Active material composite particle, electrode, producing method for active material composite particle, and producing method for electrode

Abstract

To provide an active material composite particle, an electrode, a producing method for the active material composite particle, and a producing method for the electrode, which are capable of obtaining suitable characteristics when an active material layer is formed by a dry process.SOLUTION: An active material composite particle of the present disclosure includes: a carbon material that is an electrode active material; and a nitrogen component present on a surface of the carbon material, and satisfies a ratio IG / ID of a G band intensity IG to a D band intensity ID of a Raman spectrum of 1 or more and 3 or less. A producing method for the active material composite particle may include a coating step of coating the surface of the carbon material, which is the electrode active material, with a nitrogen-containing compound to produce a coated body, and may contain a heat treatment step of heat-treating the coated body at a heat treatment temperature of 800°C or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to an active material composite particle, an electrode, a method for manufacturing an active material composite particle, and a method for manufacturing an electrode. [Background technology]

[0002] Previously, active material composite particles for use in power storage devices have been proposed in which graphite is coated with a polymer having a polycyclic structure (see, for example, Patent Document 1). It is claimed that the polycyclic structure of this active material composite particle specifically acts on the graphite surface, suppressing the activity of the graphite surface and thereby suppressing gas generation due to a reaction with the electrolyte. It is also claimed that the polymer selectively adsorbs onto the basal plane of graphite, thereby reducing the increase in resistance due to the polymer blocking the edge planes of the graphite. Another proposal has been made in which graphite is coated with petroleum pitch and heat-treated at 1000°C (see, for example, Non-Patent Document 1). It is claimed that the pitch-derived amorphous carbon coating in this active material composite particle effectively reduces irreversible capacity and improves cycle stability. Another proposal has been made in which graphite with an average particle size of 3 to 20 μm and amorphous carbon with an average particle size of 500 nm or less are rubbed together to form a composite, which is then further mixed with a carbon phase source such as coal tar pitch or polyvinyl alcohol and heat-treated at 900 to 1200°C (see, for example, Patent Document 2). The active material composite particles are said to be capable of maintaining discharge capacity and initial discharge efficiency while exhibiting excellent input / output characteristics at low temperatures. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-43316 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-100208 [Non-patent literature]

[0004] [Non-Patent Document 1] Korean J. Chem. Eng., 36(10), 1724-1731(2019) Summary of the Invention [Problem to be solved by the invention]

[0005] Recently, in order to reduce carbon dioxide emissions, attention has been focused on techniques for forming active material layers by dry film formation without using organic dispersion media. However, Patent Documents 1 and 2 and Non-Patent Document 1 do not consider forming active material layers by a dry method, and when active material layers are formed by a dry method, desired properties are not always obtained.

[0006] The present disclosure has been made to solve these problems, and has as its main object to provide an active material composite particle, an electrode, a method for manufacturing an active material composite particle, and a method for manufacturing an electrode that provide favorable properties when an active material layer is formed by a dry process. Note that "dry process" refers to using the active material in a powder state, not in a slurry or the like. [Means for solving the problem]

[0007] After extensive research to achieve the above-mentioned object, the inventors came up with the idea of ​​producing active material composite particles by coating the surface of a carbon material such as graphite with a nitrogen-containing compound such as polyacrylonitrile, and optionally heat-treating the surface at 800°C or less. The active material composite particles thus obtained have nitrogen components present on the surface of the carbon material, and a ratio IG / ID of the G-band intensity IG to the D-band intensity ID in a Raman spectrum of 1 or more and 3 or less. The inventors then discovered that the use of these active material composite particles results in favorable properties when an active material layer is formed by a dry process, leading to the completion of the invention disclosed in this specification.

[0008] That is, the active material composite particles of the present disclosure contain a carbon material that is an electrode active material and a nitrogen component present on the surface of the carbon material, and the ratio IG / ID of the G band intensity IG to the D band intensity ID in the Raman spectrum satisfies 1 or more and 3 or less.

[0009] The electrode of the present disclosure has an active material layer containing the above-described active material composite particles.

[0010] The method for producing active material composite particles of the present disclosure includes a coating step of coating the surface of a carbon material, which is an electrode active material, with a nitrogen-containing compound to produce a coated body, and may also include a heat treatment step of heat treating the coated body at a heat treatment temperature of 800°C or less.

[0011] The method for manufacturing an electrode according to the present disclosure includes a composite particle manufacturing step of manufacturing the active material composite particles by the above-described method for manufacturing active material composite particles, and an active material layer forming step of forming an active material layer by a dry process using the active material composite particles. [Effects of the Invention]

[0012] The present disclosure provides an active material composite particle, an electrode, a method for manufacturing an active material composite particle, and a method for manufacturing an electrode that exhibit favorable characteristics when an active material layer is formed by a dry process. The reason for this effect is believed to be as follows. For example, when an active material layer is formed by a wet process, the binder dissolves in a solvent and spreads evenly over the active material surface, protecting the active sites of the active material, thereby suppressing decomposition of the ion-conducting medium and the resulting degradation of properties. However, the active material is coated with the binder, resulting in reduced electrical conductivity and ionic conductivity. On the other hand, when an active material layer is formed by a dry process, the binder adheres to the active material surface in particulate form, reducing the coverage of the active material and making it difficult to protect the active sites. The present disclosure is believed to suppress decomposition of the ion-conducting medium and the resulting degradation of properties by pre-coating the surface of a carbon material, such as graphite, as an electrode active material with a nitrogen-containing compound to protect the active sites. Furthermore, the high ionic conductivity of a coating of a nitrogen-containing compound, such as polyacrylonitrile, and the high electronic conductivity of a heat-treated coating thereof, are believed to improve the properties of an electricity storage device. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 2 is a schematic diagram showing an example of an electricity storage device 20. [Figure 2]FIG. 2 is an explanatory diagram conceptually illustrating an example of a method for producing active material composite particles. [Figure 3] FIG. 2 is an explanatory diagram illustrating the structural change of polyacrylonitrile due to heat. [Figure 4] SEM images and nitrogen mapping images of samples of Experimental Examples 16, 6, and 9. [Figure 5] Raman spectra of samples from Experimental Examples 1 to 16. [Figure 6] 1 is a graph showing the relationship between the heat treatment temperature and the initial discharge capacity of the samples of Experimental Examples 1 to 16. DETAILED DESCRIPTION OF THE INVENTION

[0014] [Active material composite particles] The active material composite particles of the present disclosure are used, for example, in electrodes of electricity storage devices. The electricity storage device may be, for example, a secondary battery such as an alkali metal ion secondary battery, a hybrid capacitor, or an air battery. The electricity storage device may be one in which the charge carrier is an alkali metal or a Group 2 metal. Examples of alkali metals include lithium, sodium, and potassium, with lithium being preferred. Examples of Group 2 metals include magnesium, calcium, strontium, and barium. The electrode can be either a positive electrode or a negative electrode depending on the potential of the counter electrode, but when lithium is used as the carrier, it is preferably a negative electrode. In this embodiment, the case in which the active material composite particles are used as a negative electrode of a lithium ion secondary battery will be mainly described.

[0015] The active material composite particles contain a carbon material as an electrode active material and a nitrogen component present on the surface of the carbon material. Examples of carbon materials include activated carbon, coke, glassy carbon, graphite, non-graphitizable carbon, pyrolytic carbon, carbon fiber, and carbon nanotubes. Among these, coke, glassy carbon, graphite, non-graphitizable carbon, pyrolytic carbon, and carbon fiber, which are capable of absorbing and desorbing lithium ions, are preferred, with graphite being more preferred. Examples of graphite include natural graphite such as scaly graphite and flake graphite, and artificial graphite. The graphite may be mechanically shape-controlled, e.g., graphite particles that have been rounded, rounded, or crushed. At least a portion of the surface of the graphite may be coated with amorphous carbon or a carbonaceous material. The particle diameter of the carbon material may be 1 μm or more and 100 μm, and the particle diameter of the active material composite particles may be approximately the same as the particle diameter of the carbon material. The particle size may be the median diameter D50 measured by laser diffraction. The nitrogen component may be, for example, nitrogen in a nitrogen-containing compound or nitrogen in a thermally denatured product of a nitrogen-containing compound, as described below. Examples of nitrogen-containing compounds include polyacrylonitrile. Examples of thermally denatured products of nitrogen-containing compounds include cyclized polyacrylonitride. The thermally denatured product of a nitrogen-containing compound may have, for example, a pyridine ring structure, or may be a pyridine monomer or a compound in which multiple pyridine ring structures are linked by a condensed ring or the like. The nitrogen component is preferably uniformly dispersed on the surface of the graphite. For example, in a nitrogen mapping image obtained using a scanning electron microscope (SEM) and energy dispersive X-ray fluorescence spectroscopy (EDX), it is preferable that no aggregated portions with a diameter of 1 μm or more are observed, and more preferably no aggregated portions with a diameter of 0.5 μm or more are observed. The proportion of the nitrogen component in the active material composite particles may be 0.5% by mass or more, 0.6% by mass or more, or 1% by mass or more. The proportion of the nitrogen component in the active material composite particles may be 5% by mass or less, 3% by mass or less, or 2.5% by mass or less. The proportion of the nitrogen component in the active material composite particles may be determined, for example, by multiplying the proportion of the nitrogen-containing compound in the coating, which will be described later in the coating step, by the proportion of nitrogen in the nitrogen-containing compound.

[0016] The active material composite particles satisfy the ratio IG / ID of the G band intensity IG to the D band intensity ID in the Raman spectrum of 1 or more and 3 or less. The preferred range of the IG / ID value varies depending on, for example, the proportion of the nitrogen-containing compound in the coating, which will be explained in the coating step described later, but if the IG / ID value is 1 or more and 3 or less, relatively suitable properties can be obtained. In the Raman spectrum of a carbon material, the peak at 1580 cm -1 Near and 1350cm -1 A peak is observed around 1580 cm. -1 The G band is a peak observed around 1350 cm in the Raman spectrum, and is a peak derived from the in-plane vibration of the six-membered ring, which is common in carbon-based materials. For example, the more pure benzene ring structures there are, the stronger the intensity of this G band becomes. The D band is observed around 1350 cm in the Raman spectrum. -1 This peak is observed around 1 / 1000, and is due to defects (including bonds with heteroatoms) in the carbon hexagonal mesh (graphene sheet). For example, the intensity of this D band increases when defects are introduced into the carbon material by some kind of physical treatment or when the sp3 carbon is increased by chemical modification. The value of I / I may be, for example, 2.7 or less, 2.5 or less, 2.0 or less, or 1.7 or less. The value of I / I may also be, for example, 1.1 or more, 1.2 or more, or 1.3 or more.

[0017] [electrode] The electrode of the present disclosure has an active material layer containing the above-described active material composite particles. This active material layer may contain the above-described active material composite particles and a binder, or may further contain a conductive material. Note that, since the above-described active material composite particles are conductive, the active material layer does not need to contain a conductive material. The active material layer may be formed on a current collector. Examples of the binder include fluorine-containing resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and fluororubber, thermoplastic resins such as polypropylene and polyethylene, ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, and natural butyl rubber (NBR). Examples of the binder include aqueous binders such as cellulose-based binders and styrene butadiene rubber (SBR). Examples of the conductive material include acetylene black, carbon black, ketjen black, carbon whiskers, needle coke, carbon fiber, and metals (copper, nickel, aluminum, silver, gold, etc.). Examples of current collectors include copper, nickel, stainless steel, titanium, aluminum, calcined carbon, conductive polymers, conductive glass, and Al-Cd alloys. Examples of current collectors include those obtained by treating the surface of copper or other materials with carbon, nickel, titanium, or silver to improve adhesion, conductivity, and reduction resistance. Examples of current collector shapes include foil, film, sheet, net, punched or expanded, lath, porous, foamed, and fibrous structures. The thickness of the current collector can be, for example, 1 to 500 μm. In this electrode, the proportion of the active material composite particles may be 80% by mass or more, 90% by mass or more, or 95% by mass or more. In this electrode, the proportion of the active material composite particles may be 99% by mass or less, or 98% by mass or less. This electrode can be used, for example, as the positive or negative electrode of an electricity storage device, and is preferably used as the negative electrode of a lithium-ion secondary battery.

[0018] This electrode may also have a discharge capacity of 270 mAh / g or more per mass of the active material layer. This discharge capacity is preferably 300 mAh / g or more, more preferably 310 mAh / g or more, still more preferably 320 mAh / g or more, and even more preferably 330 mAh / g or more. This discharge capacity may be the discharge capacity in the first charge-discharge, or may be the discharge capacity after 3-cycle charge-discharge.

[0019] [Power storage device] The power storage device of the present disclosure includes the above-described electrode. This power storage device includes an electrode containing the above-described active material composite particles and an ion conductive medium that conducts carrier ions. This power storage device may be a lithium ion secondary battery having an electrode containing active material composite particles as a negative electrode. That is, this power storage device may include a positive electrode having a positive electrode active material that occludes and releases lithium ions, a negative electrode containing the above-described active material composite particles that occludes and releases lithium ions, and an ion conductive medium that is interposed between the positive electrode and the negative electrode and conducts lithium ions.

[0020] The positive electrode may be formed by, for example, mixing a positive electrode active material, a conductive material, and a binder, adding an appropriate solvent to form a paste-like positive electrode mixture, applying and drying it on the surface of a current collector, and compressing it as necessary to increase the electrode density. As the positive electrode active material, sulfides containing transition metal elements, oxides containing lithium and transition metal elements, etc. can be used. Specifically, transition metal sulfides such as TiS2, TiS3, MoS3, FeS2, basic composition formula Li (1-x) MnO2 (0 < x < 1, etc., the same hereinafter), lithium manganese composite oxides such as Li (1-x) Mn2O4, lithium cobalt composite oxides with a basic composition formula such as Li (1-x) CoO2, lithium nickel composite oxides with a basic composition formula such as Li (1-x) NiO2, basic composition formula Li (1-x) Ni a Mn b O2 (a + b = 1) or Li (1-x) Ni a Mn bLithium nickel manganese composite oxides such as O4 (a+b=2), with the basic composition formula Li (1-x) Ni a Co b Mn c Lithium nickel cobalt manganese composite oxides such as O2 (a+b+c=1), lithium vanadium composite oxides such as LiV2O3, and transition metal oxides such as V2O5 can be used. (1-x) Olivine-type lithium manganese phosphate compounds such as MnPO4, with the basic composition formula Li (1-x) Olivine-type lithium cobalt phosphate compounds such as CoPO4, with the basic composition formula Li (1-x) Olivine-type lithium nickel phosphate compounds such as NiPO4 can be used. (1-x) Inverse spinel lithium manganese vanadate compounds such as MnVO4, with the basic composition formula Li (1-x) Inverse spinel lithium cobalt vanadate compounds such as CoPO4, with the basic composition formula Li (1-x) Inverse spinel lithium vanadate nickel compounds such as NiPO4 can be used. The positive electrode active material is preferably an oxide containing one or more of nickel, manganese, and cobalt, such as LiCoO2, LiNiO2, LiMnO2, and LiNi 1 / 3 Co 1 / 3 Mn 1 / 3O2 is preferred. The term "basic composition formula" means that other elements may be included. The binder, conductive material, current collector, and other materials used in the positive electrode may be those described above for the electrode, as appropriate. Examples of solvents that can be used to disperse the active material, conductive material, and binder include organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran. Alternatively, a dispersant, thickener, or the like may be added to water, and the active material may be slurried with a latex such as SBR. Examples of thickeners that can be used include polysaccharides such as carboxymethyl cellulose and methyl cellulose, either alone or in combination. Examples of application methods include roller coating (e.g., applicator roll), screen coating, doctor blade coating, spin coating, and bar coating. Any of these methods can be used to create a desired thickness and shape.

[0021] The ion-conducting medium of the electricity storage device can be a non-aqueous electrolyte solution containing a supporting salt, a non-aqueous gel electrolyte solution, etc. Examples of the solvent for the non-aqueous electrolyte solution include carbonates, esters, ethers, nitriles, furans, sulfolanes, and dioxolanes, which can be used alone or in combination. Specific examples of carbonates include cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, and chloroethylene carbonate; chain carbonates such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t-butyl carbonate, di-i-propyl carbonate, and t-butyl-i-propyl carbonate; cyclic esters such as γ-butyl lactone and γ-valerolactone; chain esters such as methyl formate, methyl acetate, ethyl acetate, and methyl butyrate; ethers such as dimethoxyethane, ethoxymethoxyethane, and diethoxyethane; nitriles such as acetonitrile and benzonitrile; furans such as tetrahydrofuran and methyltetrahydrofuran; sulfolanes such as sulfolane and tetramethylsulfolane; and dioxolanes such as 1,3-dioxolane and methyldioxolane. Among these, a combination of cyclic carbonates and chain carbonates is preferred. This combination not only provides excellent cycle characteristics, which represent the battery characteristics during repeated charge and discharge, but also allows for a well-balanced electrolyte viscosity, the resulting battery's electrical capacity, and battery output. Note that cyclic carbonates have a relatively high dielectric constant, which is thought to increase the dielectric constant of the electrolyte, while chain carbonates are thought to suppress the viscosity of the electrolyte.

[0022] Examples of supporting salts include LiPF, LiBF, LiAsF, LiCF, SO, LiN(CF, SO), LiC(CF, SO), LiSbF, LiSiF, LiAlF, LiSCN, LiClO, LiCl, LiF, LiBr, LiI, and LiAlCl. From the viewpoint of electrical properties, it is preferable to use one or more salts selected from the group consisting of inorganic salts such as LiPF, LiBF, LiAsF, and LiClO, and organic salts such as LiCF, SO, LiN(CF, SO), and LiC(CF, SO). The concentration of this supporting salt in the nonaqueous electrolyte is preferably 0.1 mol / L to 5 mol / L, and more preferably 0.5 mol / L to 2 mol / L. A sufficient current density can be obtained when the supporting electrolyte is dissolved at a concentration of 0.1 mol / L or higher, and the electrolyte can be made more stable at a concentration of 5 mol / L or lower. In addition, a phosphorus-based, halogen-based, or other flame retardant may be added to this non-aqueous electrolyte.

[0023] Instead of a liquid ion-conducting medium, a solid ion-conducting polymer can be used as the ion-conducting medium. Examples of the ion-conducting polymer include polymer gels composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinylpyrrolidone, or vinylidene fluoride and a supporting salt. Furthermore, a combination of an ion-conducting polymer and a nonaqueous electrolyte can also be used. In addition to ion-conducting polymers, other ion-conducting media include inorganic solid electrolytes, mixed materials of organic polymer electrolytes and inorganic solid electrolytes, and inorganic solid powders bound by an organic binder.

[0024] This electricity storage device may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it has a composition that can withstand the range of use of the electricity storage device, but examples thereof include polymer nonwoven fabrics such as polypropylene nonwoven fabrics and polyphenylene sulfide nonwoven fabrics, and thin microporous films of olefin resins such as polyethylene and polypropylene. These may be used alone or in combination.

[0025] The shape of this electricity storage device is not particularly limited, and examples thereof include coin, button, sheet, laminate, cylindrical, flat, and rectangular shapes. The device may also be applied to large devices used in electric vehicles and the like. FIG. 1 is a schematic diagram showing an example of an electricity storage device 20 of the present disclosure. This electricity storage device 20 includes a cup-shaped battery case 21, a positive electrode 22 having a positive electrode active material and disposed at the bottom of the battery case 21, a negative electrode 23 having a negative electrode active material and disposed opposite the positive electrode 22 with a separator 24 interposed therebetween, a gasket 25 formed of an insulating material, and a sealing plate 26 disposed at the opening of the battery case 21 and sealing the battery case 21 via the gasket 25. In this electricity storage device 20, an ion-conducting medium 27 is filled in the space between the positive electrode 22 and the negative electrode 23. The negative electrode 23 is an electrode containing the active material composite particles described above.

[0026] [Method of manufacturing active material composite particles] The method for producing an active material composite particle according to the present disclosure may be a method for producing the above-described active material composite particle. This production method includes a coating step. This production method may further include a heat treatment step. Each step will be described below. Note that the description of the same configurations as those described for the above-described active material composite particle will be omitted as appropriate.

[0027] (Coating process) In the coating step, a nitrogen-containing compound is coated on the surface of a carbon material, which is an electrode active material, to produce a coating. The carbon material can be the same as the carbon material described above for the active material composite particles, with graphite being preferred. The nitrogen-containing compound is preferably an organic compound, and more preferably an organic polymer. For example, polyacrylonitrile can be suitably used as the nitrogen-containing compound. In the coating step, the carbon material and the nitrogen-containing compound are mixed in a solvent capable of dissolving the nitrogen-containing compound, and the solvent is removed using an evaporator or the like as necessary, thereby coating the surface of the carbon material with the nitrogen-containing compound. By dissolving the nitrogen-containing compound in a solvent, segregation of the nitrogen-containing compound can be further suppressed, and the nitrogen-containing compound can be more uniformly distributed on the surface of the carbon material. In the coating step, the carbon material may be dispersed in a solvent and stirred, and then the nitrogen-containing compound may be added. Alternatively, the nitrogen-containing compound may be dissolved in a solvent and then mixed with the carbon material. Examples of the solvent that can be used include organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran. Of these, dimethylformamide is preferred.

[0028] This coating step produces a coated body containing a carbon material as an electrode active material and a nitrogen-containing compound formed on the surface of the carbon material. This coated body contains a carbon material as an electrode active material and a nitrogen component (nitrogen in the nitrogen-containing compound) present on the surface of the carbon material, and may be the active material composite particle described above. The proportion of the nitrogen-containing compound in the coated body is preferably 2% by mass or more, more preferably 2.5% by mass or more, and even more preferably 3% by mass or more. The proportion of the nitrogen-containing compound in the coated body is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 12% by mass or less. The proportion of the nitrogen-containing compound in the coated body can be adjusted, for example, by adjusting the compounding ratio of the carbon material and the nitrogen-containing compound.

[0029] (Heat treatment process) In the heat treatment step, the coated body is heat-treated at a heat treatment temperature of 800°C or less. The preferred range of the heat treatment temperature varies depending on, for example, the proportion of the nitrogen-containing compound in the coated body. However, a heat treatment temperature of 800°C or less is thought to be able to suppress the loss of nitrogen components on the surface of the carbon material, which may occur due to decomposition of the nitrogen-containing compound. The heat treatment temperature may be, for example, 700°C or less, 650°C or less, or 600°C or less. The heat treatment temperature may be, for example, 200°C or more, 250°C or more, or 300°C or more. The heat treatment atmosphere is preferably an inert atmosphere such as an argon atmosphere. The heat treatment time may be, for example, 1 minute or more, 10 minutes or more, or 30 minutes or more. The heat treatment time may be, for example, 24 hours or less, 12 hours or less, or 2 hours or less.

[0030] In this heat treatment step, although a part or all of the nitrogen-containing compound formed on the surface of the carbon material is modified by the heat treatment, it is presumed that the nitrogen component remains on the surface of the carbon material. The particles obtained by this heat treatment step may be the above-mentioned active material composite particles. The thermally modified product of the nitrogen-containing compound may be, for example, cyclized polyacrylonitrile. The thermally modified product of the nitrogen-containing compound may be, for example, one having a pyridine ring structure, or may be a pyridine monomer or one in which multiple pyridine ring structures are linked by a condensed ring or the like.

[0031] The active material composite particle described above can be produced by the method for producing the active material composite particle described above. FIG. 2 is an explanatory diagram conceptually illustrating an example of the method for producing the active material composite particle of the present disclosure. In FIG. 2, the carbon material is graphite (G), the nitrogen-containing compound is polyacrylonitrile (PAN), and dimethylformamide (DMF) is used as the solvent for dissolving the PAN. FIG. 2A is an explanatory diagram of the coating step, and FIG. 2B is an explanatory diagram of the heat treatment step. As shown in FIG. 2A, mixing graphite with PAN in a DMF solvent produces a coated body in which the surface of the graphite is coated with PAN. This coated body is also referred to as G / PAN. In G / PAN, the PAN coating protects the active edge plane of the graphite and prevents direct contact of the active points of the graphite with the electrolyte. Furthermore, as shown in FIG. 2B, heat treatment of G / PAN causes thermal denaturation of the PAN, such as cyclization and dehydrocarbonation, resulting in particles in which a thermally denatured product of PAN is formed on the surface of the graphite. These particles are also referred to as G / PAN-XXX. Here, XXX is the heat treatment temperature (e.g., 300 to 600). In G / PAN-XXX, it is believed that one or more of the compounds shown in Figure 3 are formed on the graphite surface depending on the heat treatment temperature, etc. Figure 3 is an explanatory diagram illustrating the thermal structural changes of polyacrylonitrile. When polyacrylonitrile is heat-treated, the triple bonds of the nitrile groups are converted to double bonds, which bond with the carbon of adjacent nitrile groups to form an imine structure and cyclize. Further heat treatment generates sp2 carbons through dehydrogenation, completing the cyclization. This cyclization of PAN is thought to give it aromaticity, which is believed to suppress a decrease in electrical conductivity. However, heat treatment at temperatures exceeding 800°C results in the loss of nitrogen and a graphite-like structure. Because graphite has active edge planes as described above, it is thought that the effect of PAN coating on protecting the active edge planes of graphite is reduced. For this reason, in this disclosure, the heat treatment temperature is set to 800°C or less.

[0032] [Electrode manufacturing method] The electrode manufacturing method of the present disclosure may be a manufacturing method for manufacturing the above-described electrode. This electrode manufacturing method includes a composite particle manufacturing step and an active material layer forming step. Note that the description of the same components as those described for the above-described electrode will be omitted as appropriate.

[0033] (Composite particle manufacturing process) In the composite particle production step, active material composite particles are produced by the above-described method for producing active material composite particles. In this composite particle production step, only the coating step may be performed, or the coating step and the heat treatment step may be performed.

[0034] (Active material layer formation process) In the active material layer forming step, an active material layer is formed by a dry process using the active material composite particles described above. In the active material layer forming step, an active material layer may be formed using an electrode mixture containing active material composite particles and a binder, and the electrode mixture may further contain a conductive material. In the active material layer forming step, an active material layer may be formed on a current collector. The binder, conductive material, and active material may be the same as the binder, conductive material, and active material described above for the active material. The term "dry process" refers to using the active material composite particles or electrode mixture in a powder state rather than in a slurry state. For example, even if a liquid such as a solvent or dispersion medium is added, the amount of the liquid may be 10% by mass or less, 3% by mass or less, 2% by mass or less, or 1% by mass or less, based on the active material composite particles or electrode mixture without the addition of the liquid. In the active material layer forming step, it is more preferable not to add a liquid such as a solvent or dispersion medium to the active material composite particles or electrode mixture. Examples of methods for forming the active material layer include coating methods using roller coating such as an applicator roll, screen coating, a doctor blade method, spin coating, a bar coater, a die coater, etc. As a dry formation method, for example, electrostatic coating using an electrostatic screen or the like is suitable.

[0035] In the active material layer forming step, the active material layer may be heated after it is formed. In this case, the heating temperature may be, for example, 100°C to 250°C, or 150°C to 250°C. The heating atmosphere may be a reducing atmosphere, an inert atmosphere, or the air atmosphere. In addition, in the active material layer forming step, the active material layer may be compressed after it is formed. In this case, for example, compression may be performed using a flat press or a roll press. When compression is performed using a flat press, compression may be performed at a pressure of, for example, 1 MPa to 100 MPa, 2 MPa to 50 MPa, or 3 MPa to 10 MPa. When compression is performed using a roll press, compression may be performed at a roll speed of, for example, 0.1 m / min to 10 m / min, or 0.2 m / min to 5 m / min. When compression is performed using a roll press, compression may be performed at a linear pressure of, for example, 10 kg / cm to 110 kg / cm, or 30 kg / cm to 90 kg / cm. In the active material layer forming step, after the active material layer is formed, it may be compressed while being heated, for example, by hot pressing.

[0036] The above-described embodiments provide an active material composite particle, an electrode, a method for manufacturing an active material composite particle, and a method for manufacturing an electrode that exhibit favorable characteristics when an active material layer is formed by a dry process. The reason for this effect is believed to be as follows. For example, when an active material layer is formed by a wet process, the binder dissolves in a solvent and spreads evenly over the active material surface, protecting the active sites of the active material. This prevents decomposition of the ion-conducting medium and the associated degradation of its properties. However, the active material is coated with the binder, resulting in reduced electrical conductivity and ionic conductivity. On the other hand, when an active material layer is formed by a dry process, the binder adheres to the active material surface in particulate form, reducing the coverage of the active material and making it difficult to protect the active sites. In this embodiment, it is believed that pre-coating the surface of a carbon material, such as graphite, as an electrode active material with a nitrogen-containing compound to protect the active sites prevents decomposition of the ion-conducting medium and the associated degradation of its properties. Furthermore, it is believed that the high ionic conductivity of a coating of a nitrogen-containing compound, such as polyacrylonitrile, and the high electronic conductivity of a heat-treated coating thereof can improve the properties of an electricity storage device. In particular, PAN fibers have high ionic conductivity, and it is thought that coating them will not hinder the conduction of lithium ions or electrons, thereby increasing the initial discharge capacity, increasing film strength, and improving capacity retention.

[0037] Conventionally, electrode films (active material layers) are formed by dispersing electrode materials in a liquid dispersion medium such as an organic solvent, applying the resulting slurry to a current collector foil, drying it, and pressing it to form an electrode film. However, dry film formation, which does not use an organic dispersion medium, is gaining attention as a next-generation electrode film fabrication method to reduce CO2 emissions. However, dry film formation has been found to pose challenges, such as a reduced binder coverage on the active material, exposing active sites on the active material surface, promoting SEI formation and electrolyte decomposition, resulting in significant gas generation and a reduced initial discharge capacity. To achieve carbon neutrality through dry film formation, it is necessary to suppress this reduction in initial discharge capacity and improve capacity retention. This disclosure achieves reduced discharge capacity and improved capacity retention by appropriately protecting active sites on the active material. This is expected to not only reduce the manufacturing costs of lithium-ion batteries, but also improve electrical capacity and capacity retention. Furthermore, this disclosure eliminates the need for high-temperature heat treatment, e.g., at temperatures exceeding 800°C, thereby reducing energy costs associated with heat treatment.

[0038] It goes without saying that the present disclosure is not limited to the above-described embodiments, and can be implemented in various forms as long as they fall within the technical scope of the present disclosure.

[0039] For example, although the above-described embodiment discloses a method for manufacturing an electrode by a dry process, the electrode of the present disclosure may be manufactured by a wet process. When manufactured by a wet process, for example, the active material composite particles, a binder, and optionally a conductive material may be mixed, and an appropriate solvent may be added to form a paste-like electrode mixture. This paste may then be applied to the surface of a current collector, dried, and compressed as necessary to increase the electrode density. The solvent and application method may be appropriately selected from those described for the positive electrode of the electricity storage device.

[0040] In the above-described embodiment, the carrier of the power storage device is lithium ion, but is not limited thereto. It may be alkali metal ion such as sodium ion or potassium ion, or Group 2 element ion such as calcium ion or magnesium ion. The positive electrode active material may contain carrier ions. The electrolyte is a non-aqueous electrolyte, but may be an aqueous electrolyte.

[0041] In the above-described embodiment, the positive electrode active material is a transition metal composite oxide, but it is not particularly limited thereto, and may be, for example, a carbon material used in a capacitor. The carbon material is not particularly limited, but examples thereof include activated carbons, cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers, carbon nanotubes, and polyacenes. Among these, activated carbons exhibiting a high specific surface area are preferred. Activated carbons as carbon materials have a specific surface area of ​​1000 m 2 / g or more, and 1500m 2 / g or more is more preferable. 2 / g or more, the discharge capacity can be further increased. The specific surface area of ​​this activated carbon is 3000 m 2 / g or less, and 2 It is more preferable that the positive electrode has a capacitance of 1 / 2 sq. m / s or less. It is considered that such a positive electrode stores electricity by adsorbing and desorbing at least one of anions and cations contained in the ionically conductive medium, but it may also store electricity by inserting and desorbing at least one of anions and cations contained in the ionically conductive medium.

[0042] The present disclosure may be any of the following [1] to

[10] . [1] Active material composite particles comprising a carbon material as an electrode active material and a nitrogen component present on the surface of the carbon material, wherein the ratio IG / ID of the G band intensity IG to the D band intensity ID in a Raman spectrum satisfies 1 or more and 3 or less. [2] The active material composite particles according to [1], wherein the carbon material is graphite, and the nitrogen component is nitrogen in polyacrylonitrile or nitrogen in cyclized polyacrylonitrile. [3] An electrode having an active material layer containing the active material composite particle according to [1] or [2]. [4] The electrode according to [3], wherein the discharge capacity per mass of the active material layer is 300 mAh / g or more. [5] A method for producing active material composite particles, which includes a coating step of coating the surface of a carbon material, which is an electrode active material, with a nitrogen-containing compound to produce a coating body, and which may also include a heat treatment step of heat treating the coating body at a heat treatment temperature of 800°C or less. [6] The method for producing active material composite particles according to [5], wherein in the coating step, graphite is used as the carbon material and polyacrylonitrile is used as the nitrogen-containing compound. [7] The method for producing active material composite particles according to [5] or [6], wherein the coating step produces the coated body containing the nitrogen-containing compound in an amount of 2.5 mass % or more and 20 mass % or less. [8] The method for producing active material composite particles according to any one of [5] to [7], wherein the heat treatment step heat-treats the coating at a temperature of 300°C or higher and 600°C or lower. [9] The method for producing active material composite particles according to any one of [5] to [8], wherein in the coating step, the carbon material and the nitrogen-containing compound are mixed in a solvent capable of dissolving the nitrogen-containing compound.

[10] A method for manufacturing an electrode, comprising: a composite particle manufacturing step of manufacturing the active material composite particles by the method for manufacturing the active material composite particles according to any one of [5] to [9]; and an active material layer forming step of forming an active material layer by a dry process using the active material composite particles. [Example]

[0043] Examples in which the active material composite particles of the present disclosure were specifically examined will be described below as examples, with Experimental Examples 1 to 15 corresponding to working examples of the present disclosure and Experimental Example 16 corresponding to a comparative example.

[0044] [Production of active material composite particles] (Experimental Example 1) 9.75 g of graphite (Showa Denko, SCMG-AFC, hereafter abbreviated as G) was weighed out and dispersed in 10 mL of dimethylformamide (Fujifilm Wako, hereafter abbreviated as DMF) with stirring. 5 mL of a 5 mass% PAN / DMF solution was added to the dispersion so that the amount of polyacrylonitrile (Aldrich, Mw 150,000, hereafter abbreviated as PAN) added was equivalent to 2.5 mass%, and the DMF was removed using an evaporator while stirring. The resulting sample, G / PAN2.5, was designated Experimental Example 1.

[0045] (Experimental Examples 2 to 5) 10 g of G / PAN2.5 was prepared and heat-treated under an Ar stream at 300°C for 1 hour. The resulting sample, G / PAN2.5-300, was designated as Experimental Example 2. 10 g of G / PAN2.5 was prepared and heat-treated under an Ar stream at 400°C for 1 hour. The resulting sample, G / PAN2.5-400, was designated as Experimental Example 3. 10 g of G / PAN2.5 was prepared and heat-treated under an Ar stream at 600°C for 1 hour. The resulting sample, G / PAN2.5-600, was designated as Experimental Example 4. 10 g of G / PAN2.5 was prepared and heat-treated under an Ar stream at 800°C for 1 hour. The resulting sample, G / PAN2.5-800, was designated as Experimental Example 5.

[0046] (Experimental Example 6) 9.5 g of G was weighed out and dispersed in 10 mL of DMF, followed by stirring. 10 mL of a 5% by mass PAN / DMF solution was added thereto so that the amount of PAN added was equivalent to 5% by mass, and the DMF was removed with stirring using an evaporator. The resulting sample, G / PAN5, was designated Experimental Example 6.

[0047] (Experimental Examples 7-10) 10 g of G / PAN5 was prepared and heat-treated under an Ar stream at 300°C for 1 hour, resulting in sample G / PAN5-300, designated as Experimental Example 7. 10 g of G / PAN5 was prepared and heat-treated under an Ar stream at 400°C for 1 hour, resulting in sample G / PAN5-400, designated as Experimental Example 8. 10 g of G / PAN5 was prepared and heat-treated under an Ar stream at 600°C for 1 hour, resulting in sample G / PAN5-600, designated as Experimental Example 9. 10 g of G / PAN5 was prepared and heat-treated under an Ar stream at 800°C for 1 hour, resulting in sample G / PAN5-800, designated as Example 10.

[0048] (Experimental Example 11) 9 g of G was weighed and dispersed in 10 mL of DMF, followed by stirring. 20 mL of a 5% by mass PAN / DMF solution was added to the dispersion so that the PAN content was equivalent to 10% by mass, and the DMF was removed with stirring using an evaporator. The resulting sample, G / PAN10, was designated Experimental Example 11.

[0049] (Experimental Examples 12-15) 10 g of G / PAN10 was prepared and heat-treated under an Ar stream at 300°C for 1 hour. The resulting sample, G / PAN10-300, was designated as Experimental Example 12. 10 g of G / PAN10 was prepared and heat-treated under an Ar stream at 400°C for 1 hour. The resulting sample, G / PAN10-400, was designated as Experimental Example 13. 10 g of G / PAN10 was prepared and heat-treated under an Ar stream at 600°C for 1 hour. The resulting sample, G / PAN10-600, was designated as Experimental Example 14. 10 g of G / PAN10 was prepared and heat-treated under an Ar stream at 800°C for 1 hour. The resulting sample, G / PAN10-800, was designated as Experimental Example 15.

[0050] (Experimental Example 16) G itself was used as the sample for Experimental Example 16.

[0051] [SEM observation and EDX analysis] The samples of Experimental Examples 1 to 16 were observed using a scanning electron microscope (SEM) and analyzed by energy-dispersive X-ray fluorescence spectroscopy (EDX). Figure 4 shows the SEM observation and nitrogen mapping results for Experimental Examples 16, 6, and 9. Figures 4A and 4B show the SEM image and nitrogen mapping image, respectively, of Experimental Example 16 (G). Figures 4C and 4D show the SEM image and nitrogen mapping image, respectively, of Experimental Example 6 (G / PAN5). Figures 4E and 4F show the SEM image and nitrogen mapping image, respectively, of Experimental Example 9 (G / PAN5-600). G / PAN5 of Experimental Example 6 was synthesized by mixing G and PAN in DMF and drying the solvent. The polymer did not appear to aggregate or be unevenly distributed during the drying process (Figure 4C), and the nitrogen component was uniformly distributed over the entire surface of the G particles (Figure 4D). In Experimental Example 9, G / PAN5-600 was prepared by heat-treating G / PAN5 at 600°C. However, the particle shape and nitrogen distribution were unchanged compared to G / PAN5, and no polymer shrinkage or uneven distribution due to heat treatment occurred. In Experimental Examples 1 to 15, purchased solid PAN particles were crushed and used. However, even after crushing, the particles remained in the μm range. Therefore, when graphite particles and PAN particles were mechanically mixed in a mixer, polymer particles of the same size as the graphite were observed, which may lead to uneven distribution of nitrogen. In Experimental Examples 1 to 15, graphite particles and PAN were mixed in solution, which is presumably why nitrogen was observed more uniformly on the graphite surface, as shown in the N element distributions in Figures 4D and 4F.

[0052] [Structural analysis by Raman spectroscopy] Figure 5 shows the Raman spectra of the samples from Experimental Examples 1 to 15. Figure 5A shows the Raman spectra of Experimental Examples 1 to 5 and 16, Figure 5B shows the Raman spectra of Experimental Examples 6 to 10 and 16, and Figure 5C shows the Raman spectra of Experimental Examples 11 to 15 and 16. Experimental Example 16 was graphite, and because it had few substituents, there were few defects in the benzene ring structure, resulting in a small D band. When PAN was coated on this sample, the D band tended to increase. This is presumably due to the increased sp3 carbon on the particle surface caused by the attachment of PAN to the graphite surface. Furthermore, the D band tended to increase with increasing heat treatment temperature. This is presumably due to the increased formation of pyridine rings due to cyclization of nitrile groups. The pyridine ring, in which one carbon atom in the benzene ring is replaced by N, contributes to the broadening of the D band.

[0053] To quantify the proportion of surface substituents and defects, the intensity ratio of the G band to the D band, IG / ID, was calculated. The results are summarized in Table 1. The IG / ID value decreased with increasing heat treatment temperature. This is presumably due to the increased formation of pyridine rings. Furthermore, the pyridine rings exhibit aromaticity, which is presumably responsible for improved electrical conductivity. The IG / ID value of G / PAN2.5 was generally smaller than that of G / PAN5. However, the IG / ID value of the particles with PAN attached to G generally decreased with increasing treatment temperature. This is presumably due to the increased number of pyridine rings formed from polyacrylonitrile on the G surface. However, the D band suddenly decreased in the sample heat-treated at 800°C. This is presumably due to the substitution of N atoms with carbon to form a graphite structure during heat treatment at temperatures above 800°C, resulting in an increase in the G band and a decrease in the D band, resulting in an increase in the IG / ID value.

[0054] [Electrode preparation] 0.80 g of the powder sample of Experimental Examples 1 to 16 as the active material composite particles and 0.022 g (equivalent to approximately 2.5 wt%) of PVdF (particle diameter 200 nm) as a binder were mixed and coated onto a copper current collector foil using an electrostatic screen. The mixture was heated to 180°C and compressed into a powder film using a flat press at 5 MPa. The mixture was then punched out with a 16 mm diameter punch to prepare an electrode. The raw material composition of the electrode, where the total of graphite, PAN, and PVdF is taken as 100%, was 95.1 mass% graphite, 2.4 mass% PAN, and 2.5 mass% PVdF in Experimental Examples 1 to 5. In Experimental Examples 6 to 10, the raw material composition was 92.6 mass% graphite, 4.9 mass% PAN, and 2.5 mass% PVdF. In Experimental Examples 11 to 15, the graphite content was 87.8 mass %, the PAN content was 9.8 mass %, and the PVdF content was 2.5 mass %.

[0055] [Evaluation of energy storage devices] Using the above-described electrodes, a lithium-ion battery was fabricated as an energy storage device. Because the electrodes and electrolyte of lithium-ion batteries are susceptible to moisture and oxygen, they were assembled in an argon-filled glove box. Specifically, a polyethylene (PE) separator was sandwiched between the above-described electrodes and a Li metal counter electrode. 250 μL of electrolyte was added, and the assembly was fastened with bolts and nuts to fabricate an assembled cell. The electrolyte was a mixture of equal volumes of ethylene carbonate (EC) and diethyl carbonate (DEC), to which LiPF6 was added to achieve a 1M concentration. The fabricated lithium-ion battery was pre-discharged to 0 V at a constant current of 0.5 mA. The first cycle involved charging from 0 V to 1.2 V at a constant current of 0.5 mA. The amount of coulombs stored during this charge was calculated and used as the initial charge capacity. The battery was charged to 1.2 V and then discharged again to 0 V. The amount of coulombs released was calculated and used as the initial discharge capacity. The difference between the initial charge capacity and the initial discharge capacity is called the irreversible capacity and is often expressed as a percentage. Similar to the first cycle, charge and discharge were performed from 0 V to 1.2 V for the second and third cycles, and the ratio of the discharge capacity in the third cycle to the discharge capacity in the first cycle was compared as the capacity retention rate after three cycles. The capacity was expressed as the capacity per mass of the active material layer.

[0056] Table 1 summarizes the evaluation results of the initial discharge capacity, discharge capacity after three cycles, and capacity retention, which is the ratio of the discharge capacity after three cycles to the initial discharge capacity, of the lithium-ion batteries fabricated using each sample of Experimental Examples 1 to 16. FIG. 6 also shows graphs of the initial discharge capacity values ​​of Experimental Examples 1 to 16. FIG. 6A is a graph for Experimental Examples 16 and 1 to 5, FIG. 6B is a graph for Experimental Examples 6 to 10, and FIG. 6C is a graph for Experimental Examples 11 to 15. Experimental Examples 1 to 15 had improved initial discharge capacity, discharge capacity after three cycles, and capacity retention, compared to Experimental Example 16, which used graphite alone. Among these, Experimental Examples 7 to 9, in which G / PAN5 was heat-treated at 300 to 600°C, and Experimental Examples 12 to 14, in which G / PAN10 was heat-treated at 300 to 600°C, showed significantly increased initial discharge capacity. In Experimental Examples 6 and 11, which were not heat-treated, the initial discharge capacity increase was smaller than in Experimental Examples 7–9 and Experimental Examples 12–14. The decrease in discharge capacity of the 800°C-heat-treated samples in Experimental Examples 10 and 15 is presumed to be due to partial decomposition of the polymer consisting of linked pyridine rings formed by the heat treatment of PAN. The discharge capacity after three cycles in Experimental Example 16 was significantly lower than the initial discharge capacity, with a retention rate of approximately 66%. This decrease in capacity retention is presumed to be due to the deterioration of the electrode film and its peeling from the current collector foil. Because graphite repeatedly expands and contracts in volume due to the absorption and release of Li ions during charge and discharge, the electrode film itself is prone to cracking and peeling from the current collector foil. In contrast, in Experimental Examples 1–15, the discharge capacity after three cycles maintained more than 90% of the initial discharge capacity. This is presumed to be due to the PAN coating of the graphite particles mitigating the deterioration of the electrode film due to volume expansion and contraction during charge and discharge.

[0057] [Table 1] [Industrial Applicability]

[0058] The present disclosure is applicable to the technical field of electricity storage devices. [Explanation of symbols]

[0059] 20 Energy storage device, 21 Battery case, 22 Positive electrode, 23 Negative electrode, 24 Separator, 25 Gasket, 26 Sealing plate, 27 Ion conducting medium.

Claims

1. 1. An active material composite particle comprising a carbon material as an electrode active material and a nitrogen component present on the surface of the carbon material, wherein the ratio I / I of a G band intensity I to a D band intensity I in a Raman spectrum satisfies 1 or more and 3 or less.

2. 2. The active material composite particle according to claim 1, wherein the carbon material is graphite, and the nitrogen component is nitrogen in polyacrylonitrile or nitrogen in cyclized polyacrylonitrile.

3. An electrode having an active material layer containing the active material composite particle according to claim 1 or 2.

4. 4. The electrode according to claim 3, wherein the discharge capacity per mass of the active material layer is 300 mAh / g or more.

5. a coating step of coating a surface of a carbon material, which is an electrode active material, with a nitrogen-containing compound to produce a coating body, A heat treatment step of heat treating the coating body at a heat treatment temperature of 800°C or less may be included. A method for producing active material composite particles.

6. The method for producing active material composite particles according to claim 5 , wherein in the coating step, graphite is used as the carbon material and polyacrylonitrile is used as the nitrogen-containing compound.

7. The method for producing active material composite particles according to claim 5 or 6, wherein the coating step produces the coated body containing the nitrogen-containing compound in an amount of 2.5% by mass or more and 20% by mass or less.

8. The method for producing active material composite particles according to claim 5 or 6, wherein the heat treatment step heat-treats the coating at a temperature of 300°C or higher and 600°C or lower.

9. 7. The method for producing active material composite particles according to claim 5, wherein in the coating step, the carbon material and the nitrogen-containing compound are mixed in a solvent capable of dissolving the nitrogen-containing compound.

10. a composite particle production step of producing the active material composite particles by the method of producing the active material composite particles according to claim 5 or 6; an active material layer forming step of forming an active material layer by a dry process using the active material composite particles; A method for manufacturing an electrode, comprising:

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