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

Abstract

This negative electrode active material comprises composite particles and carbon nanotubes. The composite particles include a plurality of carbonaceous particles and a plurality of silicon particles. The plurality of carbonaceous particles and the plurality of silicon particles are amorphous. The average primary particle size of the plurality of silicon particles is 1-50 nm. The carbon nanotubes connect at least two carbonaceous particles among the plurality of carbonaceous particles on the surfaces of the composite particles.

Description

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

[0001] This disclosure relates to anode active materials, anodes for lithium-ion secondary batteries, and lithium-ion secondary batteries. This application claims priority under Japanese Patent Application No. 2025-037896, filed in Japan on March 11, 2025, the contents of which are incorporated herein by reference.

[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. Silicon (Si) is attracting attention because it has a theoretical capacity far greater than that of graphite (372 mAh / g).

[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, cracks may form in the negative electrode active material, delamination may occur at the interface between the negative electrode active material layer and the current collector, or cracks may form in the SEI (Solid Electrolyte Interphase) coating, leading to electrolyte decomposition, etc. These factors reduce the battery's cycle performance.

[0005] For example, Patent Document 1 discloses that the expansion and contraction of negative electrode active material particles can be suppressed by binding and fixing them with a graphene compound. Also, for example, Patent Document 2 discloses that in a lithium-ion secondary battery containing silicon-based active material, primary carbon-based active material particles, and secondary carbon-based active material particles, the expansion and contraction of the negative electrode active material layer can be suppressed by adjusting the particle size distribution of these materials. Furthermore, Patent Document 3 discloses a negative electrode body containing Al, in which Al increases the skeleton size of the crystal structure of Si particles, thereby suppressing expansion and contraction.

[0006] Japanese Patent Publication No. 2024-111153, International Publication No. 2024 / 202302, Japanese Patent Publication No. 2024-8702

[0007] There is a demand for lithium-ion secondary batteries excellent in rate characteristics and cycle characteristics, and studies are being conducted on new methods capable of improving these characteristics.

[0008] The present disclosure has been made in view of the above problems, and an object of the present disclosure is to provide a negative electrode active material, a negative electrode for a lithium-ion secondary battery, and a lithium-ion secondary battery that are excellent in rate characteristics and cycle characteristics.

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

[0010] The negative electrode active material according to the first aspect includes composite particles and carbon nanotubes. The composite particles include a plurality of carbonaceous particles and a plurality of silicon particles. Each of the plurality of carbonaceous particles and the plurality of silicon particles is amorphous. The average primary particle diameter of the plurality of silicon particles is 1 nm or more and 50 nm or less. The carbon nanotubes connect between two or more carbonaceous particles among the plurality of carbonaceous particles on the surface of the composite particles.

[0011] A lithium-ion secondary battery using the negative electrode active material according to the above aspect and the negative electrode for a lithium-ion secondary battery is excellent in rate characteristics and cycle characteristics.

[0012] It is a schematic diagram of the lithium-ion secondary battery according to the first embodiment. It is a schematic diagram of the negative electrode active material according to the first embodiment.

[0013] Hereinafter, embodiments will be described in detail with appropriate reference to the drawings. In the drawings used in the following description, characteristic portions may be enlarged for convenience in order to make the characteristics easy to understand, and the dimensional ratios of respective components may differ from actual ones. The materials, dimensions, and the like exemplified in the following description are merely examples, and the present disclosure is not limited thereto, and can be appropriately modified and implemented without changing the gist of the present disclosure.

[0014] "Lithium-ion secondary battery" FIG. 1 is a schematic diagram of a lithium-ion secondary battery according to a first embodiment. A lithium-ion secondary battery 100 shown in FIG. 1 includes a power generating element 40, an exterior body 50, and a non-aqueous electrolyte (not shown). 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 and 62 connected to the power generating element 40. The non-aqueous electrolyte is housed inside the exterior body 50. Although FIG. 1 illustrates a case where one power generating element 40 is provided inside the exterior body 50, a plurality of power generating elements 40 may be stacked. Further, the lithium-ion secondary battery 100 may be of any of a cylindrical type, a prismatic type, a laminated type, a button type, and the like.

[0015] (Power Generating Element) The power generating element 40 includes a separator 10, a positive electrode 20, and a negative electrode 30.

[0016] <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 in contact with at least one surface of the negative electrode current collector 32. The negative electrode 30 is an example of a negative electrode for a lithium-ion secondary battery. Here, a case where the negative electrode 30 has a two-layer configuration including the negative electrode current collector 32 and the negative electrode active material layer 34 is exemplified, but the negative electrode 30 may be a single layer in which a conductor constituting the negative electrode current collector 32 and a negative electrode active material constituting the negative electrode active material layer 34 are mixed, or may be formed only of the negative electrode active material layer 34 containing the negative electrode active material.

[0017] [Negative Electrode Current Collector] The negative electrode current collector 32 is, for example, a conductive plate material. The negative electrode current collector 32 is, for example, a thin metal plate of aluminum, copper, nickel, titanium, stainless steel, or the like. It is preferable that the negative electrode current collector 32 contains, for example, copper. The negative electrode current collector 32 may be, for example, a rolled copper foil or an electrolytic copper foil.

[0018] [Negative Electrode Active Material Layer] The negative electrode active material layer 34 contains a negative electrode active material. The negative electrode active material layer 34 may optionally contain a binder, a conductive auxiliary agent, a dispersion stabilizer, and the like.

[0019] FIG. 2 is a cross-sectional view of the negative electrode active material according to the present embodiment. FIG. 3 is an enlarged view of a characteristic portion of the negative electrode active material according to the present embodiment. The negative electrode active material 1 includes composite particles 2 and carbon nanotubes 3.

[0020] The composite particle 2 comprises a plurality of silicon particles 2A and a plurality of carbonaceous particles 2B. The composite particle 2 may also contain elements other than silicon particles 2A and carbonaceous particles 2B, such as hydrogen, oxygen, and nitrogen. Within the composite particle 2, silicon particles 2A bond with adjacent silicon particles 2A, connecting them in a network. The carbonaceous particles 2B are mixed in to fill the gaps in this network.

[0021] The composite particle 2 is a secondary particle formed by the aggregation of multiple silicon particles 2A and multiple carbonaceous particles 2B. The average secondary particle diameter of the composite particle 2 is, for example, 1 μm or more and 10 μm or less, preferably 2 μm or more and 8 μm or less, and more preferably 3 μm or more and 7 μm or less.

[0022] If the average secondary particle diameter of the composite particles 2 is within the above range, the cycle characteristics will improve. If the composite particles 2 are too small, it becomes difficult to achieve a negative electrode active material layer 34 with sufficient strength and conductivity when forming the negative electrode active material layer, and the amount of binder and conductive additive used will increase. Since the binder and conductive additive are not active materials that perform charging and discharging, an increase in their relative abundance in the negative electrode active material layer 34 will reduce the capacity of the lithium-ion secondary battery 100. Also, if the composite particles 2 are too large, the composite particles 2 may break due to expansion and contraction, and the risk of side reactions such as electrolyte decomposition occurring on the newly formed surface due to the breakage increases.

[0023] If the composite particles 2 are available in particle form, the median diameter (D50) obtained using a particle size distribution analyzer (e.g., Malvern Panalytical) can be used as the average particle diameter. When using a particle size distribution analyzer, for example, the average particle diameter of 50,000 particles is calculated. If the composite particles 2 are inside an electrode and separation of the composite particles 2 is difficult, the average particle diameter can be determined using at least 100 composite particles 2 that can be identified from a cross-sectional image. If the composite particles 2 are irregularly shaped, their major axis is used to calculate the average particle diameter. The average particle diameter measured using a particle size distribution analyzer and the average particle diameter obtained from a cross-sectional image do not deviate significantly and generally agree.

[0024] The molar ratio of silicon particles 2A in composite particle 2 is, for example, 15 mol% to 40 mol%, preferably 20 mol% to 30 mol%. The weight of silicon particles 2A in composite particle 2 can be measured by ICP (inductively coupled plasma) emission spectroscopy or the like. Alternatively, the mass of silicon particles 2A in composite particle 2 may be calculated based on the ratio of elements other than silicon (e.g., oxygen, nitrogen, hydrogen, carbon) determined using oxygen-nitrogen-hydrogen analysis (ONH analysis) and carbon-sulfur analysis (C-S analysis). The molar ratio of silicon particles 2A can be determined from the mass of silicon.

[0025] The molar ratio of carbonaceous particles 2B in composite particle 2 is, for example, 50 mol% to 80 mol%, preferably 60 mol% to 70 mol%. The mass ratio of carbonaceous particles 2B can be measured by methods such as high-frequency induction heating combustion-infrared absorption spectroscopy. The molar ratio of carbonaceous particles 2B can be determined from the mass of carbonaceous particles 2B.

[0026] By controlling the molar ratio of silicon particles 2A and carbonaceous particles 2B in the composite particle 2, the cycle characteristics of the lithium-ion secondary battery can be improved while ensuring its capacity. The silicon particles 2A are the part that contributes most to charging and discharging, and the molar ratio of silicon particles 2A affects the capacity of the lithium-ion secondary battery. The carbonaceous particles 2B function as a buffer when the volume of silicon particles 2A changes. The presence of a predetermined amount of carbonaceous particles 2B within the composite particle 2 prevents damage to the composite particle 2 and improves the cycle characteristics of the lithium-ion secondary battery 100.

[0027] The molar ratio of components other than silicon particles 2A and carbonaceous particles 2B (e.g., hydrogen, oxygen, nitrogen) in the composite particle 2 is preferably 15% or less.

[0028] The silicon particles 2A and carbonaceous particles 2B are amorphous. Lithium ions penetrate along the crystal planes of a crystal. In crystals with crystal planes, the pathways through which lithium ions can penetrate are limited. In contrast, the amorphous silicon particles 2A and carbonaceous particles 2B do not have a fixed crystal orientation, and the direction in which lithium ions can penetrate is not limited. When the silicon particles 2A and carbonaceous particles 2B constituting the composite particle 2 are amorphous, the diffusion of lithium ions in the composite particle 2 becomes uniform, and local volume changes in the composite particle 2 can be suppressed. Amorphousness can be confirmed, for example, from the diffraction spots when electron diffraction is performed. If no clear diffraction spots are observed, but an annular ring is observed, then it is amorphous.

[0029] The silicon particles 2A may be elemental silicon or silicon oxide (SiO2 ｘ : For example, x satisfies 0.8 ≤ x ≤ 2. ) Or X ｎ A silicon alloy represented by Si may also be used. 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.

[0030] The average primary particle diameter of silicon particles 2A is, for example, between 1 nm and 50 nm, but may also be between 1 nm and 30 nm, or between 10 nm, or between 1 nm and 10 nm. The average primary particle diameter of silicon particles 2A is determined from a cross-sectional image of composite particles 2. The cross-sectional image can be measured using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). For example, using a scanning electron microscope JSM-7600 (manufactured by JEOL Ltd.), composite particles 2 can be observed at a magnification of 100,000x, and the average primary particle diameter can be measured by performing image processing on the captured image. The average primary particle diameter of silicon particles 2A determined by image processing is approximately the same as the particle diameter of silicon particles 2A prepared before manufacturing.

[0031] The average primary particle diameter is determined using the image processing software HALCON (registered trademark, manufactured by MVTec Software GmbH). This software recognizes particles in the captured image and removes particles whose entirety is not captured at the edges of the observation field. When measuring the average primary particle diameter, the connecting parts between particles are excluded. For example, the parts connecting silicon particles 2A are excluded from the silicon particle 2A, which can be seen in the same way as silicon particle 2A, and only silicon particle 2A is extracted. For example, parts with a width of 1 nm or less from the white areas visible in the image are removed. Then, for each extracted particle, the shortest width (shortest diameter of the circumscribed circle of the particle) is measured, and the particle diameter is converted from the shortest width. This measurement is performed for 200 particles, the number-based cumulative particle size distribution is obtained, and the average primary particle diameter can be calculated from this distribution.

[0032] If the average primary particle diameter of the silicon particles 2A is within the above range, the increase in the film resistance of the electrolyte due to side reactions caused by contact between the silicon particles 2A and the electrolyte can be suppressed. Furthermore, if the average primary particle diameter of the silicon particles 2A is within the above range, damage to the silicon particles 2A due to expansion and contraction during charging and discharging can be suppressed.

[0033] The carbonaceous particles 2B are compounded with silicon particles 2A. Examples of carbonaceous particles 2B include carbides produced after calcining graphite, graphene, pitches, and resins. There may be two or more types of carbonaceous particles 2B.

[0034] 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.

[0035] 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.

[0036] The carbonaceous particles 2B preferably contain graphite or graphene. The sp of graphite or graphene ２ The bond has high electronic conductivity, ensuring a uniform supply of lithium ions to the silicon particle 2A.

[0037] The weight ratio of composite particles 2 in the negative electrode active material layer 34 is, for example, 5% by weight or more and 50% by weight or less, preferably 5% by weight or more and 40% by weight or less, and more preferably 5% by weight or more and 20% by weight or less. A higher weight ratio of composite particles 2 in the negative electrode active material layer 34 results in a larger capacity for the lithium-ion secondary battery 100.

[0038] The specific surface area of ​​composite particle 2 is 10 m². ２ It may be less than / g. The specific surface area of ​​composite particle 2 is 2m². ２ It may be greater than or equal to / g. The specific surface area of ​​composite particle 2 can be measured, for example, by the BET method (multilayer adsorption method). Specifically, using Gemini 2360 (manufactured by Micromeritics), the sample is pre-dried at 200°C for 20 minutes under nitrogen flow, then nitrogen gas is flowed for another 5 minutes, and the specific surface area can be determined by the BET 7-point method by nitrogen gas adsorption.

[0039] A large specific surface area of ​​composite particle 2 indicates that there are many gaps within the composite particle 2. While a large specific surface area of ​​composite particle 2 allows these gaps to alleviate stress concentration caused by the expansion and contraction of silicon particles 2A during charging and discharging, it also increases the contact area between silicon particles 2A and the electrolyte, making the electrolyte more susceptible to side reactions. Furthermore, a large specific surface area of ​​composite particle 2 requires more binder for electrode formation, resulting in a smaller capacity per unit volume of the lithium-ion secondary battery 100. If the specific surface area of ​​composite particle 2 is too small, insertion and removal of Li ions from the negative electrode active material becomes difficult.

[0040] The carbon nanotubes 3 are attached to the surface of the composite particles 2. On the surface of the composite particles 2, the carbon nanotubes 3 connect two or more carbonaceous particles among the multiple carbonaceous particles 2B.

[0041] In a single composite particle 2, electrically connecting the carbonaceous particles 2B using carbon nanotubes 3 allows for homogenization of the electronic conductivity on the surface of the composite particle 2. When the electronic properties on the surface of the composite particle 2 are homogenized, localized concentration of lithium ions is suppressed, and localized volume changes in the negative electrode active material 1 are suppressed. When the volume changes of the negative electrode active material 1 are homogenized, cracking of the negative electrode active material 1 can be further suppressed. Furthermore, when the electronic conductivity on the surface of a single composite particle is homogenized, the electrical resistance between the negative electrode active material and the electrolyte decreases, allowing for smoother charge-discharge reactions.

[0042] The average diameter of the carbon nanotubes 3 is preferably, for example, 0.5 nm to 3.0 nm. The average length of the carbon nanotubes 3 is preferably 1.0 μm to 2.0 μm. The average diameter and average length are the average values ​​of the average length and average diameter of 100 or more carbon nanotubes 3 attached to the surface of the negative electrode active material 1, as captured by a scanning electron microscope. Carbon nanotubes 3 with a short average length tend to cling to the surface of a single composite particle 2 and easily connect the carbonaceous particles 2B of a single composite particle 2.

[0043] Carbon nanotubes 3 may be single-walled or multi-walled. Short carbon nanotubes 3 are preferably single-walled.

[0044] The carbon nanotubes 3 may connect different composite particles 2. By connecting different composite particles 2, the electron conductivity between the negative electrode active material 1 is improved, and electrons generated in the negative electrode active material layer 34 can be quickly propagated to the negative electrode current collector 32. As a result, the rate characteristics of the lithium-ion secondary battery 100 are improved.

[0045] The average length of the carbon nanotubes 3 connecting the different composite particles 2 is, for example, 5 μm or more, and may be 20 μm or more. The average diameter of the carbon nanotubes 3 connecting the different composite particles 2 may be, for example, 0.5 nm or more and 100.0 nm or less. The carbon nanotubes 3 connecting the different composite particles 2 may be single-walled carbon nanotubes or multi-walled carbon nanotubes.

[0046] The carbon nanotubes 3 may include both short ones with an average length of 1.0 μm or more and 2.0 μm or less, and long ones with an average length of 5 μm or more.

[0047] The binder binds the negative electrode active materials 1 together and the negative electrode active materials 1 together with the negative electrode current collector 32. Known binders can be used. The binder is a compound different from organic additives. Preferably, the binder does not dissolve in the electrolyte, is oxidation-resistant, and has adhesive properties. For example, the binder is a fluororesin. Examples of binders include polyvinylidene fluoride (PVDF), 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, polypropylene (PP) or polyethylene (PE) grafted with maleic anhydride, and mixtures thereof. The binder may also be, for example, cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, polyamideimide resin, acrylic resin, etc. For example, carboxymethylcellulose (CMC) may be used instead of cellulose.

[0048] The binder content in the negative electrode active material layer 34 is not particularly limited. For example, the binder content relative to the total mass of the negative electrode active material, organic additives, conductive additives, and binder is 1% by mass or more and 15% by mass or less, preferably 4% by mass or more and 10% by mass or less. If the binder content is low, the adhesive strength of the negative electrode 30 will be weakened. If the binder content is high, the binder is electrochemically inert and does not contribute to the discharge capacity, so the energy density of the lithium-ion secondary battery 100 will be low.

[0049] The conductive additive in the negative electrode active material layer 34 enhances the electronic conductivity between the negative electrode active materials. Examples of conductive additives include carbon powder, carbon nanotubes, carbon materials, metal powders, mixtures of carbon materials and metal powders, and conductive oxides. Examples of carbon powders include carbon black, acetylene black, and Ketjen black. Examples of metal powders include copper, nickel, stainless steel, and iron powders.

[0050] The content rate of the conductive aid in the negative electrode active material layer 34 is not particularly limited. For example, the content rate of the conductive aid relative to the total mass of the negative electrode active material, organic additive, conductive aid and binder is 5 mass% or more and 20 mass% or less, preferably 1 mass% or more and 12 mass% or less.

[0051] <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.

[0052] [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 of aluminum, copper, nickel, titanium, stainless steel or the like. 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.

[0053] [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 aid and a binder as necessary.

[0054] The positive electrode active material includes an electrode active material that can reversibly progress occlusion and release of lithium ions, desorption and insertion (intercalation) of lithium ions, or doping and dedoping of lithium ions and counter anions.

[0055] The positive electrode active material is, for example, a composite metal oxide. The composite metal oxide is, for example, lithium cobaltate (LiCoO ２ ), lithium nickelate (LiNiO ２ ), lithium manganate (LiMnO ２ ), lithium manganese spinel (LiMn ２ O ４ ), and a compound represented by the general formula: LiNi ｘ Co ｙ Mn ｚ M ａ O ２ (in the general formula, 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 compound (LiV ２O ５ ), olivine-type LiMPO ４ (However, M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, 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 substance. For example, the positive electrode active material may be polyacetylene, polyaniline, polypyrrole, polythiophene, or polyacene.

[0056] The positive electrode active material may be a lithium-free material. Examples of lithium-free materials include FeF ３ These include conjugated polymers containing organic conductive materials, Schevrel phase compounds, transition metal chalcogenides, vanadium oxides, niobium oxides, etc. Lithium-free materials may be used individually or in combination. If the positive electrode active material is lithium-free, for example, a discharge is performed first. Lithium is inserted into the positive electrode active material by the discharge. Alternatively, lithium may be pre-doped chemically or electrochemically into lithium-free positive electrode active materials.

[0057] Conductive additives enhance the electronic conductivity between positive electrode active materials. Examples of conductive additives include carbon powder, carbon nanotubes, carbon materials, metal powders, mixtures of carbon materials and metal powders, and conductive oxides. Examples of carbon powders include carbon black, acetylene black, and Ketjen black. Examples of metal powders include copper, nickel, stainless steel, and iron powders.

[0058] The content of the conductive additive in the positive electrode active material layer 24 is not particularly limited. For example, the content of the conductive additive relative to the total mass of the positive electrode active material, conductive additive, and binder is 0.5% by mass or more and 20% by mass or less, preferably 1% by mass or more and 5% by mass or less.

[0059] The binder in the positive electrode active material layer 24 binds the positive electrode active materials together. The binder may be the same as the one used in the negative electrode active material layer 34.

[0060] 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, 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 is electrochemically inert and does not contribute to the discharge capacity, so the energy density of the lithium-ion secondary battery 100 will be low.

[0061] <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 and prevents 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.

[0062] 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.

[0063] <Electrolyte> The electrolyte is sealed inside the outer casing 50 and impregnates the power generation element 40. If the separator 10 is a solid electrolyte, it does not need to contain an electrolyte. A known electrolyte can be used. The electrolyte includes, for example, a non-aqueous solvent and an electrolyte.

[0064] The electrolyte is, for example, a lithium salt. The electrolyte is, for example, LiPF ６ LiClO ４ LiBF ４ LiCF ３ SO ３ LiCF ３ CF ２ SO ３ LiC (CF ３ SO ２ ) ３ ,LiN(CF ３ SO ２ ) ２ ,LiN(CF ３ CF ２ SO ２ ) ２ ,LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN (CF ３ CF ２ CO) ２ , LiBOB, LiN(FSO ２ ) ２ These are examples. A single lithium salt may be used alone, or two or more may be used in combination. From the viewpoint of degree of ionization, the electrolyte is LiPF ６ It is preferable that it contains [the specified element]. The concentration of the electrolyte is, for example, 0.8 mol / L or more and 5.0 mol / L or less.

[0065] The non-aqueous 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), fluoroethylene carbonate, vinylene carbonate, etc. Examples of linear carbonate compounds include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. Examples of cyclic ester compounds include γ-butyrolactone, etc. Examples of linear ester compounds include propyl propionate, ethyl propionate, ethyl acetate, etc.

[0066] <Outer casing> 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.

[0067] 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).

[0068] 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.

[0069] <Terminals> Terminals 60 and 62 are connected to the negative electrode 30 and the positive electrode 20, 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.

[0070] "Method for Manufacturing a Lithium-Ion Secondary Battery" 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 assembling them. An example of the manufacturing method for a lithium-ion secondary battery 100 is described below.

[0071] First, the negative electrode active material is prepared. The negative electrode active material can be produced by performing a composite particle fabrication process and an adhesion process.

[0072] In the composite particle manufacturing process, silicon particles 2A and a carbon source are mixed in an organic solvent. The molar ratio of silicon particles 2A to carbonaceous particles 2B in the composite particle 2 can be adjusted by changing the mixing ratio of silicon particles 2A to the carbon source.

[0073] 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.

[0074] Organic solvents include methanol, ethanol, and tetrahydrofuran. Dispersants may be added to the organic solvent. By adding a dispersant, the carbonaceous particles 2B uniformly cover the silicon particles 2A during composite formation. Such composite particles 2 have excellent electronic conductivity and are less likely to undergo side reactions with the electrolyte during charging and discharging.

[0075] Next, the mixed mixture 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.

[0076] Next, the dried powder is subjected to heat treatment. During the heat treatment process, the resin or resin composition that serves as the carbon source undergoes incomplete combustion and carbonization, resulting in carbonaceous particles 2B. This creates a composite of silicon particles 2A and carbonaceous particles 2B.

[0077] 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, and an insufficiently carbonized carbon source 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 2A and carbonaceous particles 2B react, and an excess of silicon carbide is produced. Among silicon compounds, silicon carbide has a small contribution to charging and discharging, and it reduces the conductivity of lithium ions, causing a decrease in the discharge capacity of the lithium-ion secondary battery.

[0078] 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.

[0079] Next, the composite particles are irradiated with a laser or electron beam to perform a thermal welding treatment between the silicon particles 2A and the carbonaceous particles 2B. This thermal welding treatment forms a network structure in which the silicon particles 2A are connected.

[0080] In the adhesion process, carbon nanotubes 3 are formed on the surface of composite particles 2. The carbon nanotubes 3 selectively form chemical bonds with the carbon in the composite particles by introducing functional groups through chemical modification of the surface of carbon nanotubes (CNTs). For example, by introducing carboxyl groups or hydroxyl groups to CNTs and reacting them with the carbon in the composite particles, strong C-C bonds are formed, causing them to adhere to the surface of composite particles 2.

[0081] Furthermore, carbon nanotubes 3 with a short average length tend to adhere along the surface of composite particles 2. For example, if carbon nanotubes 3 with an average length of 1.0 μm or more and 2.0 μm or less are used, the carbon nanotubes 3 will adhere along the surface of one composite particle 2, and the carbon nanotubes 3 will easily connect the carbonaceous particles 2B of one composite particle 2.

[0082] Next, the negative electrode 30 is manufactured using this negative electrode active material. The negative electrode 30 is manufactured, for example, by sequentially performing a slurry manufacturing step, an electrode coating step, a drying step, and a rolling step.

[0083] In the slurry preparation process, a slurry is created by mixing the negative electrode active material, binder, conductive additive, and solvent. The solvent is, for example, water or N-methyl-2-pyrrolidone. The composition ratio of the negative electrode active material, conductive material, and binder is preferably 70% to 100% by mass: 0% to 10% by mass: 0% to 20% by mass. These mass ratios are adjusted so that the total mass is 100% by mass. A metal container such as stainless steel is preferred for slurry preparation.

[0084] Next, the electrode coating process is performed. The electrode coating process involves applying a slurry to the surface of the negative electrode current collector 32. There are no particular restrictions on the method of applying the slurry. For example, the slit die coating method and the doctor blade method can be used as slurry coating methods.

[0085] Next, a drying process is performed. 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 an atmosphere 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 or the like.

[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 a power generation element 40. 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 of each 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.

[0090] In this embodiment, the lithium-ion secondary battery 100 uses carbon nanotubes 3 to electrically connect the carbonaceous particles 2B of a single composite particle 2, thereby homogenizing the electronic conductivity on the surface of a single composite particle 2. When the electronic properties on the surface of the composite particle 2 become homogenized, local concentration of lithium ions is suppressed, and localized volume changes in the negative electrode active material 1 can be suppressed. When the volume change of the negative electrode active material 1 is homogenized, cracking of the negative electrode active material 1 can be further suppressed. Furthermore, when the electronic conductivity on the surface of a single composite particle is homogenized, the electrical resistance between the negative electrode active material and the electrolyte decreases, and the charge-discharge reaction occurs smoothly. As a result, the lithium-ion secondary battery 100 in this embodiment has excellent rate characteristics and excellent cycle characteristics.

[0091] Although an example of this embodiment has been described in detail above with reference to the drawings, the configurations and their combinations in this embodiment are merely examples, and additions, omissions, substitutions, and other modifications to the configurations are possible without departing from the spirit of this disclosure.

[0092] "Example 1" First, composite particles were prepared by mixing silicon particles with an average primary particle diameter of 5 nm with a carbon source and firing them. After preparing the composite particles, a laser-based heat welding treatment was performed. Next, carbon nanotubes were attached to the surface of the composite particles. The carbon nanotubes were attached to the surface of the composite particles by immersing and mixing them in a solvent containing carbon nanotubes and carboxymethylcellulose (dispersant). The carbon nanotubes had an average diameter of 1.6 nm and an average length of 1.5 μm.

[0093] The surface of the negative electrode active material was examined using a scanning electron microscope (SEM). SEM examination revealed carbon nanotubes attached between the carbonaceous particles of the negative electrode active material. In other words, it was confirmed that the carbonaceous particles of the negative electrode active material were connected by carbon nanotubes. Furthermore, the average diameter and length of the attached carbon nanotubes were found to be similar to those added to the solvent during manufacturing.

[0094] Furthermore, the cross-section of the negative electrode active material was examined using SEM. The primary particle size of Si in the cross-section examined by SEM was 5.0 nm, which was consistent with that at the time of manufacture. In addition, the negative electrode active material was irradiated with an electron beam, and electron diffraction confirmed that the silicon particles and carbonaceous particles were amorphous.

[0095] 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 the negative electrode slurry.

[0096] The negative electrode slurry was applied to one surface of the negative electrode current collector and dried. The amount of negative electrode active material supported in the negative electrode active material layer after drying was 2.5 mg / cm³. ２ The negative electrode active material layer was pressurized with a roll press and then fired at over 300°C for 5 hours under a nitrogen atmosphere.

[0097] Furthermore, a positive electrode slurry was applied to one surface of the positive electrode current collector. The positive electrode current collector was made of 15 μm thick aluminum foil. The positive electrode slurry was prepared by mixing the positive electrode active material, a conductive additive, a binder, and a solvent.

[0098] Lithium cobalt oxide was used as the positive electrode active material. Acetylene black was used as the conductive additive. Polyvinylidene fluoride (PVDF) was used as the binder. N-methyl-2-pyrrolidone was used as the solvent. A positive electrode slurry was prepared by mixing 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. The amount of positive electrode active material supported in the positive electrode active material layer after drying was 25 mg / cm³. ２ The solvent was removed from the cathode slurry in a drying oven to create a cathode active material layer. The cathode active material layer was pressed with a roll press to produce the cathode.

[0099] Next, the electrolyte was prepared. The solvent for the electrolyte was fluoroethylene carbonate (FEC): ethylene carbonate (EC): diethyl carbonate (DEC) = 10% by volume: 20% by volume: 70% by volume. In addition, additives for improving output, gas suppression, cycle characteristic improvement, and safety performance improvement were added to the electrolyte. The electrolyte was LiPF ６ LiPF was used. ６ The concentration was set to 1 mol / L.

[0100] (Fabrication of Lithium-ion Secondary Battery for Evaluation) The fabricated negative electrode and positive electrode were stacked with a separator (porous polyethylene sheet) in between, so that the positive electrode active material layer and the negative electrode active material layer faced each other, to obtain a laminate. A nickel negative electrode lead was attached to the negative electrode of the laminate. An aluminum positive electrode lead was attached to the positive electrode of the laminate. The positive electrode lead and negative electrode lead were welded together using an ultrasonic welding machine. This laminate was inserted into an aluminum laminate film enclosure and a closed section was formed by heat sealing all but one corner around the perimeter. Finally, after injecting the electrolyte into the enclosure, the remaining corner was sealed by heat sealing under reduced pressure using a vacuum sealing machine to fabricate a lithium-ion secondary battery.

[0101] (Measurement of capacity retention rate after 300 cycles) The cycle characteristics of the lithium-ion secondary battery were measured. The cycle characteristics were measured using a secondary battery charge / discharge test device (manufactured by Hokuto Denko Co., Ltd.).

[0102] The battery was charged at a constant current charge rate of 1C (the current value at which charging is completed in one hour when constant current charging is performed at 25°C) until the battery voltage reached 4.2V, and then discharged at a constant current discharge rate of 1.0C until the battery voltage reached 2.5V. The discharge capacity after the end of charging and discharging was detected and divided by the weight of the negative electrode active material to determine the battery capacity Q before the cycle test. １ We calculated the battery capacity Q. １ This corresponds to the discharge capacity, which was 1934 mAh / g.

[0103] The secondary battery charge / discharge test device was used again to charge the battery at a constant current charge rate of 1C until the battery voltage reached 4.2V, and then discharge it at a constant current discharge rate of 1C until the battery voltage reached 2.5V. Each of these charge / discharge cycles was counted as one cycle, and 300 charge / discharge cycles were performed. Afterward, the discharge capacity was detected, and the battery capacity Q after 300 cycles was determined. ２ The following was calculated: The battery capacity Q calculated above. １ Q ２ From this, the capacity retention rate E after 300 cycles was calculated. This capacity retention rate is the cycle characteristic, and "cycle characteristic" = Q ２ / Q １ It can be calculated by multiplying by 100. The cycle characteristics of Example 1 were 85%.

[0104] (Rate Characteristics) The rate characteristics of the lithium-ion secondary battery were also determined. The rate characteristics were measured using a secondary battery charge / discharge test apparatus. The rate characteristics were evaluated as a percentage (%) with a voltage range of 4.2V to 2.5V, and 1C = 1000mAh per full cell design capacity. The rate characteristics are the ratio of the discharge capacity when CCCV charging (constant current constant voltage charging, termination current value of 0.05C) is performed at a current of 0.2C and discharged at a current of 0.2C to the discharge capacity when CCCV charging (constant current constant voltage charging, termination current value of 0.05C) is performed at a current of 0.2C and discharged at a current of 0.2C, and are expressed by the following formula (1): (2C capacity retention rate (%)) = (2C discharge capacity) / (0.2C discharge capacity) × 100 ... (1) The rate characteristics of Example 1 were 84%.

[0105] "Examples 2-4" In Examples 2-4, the average primary particle size of the silicon particles constituting the composite particles was changed from that in Example 1. The average primary particle size of the silicon particles was adjusted by changing the particle size of the raw materials used when producing the composite particles. In Examples 2-4, lithium-ion secondary batteries were produced in the same manner as in Example 1, and the characteristics of each lithium-ion secondary battery were evaluated.

[0106] Examples 5-12 differ from Example 1 in that the average diameter or average length of the carbon nanotubes attached to the composite particles was changed. The size of the carbon nanotubes was adjusted by changing the size of the carbon nanotubes added to the solution during manufacturing. In Examples 5-12, lithium-ion secondary batteries were fabricated in the same manner as in Example 1, and the characteristics of each lithium-ion secondary battery were evaluated.

[0107] "Examples 13 and 14" Examples 13 and 14 differ from Example 1 in that two types of carbon nanotubes with different diameters and lengths were prepared to be attached to the composite particles. Of the two types of carbon nanotubes, the shorter one was designated as the first carbon nanotube, and the longer one as the second carbon nanotube. The presence of two types of carbon nanotubes can be confirmed by creating a distribution of their frequency relative to diameter or length. If two peaks are observed, it can be said that two types of carbon nanotubes are present. In this case, the average diameter and average length of each carbon nanotube can be obtained as the mode of the respective peaks. In Examples 13 and 14, lithium-ion secondary batteries were fabricated in the same manner as in Example 1, and the characteristics of the lithium-ion secondary batteries were evaluated.

[0108] "Comparative Example 1" Comparative Example 1 differs from Example 1 in that carbon nanotubes were not attached to the composite particles. In Comparative Example 1, a lithium-ion secondary battery was fabricated in the same manner as in Example 1, and the characteristics of the lithium-ion secondary battery were evaluated.

[0109] "Comparative Example 2" Comparative Example 2 differs from Example 1 in that the conditions for attaching carbon nanotubes to the composite particles were changed in that the pH of the solvent was altered. In Comparative Example 2, a lithium-ion secondary battery was fabricated in the same way as in Example 1, and the characteristics of the lithium-ion secondary battery were evaluated. When the surface of the negative electrode active material of Comparative Example 2 was examined by SEM, it was found that the carbon nanotubes were not attached across multiple carbonaceous particles, and the multiple carbonaceous particles were not connected by carbon nanotubes.

[0110] "Comparative Examples 3-6" Comparative Examples 3-6 involved changing the crystallinity of the silicon particles and carbonaceous materials constituting the composite particles. These crystallinities were adjusted by changing the heat treatment conditions after the composite particles were fabricated. In Comparative Examples 3-6, lithium-ion secondary batteries were fabricated in the same manner as in Example 1, and the characteristics of each lithium-ion secondary battery were evaluated.

[0111] "Comparative Examples 7 and 8" In Comparative Examples 7 and 8, the average primary particle size of the silicon particles constituting the composite particles was changed from that in Example 1. The average primary particle size of the silicon particles was adjusted by changing the particle size of the raw materials used when producing the composite particles. In Comparative Examples 7 and 8, lithium-ion secondary batteries were produced in the same manner as in Example 1, and the characteristics of each lithium-ion secondary battery were evaluated.

[0112] The conditions and measurement results for Examples 1-14 and Comparative Examples 1-8 are summarized in Table 1.

[0113]

[0114] Examples 1 to 14 all exhibited superior cycle characteristics and rate characteristics compared to Comparative Examples 1 to 8. Comparative Example 1 is thought to have failed to achieve sufficient cycle characteristics and rate characteristics because it did not contain carbon nanotubes. Comparative Example 2 is thought to have failed to achieve sufficient improvement in cycle characteristics and rate characteristics because the attachment position of the carbon nanotubes was inappropriate. Comparative Examples 3 to 6 are thought to have had high crystallinity of the primary particles, which fixed the lithium ion conduction pathway and reduced the rate characteristics. Comparative Examples 7 and 8 are thought to have had inappropriate average primary particle diameters of the negative electrode active material, which led to side reactions between the electrolyte and the negative electrode active material and reduced cycle characteristics.

[0115] 1. Negative electrode active material 2. Composite particles 2A. Silicon particles 2B. Carbonaceous particles 3. Carbon nanotubes 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 generation element 50. Outer casing 52. Metal foil 54. Resin layer 60, 62. Terminals 100. Lithium-ion secondary battery

Claims

1. A negative electrode active material comprising composite particles and carbon nanotubes, wherein the composite particles have a plurality of carbonaceous particles and a plurality of silicon particles, the plurality of carbonaceous particles and the plurality of silicon particles are each amorphous, the average primary particle diameter of the plurality of silicon particles is 1 nm or more and 50 nm or less, and the carbon nanotubes connect two or more of the plurality of carbonaceous particles on the surface of the composite particles.

2. The negative electrode active material according to claim 1, wherein the average diameter of the carbon nanotubes is 0.5 nm or more and 3.0 nm or less, and the average length of the carbon nanotubes is 1.0 μm or more and 2.0 μm or less.

3. The negative electrode active material according to claim 1, wherein the primary particle diameter of the silicon particles is 10 nm or less.

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

5. A lithium-ion secondary battery comprising a negative electrode for a lithium-ion secondary battery, a positive electrode, and an electrolyte, wherein the electrolyte is located between the negative electrode and the positive electrode for the lithium-ion secondary battery.

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