active material is, for example, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2), lithium manganese spinel (LiMn2O4), and LiNi x Co y Mn z M aA compound of O2 (where x + y + z + a = 1, 0 ≦ x < 1, 0 ≦ y < 1, 0 ≦ z < 1, 0 ≦ a < 1 in the general formula, and M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, Cr), a lithium vanadium compound (LiV2O5), olivine-type LiMPO4 (where M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, 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 may be an organic substance. For example, the positive electrode active material may be polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene.

[0029] The positive electrode active material may also be a lithium-free material. Examples of the lithium-free material include FeF3, a conjugated polymer containing an organic conductive substance, Chevrel phase compound, transition metal chalcogenide, vanadium oxide, niobium oxide, etc. The lithium-free material may use only one of the materials, or may be used in combination of a plurality. When the positive electrode active material is a lithium-free material, for example, discharge is first performed. Lithium is inserted into the positive electrode active material by discharge. In addition, the lithium-free material of the positive electrode active material may be pre-doped with lithium chemically or electrochemically.

[0030] The conductive assistant enhances the electron conductivity between the positive electrode active materials. The conductive assistant is, for example, carbon powder, carbon nanotube, carbon material, metal fine powder, a mixture of carbon material and metal fine powder, conductive oxide. The carbon powder is, for example, carbon black, acetylene black, ketjen black, etc. The metal fine powder is, for example, powder of copper, nickel, stainless steel, iron, etc.

[0031] The binder in the positive electrode active material layer 24 binds the positive electrode active materials together. Known binders can be used. Preferably, the binder does not dissolve in the electrolyte, has oxidation resistance, and adhesive properties. Examples of binders include fluororesins. Examples of binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), polyamide (PA), polyimide (PI), polyamideimide (PAI), polybenzimidazole (PBI), polyethersulfone (PES), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), etc.

[0032] The binder may be fluororubber. For example, the binder may be vinylidene fluoride-hexafluoropropylene fluororubber (VDF-HFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFPTFE fluororubber), vinylidene fluoride-pentafluoropropylene fluororubber (VDF-PFP fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene fluororubber (VDF-PFP-TFE fluororubber), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene fluororubber (VDF-PFMVE-TFE fluororubber), or vinylidene fluoride-chlorotrifluoroethylene fluororubber (VDF-CTFE fluororubber).

[0033] <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.

[0034] [Negative electrode current collector] The negative electrode current collector 32 is, for example, a conductive plate material. The negative electrode current collector 32 can be the same as the positive electrode current collector 22.

[0035] [Negative electrode active material layer] Figure 2 is a cross-sectional view of the negative electrode active material layer 34 according to the first embodiment, as captured by a scanning electron microscope (SEM). The negative electrode active material layer 34 contains composite particles 1, which are composites of carbonaceous particles and silicon particles. There are multiple composite particles 1 within the negative electrode active material layer 34. The negative electrode active material layer 34 is an example of a negative electrode material. The composite particles 1 function as a negative electrode active material. In addition to the composite particles 1, the negative electrode active material layer 34 may also contain a binder, a conductive additive, etc.

[0036] The composite particle 1 includes a first composite particle 1A and a second composite particle 1B. The first composite particle 1A has a silicon content of 0.5% by weight or more and 5% by weight or less. Multiple first composite particles 1A are present in the negative electrode active material layer 34. The second composite particle 1B has a silicon content greater than that of the first composite particle 1A. The second composite particle 1B has a silicon content of 60% by weight or more and 70% by weight or less. Multiple second composite particles 1B are present in the negative electrode active material layer 34. The composite particle 1 may also contain composite particles other than the first composite particle 1A and the second composite particle 1B.

[0037] The silicon content of composite particle 1 can be measured by energy-dispersive X-ray spectroscopy (EDS) by irradiating composite particle 1, which can be seen in cross-sectional SEM images, with an electron beam. The weight ratio of carbonaceous particles in composite particle 1 can also be measured by methods such as high-frequency induction heating combustion-infrared absorption spectroscopy, and the weight ratio of silicon particles in composite particle 1 can be measured by methods such as ICP (inductively coupled plasma) emission spectroscopy.

[0038] The average silicon content of composite particle 1 is, for example, 40% by weight or more and 60% by weight or less, preferably 45% by weight or more and 50% by weight or less. The average silicon content of composite particle 1 is, for example, the average value of at least 50 composite particles 1.

[0039] In the negative electrode active material layer 34, the proportion of the first composite particle 1A is, for example, 3% by volume or more and 40% by volume or less, preferably 6% by volume or more and 30% by volume or less. In the negative electrode active material layer 34, the proportion of the second composite particle 1B is, for example, 1% by volume or more and 20% by volume or less, preferably 2% by volume or more and 15% by volume or less. For example, if the silicon content of the composite particle 1 contained in the negative electrode active material layer 34 is plotted on the horizontal axis and the number of composite particles 1 whose silicon content is within a predetermined range is plotted on the vertical axis, the graph will have two peaks: one in the range where the silicon content is 0.5% by weight or more and 5% by weight or less, and another in the range where the silicon content is 60% by weight or more and 70% by weight or less.

[0040] The average secondary particle diameter of composite particle 1 is between 3 μm and 10 μm. If the average secondary particle diameter of composite particle 1 is 3 μm or more, conductivity between composite particles 1 can be ensured even with a small amount of binder and conductive additive. Furthermore, if the average secondary particle diameter of composite particle 1 is 10 μm or less, the composite particles 1 are less likely to be damaged during charging and discharging of the lithium-ion secondary battery 100.

[0041] The average secondary particle diameter of composite particle 1 can be determined, for example, from a cross-sectional image of the negative electrode active material layer 34. The cross-sectional image can be measured using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). For example, by observing composite particle 1 at a magnification of 100,000x using a scanning electron microscope JSM-7600 (manufactured by JEOL Ltd.) and performing image processing on the captured image, the average secondary particle diameter of composite particle 1 can be measured.

[0042] For example, the average secondary particle diameter can be determined using the image processing software HALCON (registered trademark, manufactured by MVTec Software GmbH). This software recognizes particles in the captured image, removes particles whose entirety is not captured at the edges of the observation field, measures the maximum length (diameter of the circumscribed circle of the particle) for each particle, and converts the particle diameter from the maximum length. By performing such measurements for 200 particles and obtaining the numerically-based cumulative particle size distribution, the average secondary particle diameter of composite particle 1 can be calculated from this distribution.

[0043] The specific surface area of ​​composite particle 1 is, for example, 3 m². 2 / g or more 25m 2 The specific surface area is less than or equal to / g. The specific surface area can be measured, for example, by the BET method (multilayer adsorption method). Specifically, using Gemini2360 (Micromeritics), the sample is pre-dried at 200°C for 20 minutes under nitrogen flow, followed by flowing nitrogen gas for another 5 minutes, and then determined by the BET 7-point method by nitrogen gas adsorption.

[0044] A sufficiently large specific surface area of ​​composite particle 1 indicates that there are many gaps within the composite particle 1. These gaps between composite particles 1 alleviate stress concentration caused by the expansion and contraction of silicon particles during charging and discharging, preventing damage to the composite particles 1. Furthermore, by ensuring that the specific surface area of ​​composite particle 1 is not too large, excessive side reactions between the silicon particles of composite particle 1 and the electrolyte can be suppressed.

[0045] Figure 3 is a schematic diagram of composite particle 1 contained in the negative electrode material according to the first embodiment. Composite particle 1 is a composite of carbonaceous particle 2 and silicon particle 3. The first composite particle 1A and the second composite particle 1B each contain carbonaceous particle 2 and silicon particle 3, respectively. The first composite particle 1A has a lower proportion of silicon particle 3 than the second composite particle 1B.

[0046] Both carbonaceous particles 2 and silicon particles 3 are amorphous. Here, amorphous refers to a structure that does not have a regular arrangement of atoms over long distances on the scale of interatomic distance, and is a material that does not show a clear X-ray diffraction pattern. Not showing a clear X-ray diffraction pattern means, for example, that the X-ray diffraction spectrum does not have peaks with a full width at half maximum of 5° or less.

[0047] The amorphous carbonaceous particles 2 and silicon particles 3 exhibit excellent input / output characteristics because they do not exhibit anisotropy in the direction of lithium ion insertion and deinsertion. Furthermore, the amorphous silicon particles are less prone to breakage during volume expansion.

[0048] The silicon particles 3 are not limited to elemental silicon, but may also be silicon alloys or silicon oxides. The silicon particles 3 may be crystalline or amorphous.

[0049] For example, silicon alloys are X n It is represented by Si. X is a cation. X can be, for example, Ba, Mg, Al, Zn, Sn, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, Y, Zr, Nb, Mo, W, Au, Ti, Na, K, etc. n satisfies 0 ≤ n ≤ 0.5.

[0050] Silicon oxides are, for example, SiO x This is expressed as such, where x satisfies, for example, 0.8 ≤ x ≤ 2. The silicon oxide may consist only of SiO2, only of SiO, or a mixture of SiO and SiO2. Furthermore, the silicon oxide may have some oxygen deficiencies.

[0051] The average primary particle diameter of silicon particles 3 is, for example, between 1 nm and 50 nm. The average primary particle diameter of silicon particles 3 can be determined by the same method as the average secondary particle diameter of composite particles 1. For example, the average primary particle diameter of silicon particles 3 can be measured by observing a cross-section of composite particles 1 using a scanning electron microscope JSM-7600 (manufactured by JEOL Ltd.) and performing image processing on the captured image. In this case, the average primary particle diameter is calculated by removing carbonaceous particles 2 through image processing.

[0052] If the average primary particle diameter of the silicon particles 3 is within the above range, it is possible to suppress the decomposition of the electrolyte, which is one of the side reactions caused by the contact between the silicon particles 3 and the electrolyte, and the resulting increase in the film resistance of the formed film. In addition, if the average primary particle diameter of the silicon particles 3 is within the above range, the silicon particles 3 are less likely to be damaged by expansion and contraction during charging and discharging.

[0053] The carbonaceous particles 2 are compounded with silicon particles 3. The carbonaceous particles 2 are, for example, carbides produced after firing graphite, graphene, pitches, or resins. There may be two or more types of carbonaceous particles 2.

[0054] Pitches may include coal-based pitches, petroleum-based pitches, or synthetic pitches, such as coal tar, tar light oil, tar intermediate oil, tar heavy oil, naphthalene oil, anthracene oil, coal tar pitch, pitch oil, mesophase pitch, oxygen-crosslinked petroleum pitch, heavy oil, coke, low molecular weight heavy oil, and their derivatives.

[0055] Resins include, for example, thermoplastic resins such as polyvinyl alcohol, phenolic resins, epoxy resins, melamine resins, urea resins, aniline resins, cyanate resins, furan resins, ketone resins, unsaturated polyester resins, urethane resins, and modified versions thereof. Phenolic resins include, for example, novolac-type phenolic resins and resol-type phenolic resins. Epoxy resins include, for example, bisphenol-type epoxy resins and novolac-type epoxy resins. Resins include, for example, polyethylene, polystyrene, acrylonitrile-styrene (AS) resin, acrylonitrile-butadiene-styrene (ABS) resin, polypropylene, polyethylene terephthalate, polycarbonate, polyacetal, polyphenylene ether, polybutylene terephthalate, polyphenylene sulfide, polysulfone, polyethersulfone, polyetheretherketone, and polyvinyl chloride.

[0056] The conductive additive and binder can be the same as those used for the positive electrode 20. In addition to those listed for the positive electrode 20, the binder for the negative electrode 30 may be, for example, cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamide-imide resin, acrylic resin, etc. For example, carboxymethylcellulose (CMC) may be used instead of cellulose.

[0057] <Separator> The separator 10 is sandwiched between the positive electrode 20 and the negative electrode 30. The separator 10 isolates the positive electrode 20 and the negative electrode 30, preventing a short circuit between them. The separator 10 spreads in plane along the positive electrode 20 and the negative electrode 30. Lithium ions can pass through the separator 10.

[0058] The separator 10 may have, for example, an electrically insulating porous structure. The separator 10 may be, for example, a single layer or laminate of a polyolefin film. The separator 10 may also be a stretched film of a mixture of polyethylene or polypropylene. The separator 10 may also 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 also 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. An inorganic coated separator is obtained by coating the surface of the above film with a mixture of resin such as PVDF or CMC and inorganic substances such as alumina or silica. Inorganic coated separators have excellent heat resistance and suppress the deposition of transition metals eluted from the positive electrode onto the negative electrode surface.

[0059] <Electrolyte> The electrolyte is sealed within the outer casing 50 and impregnates the power generation element 40. The electrolyte is not limited to a liquid electrolyte, but may also be a solid electrolyte. A non-aqueous electrolyte, for example, comprises a non-aqueous solvent and an electrolytic salt. The electrolytic salt is dissolved in the non-aqueous solvent.

[0060] The solvent is not particularly limited as long as it is a solvent commonly used in lithium-ion secondary batteries. The solvent may include, for example, a cyclic carbonate compound, a linear carbonate compound, a cyclic ester compound, or a linear ester compound. The solvent may also contain a mixture of these in any proportion. Examples of cyclic carbonate compounds include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, fluoroethylene carbonate, and vinylene carbonate. Examples of linear carbonate compounds include diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). Examples of cyclic ester compounds include γ-butyrolactone. Examples of linear ester compounds include propyl propionate, ethyl propionate, and ethyl acetate. The ratio of cyclic carbonate to linear carbonate in the non-aqueous solvent is preferably, for example, 1:9 to 1:1 by volume.

[0061] The electrolytic salt is, for example, a lithium salt. The electrolyte is, for example, LiPF6, LiClO4, LiBF4, LiCF3SO3, LiCF3CF2SO3, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(CF3CF2CO)2, LiBOB, LiN(FSO2)2, etc. One lithium salt may be used alone, or two or more may be used in combination. From the viewpoint of degree of ionization, it is preferable that the electrolyte contains LiPF6. It is preferable that the degree of ionization of the electrolytic salt at room temperature in a carbonate solvent is 10% or more.

[0062] When dissolving LiPF6 in a non-aqueous solvent, it is preferable to adjust the electrolyte concentration in the non-aqueous electrolyte to between 0.5 mol / L and 2.0 mol / L. A concentration of 0.5 mol / L or higher ensures sufficient conductivity of the non-aqueous electrolyte, making it easier to obtain sufficient capacity during charging and discharging. Furthermore, keeping the electrolyte concentration within 2.0 mol / L suppresses the increase in viscosity of the non-aqueous electrolyte, ensuring sufficient lithium ion mobility, and making it easier to obtain sufficient capacity during charging and discharging.

[0063] Even when LiPF6 is mixed with other electrolytes, it is preferable to adjust the lithium ion concentration in the non-aqueous electrolyte to 0.5 mol / L or more and 2.0 mol / L or less, and it is even more preferable that the lithium ion concentration from LiPF6 accounts for 50 mol% or more of the total.

[0064] <Exterior> The outer casing 50 seals the power generation element 40 and the non-aqueous electrolyte inside. The outer casing 50 prevents leakage of the non-aqueous electrolyte to the outside and prevents moisture and other substances from entering the lithium-ion secondary battery 100 from the outside.

[0065] The outer casing 50, as shown in Figure 1 for example, has a metal foil 52 and a resin layer 54 laminated on each surface of the metal foil 52. The outer casing 50 is a metal laminate film in which the metal foil 52 is coated on both sides with a polymer film (resin layer 54).

[0066] For example, aluminum foil can be used as the metal foil 52. A polymer film such as polypropylene can be used for the resin layer 54. The materials constituting the resin layer 54 may differ between the inside and outside. For example, a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide (PA), can be used as the outer material, while polyethylene (PE) or polypropylene (PP) can be used as the material for the inner polymer film.

[0067] <Terminal> Terminals 62 and 60 are connected to the positive electrode 20 and the negative electrode 30, respectively. Terminal 62 connected to the positive electrode 20 is the positive terminal, and terminal 60 connected to the negative electrode 30 is the negative terminal. Terminals 60 and 62 are responsible for electrical connections to the outside. Terminals 60 and 62 are made of conductive materials such as aluminum, nickel, and copper. The connection method may be welding or screw fastening. It is preferable to protect terminals 60 and 62 with insulating tape to prevent short circuits.

[0068] "Manufacturing method for lithium-ion secondary batteries" A lithium-ion secondary battery 100 is manufactured by preparing a negative electrode 30, a positive electrode 20, a separator 10, an electrolyte, and an outer casing 50, and then assembling them. An example of a manufacturing method for a lithium-ion secondary battery 100 is described below.

[0069] The negative electrode 30 is manufactured, for example, by sequentially performing a slurry preparation process, an electrode coating process, a drying process, and a rolling process.

[0070] The slurry preparation process involves mixing the negative electrode active material, binder, conductive additive, and solvent to create a slurry.

[0071] First, let's explain the manufacturing method for the negative electrode active material. The negative electrode active material is produced through a composite particle manufacturing process, a mixing process, a drying process, and a heat treatment process.

[0072] First, in the composite particle manufacturing process, silicon particles and a carbon source are mixed in an organic solvent. The weight ratio of silicon particles to carbonaceous particles in the composite particle can be adjusted by changing the mixing ratio of silicon particles to carbon source. For example, a first composite particle 1A with a silicon content of 0.5% to 5% by weight and a second composite particle 1B with a silicon content of 60% to 70% by weight are manufactured separately.

[0073] Silicon particles are produced, for example, by pulverizing a silicon ingot in multiple stages to achieve an average particle size of 1 nm to 50 nm. Silicon ingots are produced, for example, by rapidly cooling molten silicon. Rapid cooling of the molten silicon prevents excessive crystallization of the silicon particles, making them amorphous. The cooling rate is, for example, 10 3 K / s or more 10 8 It is less than K / s.

[0074] The silicon ingots can be ground using, for example, a ball mill or a media mill. Ball mills can include, for example, planetary mills, vibrating ball mills, conical mills, and tube mills. Media mills can include, for example, attritor type, sand grinder type, aniller mill type, and tower mill type. The particle size of the silicon particles may be controlled using a sieve.

[0075] The method for producing silicon particles is not limited to this method. For example, silicon particles may also be produced by the atomization method. The atomization method is a method of producing fine particles by melting and spraying molten metal. Atomization methods include forming powder by spraying with an inert gas (gas atomization method), forming powder by spraying onto a rotating disk, forming powder by spraying with high-pressure water (water atomization method), and pouring sprayed metal into a high-speed rotating water stream (post-atomization method).

[0076] The molten metal may also be cooled using the gun method, single-roll method, or double-roll method. These methods can accelerate the cooling rate of the molten metal. The average particle size can be adjusted by further grinding the powder or ribbon produced by this method. Alternatively, the resulting powder may be subjected to a ball mill or the like to further amorphize the silicon particles.

[0077] The carbon source may be graphite, graphene, pitches, resins, etc. The pitches and resins mentioned above can be used. Preferably, the carbon source is at least one selected from the group consisting of graphite, graphene, novolac-type phenolic resin, resol-type phenolic resin, coal-based pitch, and petroleum-based pitch. Two or more types of carbon sources may be used.

[0078] Organic solvents include methanol, ethanol, and tetrahydrofuran. Dispersants may be added to the organic solvent. Adding a dispersant allows for uniform compounding of carbonaceous particles and silicon particles. Such composite particles have excellent electronic conductivity and are less likely to undergo side reactions with the electrolyte during charging and discharging.

[0079] Next, the mixture of silicon particles and a carbon source in an organic solvent is dried. Drying removes the organic solvent from the mixture, yielding a powder. The drying method is not particularly limited, but for example, spray drying is used.

[0080] Next, the dried powder is subjected to heat treatment. During the heat treatment process, the resin or resin composition, which serves as the carbon source, undergoes incomplete combustion and carbonization, resulting in the formation of carbonaceous particles. This creates a composite of silicon particles and carbonaceous particles.

[0081] The heat treatment is preferably carried out at a heat treatment temperature of 350 to 1200°C for the mixture. If the heat treatment temperature is too low, the carbon source will not be sufficiently carbonized, which can cause lithium to be trapped during charging and discharging. When lithium is trapped, the initial efficiency of the lithium-ion secondary battery decreases. If the heat treatment temperature is too high, the silicon particles and carbonaceous particles react, and an excess of silicon carbide is produced. Among silicon compounds, silicon carbide has a small contribution to charging and discharging, which reduces the conductivity of lithium ions and causes a decrease in the discharge capacity of the lithium-ion secondary battery.

[0082] Furthermore, the heat treatment time is preferably between 1 hour and 72 hours. The heat treatment atmosphere is preferably a reducing atmosphere such as a nitrogen atmosphere or an argon atmosphere.

[0083] Next, a slurry is prepared using the composite particles prepared in the above procedure. The slurry can be prepared by mixing the first composite particles, the second composite particles, a binder, a conductive additive, and a solvent. Examples of solvents include water and N-methyl-2-pyrrolidone. By adjusting the mixing ratio of the first composite particles 1A and the second composite particles 1B in the slurry, the volume ratio of the first composite particles 1A and the second composite particles 1B in the negative electrode active material layer 34 can be adjusted.

[0084] The electrode coating process involves coating the surface of the negative electrode current collector 32 with slurry. There are no particular restrictions on the method of slurry coating. For example, the slit die coating method and the doctor blade method can be used as slurry coating methods. The slurry is coated, for example, at room temperature.

[0085] The drying process is a process of removing the solvent from the slurry. For example, the negative electrode current collector 32 coated with slurry is dried in a temperature environment between 80°C and 350°C.

[0086] The rolling process is performed as needed. The rolling process involves applying pressure to the negative electrode active material layer 34 to adjust its density. The rolling process is performed, for example, using a roll press. The linear pressure of the roll press is, for example, between 100 kgf / cm and 2500 kgf / cm.

[0087] The positive electrode 20 can be manufactured using the same procedure as the negative electrode 30. The separator 10 and the outer casing 50 can be commercially available.

[0088] Next, the positive electrode 20 and negative electrode 30 are stacked so that the separator 10 is positioned between them to create the power generation element 40. For example, the positive electrode 20, separator 10, and negative electrode 30 are stacked and pressed together to make them tightly bonded. If the power generation element 40 is a wound body, the positive electrode 20, negative electrode 30, and separator 10 are wound around one end as an axis.

[0089] Finally, the power generation element 40 is sealed in the casing 50. The non-aqueous electrolyte is injected into the casing 50. After injecting the non-aqueous electrolyte, the non-aqueous electrolyte is impregnated into the power generation element 40 by applying reduced pressure, heating, etc. By sealing the casing 50 with heat, etc., a lithium-ion secondary battery 100 is obtained. Alternatively, instead of injecting the electrolyte into the casing 50, the power generation element 40 may be impregnated in the electrolyte. After injecting the electrolyte into the power generation element, it is preferable to leave it undisturbed for 24 hours.

[0090] The lithium-ion secondary battery 100 according to the first embodiment has composite particles with different silicon content. The first composite particle 1A has a low silicon content, resulting in minimal volume change during charging and discharging of the lithium-ion secondary battery 100. On the other hand, because the first composite particle 1A contains silicon, it functions as an active material. That is, the first composite particle 1A functions as an active material while also acting as a buffer against volume changes of other composite particles. The second composite particle 1B has a high silicon content, increasing the discharge capacity of the negative electrode 30. The presence of composite particles with different characteristics within the negative electrode active material layer 34 allows for the utilization of the advantages of each composite particle. Furthermore, when the second composite particle 1B expands in volume, the first composite particle 1A acts as a buffer, suppressing damage to the composite particle 1 and preventing a decrease in the cycle characteristics of the lithium-ion secondary battery 100. Additionally, when the composite particle 1 expands in volume, the first composite particle 1A acts as a buffer, and the composite particles 1 come into close contact with each other, resulting in a smoother conductive path between the composite particles 1.

[0091] Although embodiments of the present invention have been described in detail above with reference to the drawings, the configurations and combinations thereof in each embodiment are merely examples, and additions, omissions, substitutions, and other modifications to the configurations are possible without departing from the spirit of the present invention. [Examples]

[0092] "Example 1" (Fabrication of negative electrode active material) <Fabrication of the first composite particle> Silicon (Aldrich, purity over 99%) was melted under vacuum using an arc melting apparatus. The molten metal was then rapidly cooled by blowing argon gas onto a rotating copper roller to produce silicon powder. Next, the silicon powder was pulverized in a planetary ball mill in an argon gas atmosphere using φ0.1 mm silicon nitride balls for 24 hours to produce silicon particles with an average primary particle size of 5 nm.

[0093] Next, 5 g of silicon particles and 200 g of tetrahydrofuran (manufactured by Kanto Chemical Co., Ltd., special grade) were placed in a 500 ml beaker and stirred with a magnetic stirrer. Subsequently, 6.5 g of phenolic resin (manufactured by DIC Co., Ltd.) was added and stirring continued for approximately 0.5 hours. Then, 180 g of furan resin was added and stirring continued for 3 hours. After that, the mixture was treated with a homogenizer for 1 hour. The resulting mixture was heat-treated in an oven at 90°C for approximately 24 hours, and then ground to obtain a powder.

[0094] Next, 10.00 g of the powder was placed in an alumina crucible, and the temperature was raised from room temperature to 900°C over 5 hours in an argon atmosphere, followed by heat treatment at 900°C for 4 hours to produce the first composite particles.

[0095] <Fabrication of the second composite particle> The second composite particle was prepared using the same procedure as the first composite particle. The method for preparing the second composite particle differed from that of the first composite particle in the mixing ratio of silicon particles, phenolic resin, and furan resin.

[0096] <Preparation of other composite particles> Other composite particles were prepared using the same procedure as for the first composite particle. The preparation method for the other composite particles differed from that for the first composite particle in the mixing ratio of silicon particles, phenolic resin, and furan resin.

[0097] <Measurement of carbon content and weight ratio of silicon particles> The weight ratio of carbonaceous particles to silicon particles in the first and second composite particles was determined using high-frequency induction heating combustion-infrared absorption spectroscopy and ICP (inductively coupled plasma) emission spectroscopy. First, the weight of carbonaceous particles contained in the first and second composite particles was determined using high-frequency induction heating combustion-infrared absorption spectroscopy. Next, the weight of silicon particles contained in the first and second composite particles was measured using ICP (inductively coupled plasma) emission spectroscopy.

[0098] <Measurement of specific surface area of ​​composite particles> The specific surface area of ​​the composite particles was determined using Gemini2360 (Micromeritics). The specific surface area was determined by pre-drying the sample at 200°C for 20 minutes under nitrogen flow, followed by flowing nitrogen gas for another 5 minutes, and then using the BET 7-point method by nitrogen gas adsorption.

[0099] [Creating evaluation cells] A slurry was prepared by dispersing a mixture of the first composite particles, the second composite particles, and other composite particles with a binder consisting of polyimide (PI) and acetylene black in the solvent N-methyl-2-pyrrolidone (NMP). The slurry was prepared so that the weight ratio of the composite particles to the acetylene black and polyimide was 80:10:10. The mixing ratio of the first and second composite particles was also adjusted so that the volume ratio of the first and second composite particles in the negative electrode active material layer was a predetermined ratio. This slurry was applied to a copper foil current collector, dried, and then rolled to produce an electrode (negative electrode) with the negative electrode active material layer of Example 1.

[0100] Next, a separator made of polyethylene microporous membrane was sandwiched between the fabricated negative electrode and a Li foil, which served as its counter electrode, to obtain a laminate (power generation element). This laminate was placed in an aluminum laminator pack, and an electrolyte solution was poured into the pack, which was mixed with LiPF6 to a concentration of 1.3 mol / L in a volume ratio of FEC:VC:EMC = 1:1:8. The pack was then vacuum-sealed to produce the evaluation cell for Example 1.

[0101] Next, the discharge capacity and cycle characteristics of the evaluation cell were determined. The discharge capacity was determined by first measuring the charge capacity in a constant temperature bath at 25°C with a charge rate of 0.1C (the current value at which discharge is completed in 10 hours when constant current discharge is performed at 25°C), and then measuring the initial discharge capacity in a constant temperature bath at 25°C with a discharge rate of 0.1C.

[0102] Furthermore, the cycle characteristics were determined by repeating 0.5C charge / 1C discharge 50 times using the battery cell after the initial discharge capacity measurement, following the charge / discharge procedure described above. Charging and discharging were performed in a constant temperature bath at 45°C. The initial discharge capacity was set to 100%, and the value of the discharge capacity after 50 cycles was defined as the cycle characteristic. Larger initial discharge capacity and cycle characteristics are preferable.

[0103] Examples 2-8 Examples 2-8 altered the volume ratio of the first and second composite particles in the negative electrode active material layer by changing the mixing ratio of the first and second composite particles when preparing the negative electrode slurry. In Examples 2-8, the same evaluation was performed as in Example 1.

[0104] Examples 9 and 10 Examples 9 and 10 altered the volume ratio of the first composite particles, the volume ratio of the second composite particles, and the average secondary particle diameter of the composite particles in the negative electrode active material layer by changing the mixing ratio of the first composite particles, the second composite particles, and other composite particles when preparing the negative electrode slurry. In Examples 9 and 10, the same evaluation as in Example 1 was performed in the same manner as in Example 1.

[0105] Examples 11-14 Examples 11-14 varied the particle size of the silicon particles constituting the composite particles. Examples 11 and 12 differed from Example 5 in terms of silicon particle particle size. Examples 13 and 14 also differed from Example 6 in terms of silicon particle particle size. In Examples 11-14, the same evaluation as in Example 1 was performed in the same manner as in Example 1.

[0106] Examples 15 and 16 Examples 15 and 16 differ from Example 1 in that the average specific surface area of ​​the composite particles was changed. In Examples 15 and 16, the same evaluation was performed as in Example 1.

[0107] "Comparative Examples 1 and 2" Comparative Examples 1 and 2 varied the particle size of the silicon particles constituting the composite particles. Furthermore, the mixing ratio of the first and second composite particles during the preparation of the negative electrode slurry was changed, altering the volume ratio of the first and second composite particles in the negative electrode active material layer. Comparative Examples 1 and 2 were evaluated in the same manner as in Example 1.

[0108] "Comparative Examples 3-5" In Comparative Examples 3 to 5, neither the first composite particle nor the second composite particle was added when preparing the negative electrode slurry. In Comparative Examples 3 to 5, the same procedure and evaluation as in Example 1 were followed.

[0109] "Comparative Examples 6-8" In Comparative Examples 6-8, crystalline silicon particles or carbonaceous materials were used as the constituent silicon particles or carbonaceous materials of the composite particles. In Comparative Examples 6-8, the same evaluation as in Example 1 was performed in the same manner as in Example 1.

[0110] The evaluation results for Examples 1-18 and Comparative Examples 1-8 are summarized in Tables 1 and 2.

[0111] [Table 1]

[0112] [Table 2] [Explanation of Symbols]

[0113] 1 Composite particles 1A 1st composite particle 1B 2nd composite particle 2 Carbonaceous materials 3. Silicon particles 10 Separators 20 positive electrode 22 Positive electrode current collector 24 Cathode active material layer 30 negative electrode 32 Negative electrode current collector 34 Negative electrode active material layer 40 Power generation element 50 Exterior 52 Metal foil 54 Resin layer Terminals 60, 62 100 Lithium-ion rechargeable batteries

Claims

1. It contains composite particles formed by compounding amorphous carbonaceous particles and amorphous silicon particles. The average primary particle diameter of the silicon particles is 1 nm or more and 50 nm or less. The composite particles comprise a first composite particle having a silicon content of 0.5% by weight or more and 5% by weight or less, and a second composite particle having a silicon content of 60% by weight or more and 70% by weight or less, in a negative electrode material for a lithium-ion secondary battery.

2. The negative electrode material for a lithium-ion secondary battery according to claim 1, wherein the proportion of the first composite particles is 3% by volume or more and 40% by volume or less, and the proportion of the second composite particles is 1% by volume or more and 20% by volume or less.

3. The negative electrode material for a lithium-ion secondary battery according to claim 2, wherein the proportion of the first composite particles is 6% by volume or more and 30% by volume or less, and the proportion of the second composite particles is 2% by volume or more and 15% by volume or less.

4. The negative electrode material for a lithium-ion secondary battery according to claim 1, wherein the average secondary particle diameter of the composite particles is 3 μm or more and 10 μm or less.

5. The specific surface area of ​​the composite particles is 3 m². ２ / g or more 25m ２ The negative electrode material for a lithium-ion secondary battery according to claim 1, wherein the amount is less than or equal to / g.

6. A negative electrode for a lithium-ion secondary battery, comprising the negative electrode material for a lithium-ion secondary battery described in claim 1.

7. A lithium-ion secondary battery comprising a negative electrode, a positive electrode, and an electrolyte as described in claim 6.

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