Electrode, power storage device, and method for manufacturing electrode

Abstract

To provide an electrode, an electricity storage device, and an electrode manufacturing method that can achieve both high charge / discharge efficiency and high input / output.SOLUTION: An electrode according to the present disclosure is an electrode used in an electricity storage device, which contains a carbon material containing graphite as an electrode active material, and in a real value C' of capacitance obtained by measuring the impedance of an uncharged electrode, the real value C' per unit mass of the carbon material at -30°C and 1 Hz is 50 mF / g or more, and the real value C' per unit mass of the carbon material at 60°C and 0.01 Hz is 150 mF / g or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] Disclosed herein are an electrode, an electricity storage device, and a method for manufacturing the electrode. [Background technology]

[0002] Conventionally, when lithium-ion secondary batteries are used in vehicles as energy storage devices, they require high input / output characteristics to improve fuel efficiency, rapid charging characteristics for short charging times, and high energy density, which affects driving range and vehicle compatibility. Graphite particles are often used as the negative electrode active material in lithium-ion secondary batteries. Graphite can insert and extract lithium ions at a low potential of 0.1 V relative to the lithium reference potential, enabling high battery voltages. Graphite used as a negative electrode can be broadly divided into natural graphite, which is extracted and refined from mines, and artificial graphite, which is made by graphitizing coke. Generally, as shown in Non-Patent Document 1, artificial graphite is more durable, but the manufacturing process, which requires graphitization, is complex. On the other hand, natural graphite does not require a graphitization process, but suffers from reduced charge / discharge efficiency due to excessive edge surface activity. In other words, achieving both input / output and energy density for electrodes containing graphite as an active material is a challenge for high-performance batteries. Various technologies have been proposed to improve the charge-discharge efficiency of natural graphite anodes. Technologies reported include coating the graphite particle surface with an amorphous carbon coating (see, for example, Patent Documents 1 and 2), controlling the sphericity (Patent Document 3), doping with heteroelements (Patent Document 4), and having Si-O-C bonds on the particle surface (Patent Document 5). It has also been reported that high-rate charge-discharge characteristics can be improved by using a graphite anode with controlled capacitance (see, for example, Patent Document 6). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-19048 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-10651 [Patent Document 3] Japanese Patent Application Publication No. 2019-175852 [Patent Document 4] Japanese Patent Application Laid-Open No. 2016-213204 [Patent Document 5] Japanese Patent Application Publication No. 2020-64876 [Patent Document 6] Japanese Patent Application Laid-Open No. 2015-53116 [Non-patent literature]

[0004] [Non-Patent Document 1] J. Electrochem. Soc., 164, A3545-3555 (2017) Summary of the Invention [Problem to be solved by the invention]

[0005] However, while it is possible to improve charge / discharge efficiency using improved natural graphite, as exemplified by Patent Documents 1 to 5, the tradeoff is often a decrease in input / output characteristics. In particular, when the specific surface area of ​​graphite particles is reduced by coating the surface with amorphous carbon, thereby reducing the amount of coating formed on the graphite surface, there is a problem in that the surface area contributing to lithium insertion and desorption reactions is also reduced. Thus, there is a need for an electrode for an energy storage device that achieves both charge / discharge efficiency and input / output characteristics. Patent Document 6 states that capacitance value is affected by morphological defects such as coating unevenness on the negative electrode, but does not consider the surface structure of graphite particles.

[0006] The present disclosure has been made in consideration of such problems, and has as its main object to provide an electrode, an electricity storage device, and a method for manufacturing the electrode that can achieve both high charge / discharge efficiency and high input / output. [Means for solving the problem]

[0007] As a result of intensive research to achieve the above-mentioned object, the inventors have found that by coating the surface of graphite with amorphous carbon through a more appropriate treatment, it is possible to provide an electrode, an electricity storage device, and a method for manufacturing an electrode that achieve both high charge / discharge efficiency and high input / output, and have thereby completed the invention disclosed in this specification.

[0008] That is, the electrode of the present disclosure is An electrode used in an electricity storage device, The electrode contains a carbon material containing graphite as an electrode active material, and when impedance measurement is performed on an uncharged electrode, the real value C' of capacitance obtained is such that the real value C' per unit mass of the carbon material at -30°C and 1 Hz is 50 mF / g or more, and the real value C' per unit mass of the carbon material at 60°C and 0.01 Hz is 150 mF / g or less.

[0009] Alternatively, the electrode of the present disclosure may comprise: An electrode used in an electricity storage device, The electrode contains a carbon material containing graphite as an electrode active material, and the carbon material exhibits a mass loss rate of 10% by mass or more at 600°C and 70% by mass or less at 700°C when heated in air at a rate of 2°C / min in thermogravimetry.

[0010] The electricity storage device of the present disclosure includes any one of the electrodes described above.

[0011] The method for manufacturing an electrode according to the present disclosure includes: A method for manufacturing any of the electrodes described above, a coating step of coating the surface of graphite with amorphous carbon using a multi-purpose mixer and then performing a predetermined heat treatment to obtain the carbon material; an electrode formation step of producing an electrode using the carbon material; It includes: [Effects of the Invention]

[0012] The present disclosure provides an electrode, an energy storage device, and a method for manufacturing an electrode that achieve both high charge / discharge efficiency and high input / output. The reason for this effect is believed to be as follows: By appropriately coating the surface with amorphous carbon, the specific surface area of ​​graphite is reduced, thereby reducing the amount of coating formed on the graphite surface, while suppressing the reduction in the surface area contributing to lithium insertion and desorption reactions, thereby achieving both charge / discharge efficiency and input / output characteristics. The real value C' of capacitance obtained by measuring the impedance of the electrode during charging can be used as an index representing the electrode characteristics depending on the temperature and frequency. When this capacitance is within a predetermined range, the electrode can achieve both charge / discharge efficiency and input / output characteristics. Furthermore, the mass loss rate determined by thermogravimetric analysis of a carbon material used as an electrode active material is related to its reactivity with oxygen and can be used as an index representing the surface characteristics of the electrode active material. When this mass loss rate determined by thermogravimetric analysis is within a predetermined range, the electrode can achieve both charge / discharge efficiency and input / output characteristics. [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] 1 shows a temperature profile illustrating an example of heat treatment in Experimental Examples 1 to 11. [Figure 3] Thermogravimetric measurement results of carbon materials in Experimental Examples 1, 3, 5, 7, and 11. [Figure 4] Capacitance evaluation results for Experimental Examples 1, 3, 5, and 7 at -30°C. [Figure 5] Capacitance evaluation results for Experimental Examples 1, 3, 5, and 7 at 60°C. [Figure 6] FIG. 10 is a graph showing the relationship between the relative resistance value Rct and the initial charge-discharge efficiency in Experimental Examples 1 to 11. [Figure 7] Relationship diagram of capacitance real value C' at -30℃ and 60℃. [Figure 8] Thermogravimetric analysis diagram showing the relationship between mass loss rates at 600°C and 700°C. DETAILED DESCRIPTION OF THE INVENTION

[0014] [Electrodes for energy storage devices] The electrode for an electricity storage device of the present disclosure contains a carbon material containing graphite as an electrode active material. Because this electrode contains an electrode active material that inserts and desorbs carrier ions, this electricity storage device can be used in electricity storage devices such as alkali metal ion secondary batteries, hybrid capacitors, and air batteries. This electricity storage device electrode can also be used in electricity storage devices that use alkali metal ions, Group 2 ions, or the like as carrier ions. Examples of alkali metal ions include lithium ions, sodium ions, and potassium ions, with lithium ions being more preferred. Examples of Group 2 ions include magnesium ions, calcium ions, strontium ions, and barium ions. This electricity storage device electrode functions as either a positive electrode or a negative electrode depending on the potential of the electrode active material relative to the potential of the electrode active material. When lithium is used as the carrier, it is preferably used as a negative electrode. Here, lithium ion secondary batteries will be primarily described.

[0015] In this electrode, the carbon material may be graphite, at least a portion of whose surface is coated with amorphous carbon. The amorphous carbon coating the graphite may be partially or completely crystallized by heat treatment. That is, the graphite may be at least partially coated with a carbonaceous material derived from amorphous carbon. In this electrode, the coating with amorphous carbon can optimize film formation and reactivity.

[0016] Examples of graphite include natural graphite such as scaly graphite and flake graphite, and artificial graphite. Furthermore, graphite may be mechanically shape-controlled, for example, graphite particles that have been rounded to a spherical shape, or crushed. Such shape control is believed to suppress selective orientation and inhibit the insertion and desorption of carrier ions. Natural graphite, which may or may not be shape-controlled, is preferred. This is because natural graphite generally requires a simpler processing process and is less expensive than artificial graphite, which requires graphitization. Furthermore, natural graphite may have high edge surface activity, so it is preferable to adjust the edge surface activity by coating it with amorphous carbon or the like. The graphite particles may be one or more of flake natural graphite and spherical natural graphite.

[0017] The amorphous carbon preferably uniformly covers the entire surface of the graphite particles. This carbon material is believed to produce a material with less variation and allow for more appropriate control of the electrode capacitance. The thickness of this amorphous carbon coating is not particularly limited, but is preferably in the range of 10 nm to 200 nm, and more preferably in the range of 20 nm to 150 nm. This range facilitates the insertion and desorption of carrier ions and allows the specific surface area to be controlled to an appropriate value. The amorphous carbon is not particularly limited in terms of raw materials or manufacturing method, but may be, for example, carbonized coal- or petroleum-based pitch and tar (pitch, tar, tar pitch, heavy oil (e.g., asphalt), etc.). It may also be carbonized resins such as thermosetting resins (e.g., resol-type phenolic resins and furan resins) and thermoplastic resins (e.g., novolac-type phenolic resins and polyacrylonitrile (PAN) resins). In addition to the amorphous carbon, the amorphous carbon coating may also contain residues of the amorphous carbon raw materials. The amorphous carbon coating may also contain crystalline portions.

[0018] This carbon material preferably contains less than 5% by mass, more preferably 4% by mass or less, and even more preferably 3.5% by mass or less of amorphous carbon based on the total weight of graphite and amorphous carbon. Furthermore, this carbon material preferably contains 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more of amorphous carbon based on the total weight of graphite and amorphous carbon. This carbon material functions as an electrode active material, and does not contain, for example, graphite or any carbonaceous material serving as a conductive material or binder.

[0019] This electrode may be one in which, when impedance measurements are performed on the uncharged electrode, the real capacitance C' per unit mass of the carbon material at -30°C and 1 Hz is 50 mF / g or more, and the real capacitance C' per unit mass of the carbon material at 60°C and 0.01 Hz is 150 mF / g or less. This measured capacitance C' is presumed to reflect the electric double layer capacitance formed at the interface between the electrode containing the carbon material and the ion-conducting medium of the electricity storage device. A higher measured capacitance C' indicates a larger electric double layer capacitance at the interface and a higher proportion of graphite edges that serve as reaction active sites. When the measured capacitance C' exceeds 1 Hz, no electric double layer is formed, and when it is below 1 Hz, the effect of superimposition of Faraday current reduces the accuracy of capacitance evaluation. By using an electrode with a real capacitance C' of 50 mF / g or more at -30°C and 1 Hz, reaction resistance can be reduced, further improving the input / output characteristics of the electricity storage device. Furthermore, when the real value C' at 60°C and 0.01 Hz is 150 mF / g or less, the measured capacitance C' is presumed to reflect the amount of Faradaic reaction that initiates side reactions at the interface between the electrode and the ionically conductive medium of the energy storage device. It is presumed that a lower real value C' indicates that side reactions at the interface can be suppressed. By using an electrode with a real value C' of 150 mF / g or less, it is presumed that side reactions that reduce the initial charge / discharge efficiency can be suppressed, making it possible to further increase the energy density of the energy storage device.

[0020] In the real value C' of capacitance obtained by performing impedance measurement of the uncharged electrode, the real value C' at -30°C and 1 Hz is preferably higher, more preferably 53.0 mF / g or higher, and even more preferably 56.3 mF / g or higher. This real value C' may be 100 mF / g or lower. In addition, in the real value C' of capacitance obtained by performing impedance measurement of the uncharged electrode, the real value C' at 60°C and 0.01 Hz is preferably lower, more preferably 121.4 mF / g or lower, and even more preferably 118.6 mF / g or lower. This real value C' may be 10 mF / g or higher. Within such a range, a good electrode can be obtained that further achieves both charge / discharge efficiency and input / output characteristics.

[0021] Impedance measurements to determine capacitance, which indicates the physical property of an electrode, are performed as follows. Electrodes containing the above-described carbon material are placed opposite each other with a separator interposed between them to prepare a symmetrical cell. An ion-conducting medium, such as that used in electricity storage devices, is interposed between each electrode. The ion-conducting medium is a non-aqueous electrolyte solution in which a supporting salt containing Li is dissolved in a carbonate-based solvent. After performing impedance measurements using an impedance measuring device at 60°C and -30°C, the real value C' of capacitance is calculated from the obtained impedance spectrum using the following formula (1). Using the basis weight of the electrode composite, the real value C' per unit mass of the carbon material at -30°C and 1 Hz and the real value C' per unit mass of the carbon material at 60°C and 0.01 Hz were calculated.

[0022]

number

[0023] In the electrode of the present disclosure, the carbon material may be one that, when heated in air at a rate of 2°C / min, exhibits a mass loss rate of 10% by mass or more at 600°C and a mass loss rate of 70% by mass or less at 700°C, as measured by thermogravimetry. Furthermore, the carbon material preferably exhibits a mass loss rate of 11.8% by mass or more at 600°C and a mass loss rate of 762.1% by mass or less at 700°C, as measured by thermogravimetry. It is believed that the mass loss rate measured by thermogravimetry reflects the quantity and quality of edge faces that serve as reaction initiation sites on the surface of the carbon material. In this thermogravimetry, in the low-temperature range of 500 to 600°C, the lower the crystallinity, the more active sites for carrier ion insertion and desorption. On the other hand, if the mass loss rate at 700°C is high, the number of edge faces that serve as combustion reaction initiation sites is likely to be excessively large, resulting in a decrease in the initial charge / discharge efficiency. Incidentally, factors that contribute to a decrease in the initial charge / discharge efficiency include excessive film formation.

[0024] The carbon material may have a median diameter D50 measured by laser diffraction of 1 μm to 100 μm, 3 μm to 30 μm, or 5 μm to 25 μm, and may have a true specific gravity of 1.5 to 2.3, or 2.0 to 2.3.

[0025] The electrode of the present disclosure may be formed by using the above-mentioned carbon material as the electrode active material and by closely adhering the electrode active material to a current collector. Alternatively, the electrode may be formed by mixing the carbon material electrode active material with a binder and, if necessary, a conductive material, adding an appropriate solvent to form a paste-like electrode mixture, applying it to the surface of a current collector, drying it, and compressing it as necessary to increase the electrode density. The binder serves to bind the active material particles and the conductive material particles together, and may be, for example, 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; natural butyl rubber (NBR); or the like, either alone or as a mixture of two or more thereof. Aqueous binders such as cellulose-based binders or aqueous dispersions of styrene butadiene rubber (SBR) can also be used. Examples of solvents that can be used to disperse the electrode 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 can be added to water, and the active material can 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 achieve the desired thickness and shape. The conductive material is not particularly limited as long as it is an electron-conductive material that does not adversely affect the battery performance of the electrode, and for example, acetylene black, carbon black, ketjen black, carbon whisker, needle coke, carbon fiber, metals (copper, nickel, aluminum, silver, gold, etc.), or a mixture of two or more thereof can be used.Among these, as the conductive material, carbon black and acetylene black are preferable from the viewpoints of electron conductivity and coating properties. As the current collector, in addition to copper, nickel, stainless steel, titanium, aluminum, fired carbon, conductive polymers, conductive glass, Al-Cd alloys, etc., for the purpose of improving adhesion, conductivity, and reduction resistance, for example, those obtained by treating the surface of copper or the like with carbon, nickel, titanium, silver, or the like can also be used. It is also possible to subject these to surface oxidation treatment. Regarding the shape of the current collector, examples include foil-like, film-like, sheet-like, net-like, punched or expanded ones, lath bodies, porous bodies, foams, and formed bodies of fiber groups. The thickness of the current collector is, for example, 1 to 500 μm. This electrode can be used, for example, as the positive electrode or negative electrode of an energy storage device, and is preferably used as the negative electrode of a lithium-ion secondary battery.

[0026] (Energy storage device) The energy storage device of the present disclosure includes any of the electrodes described above. This energy storage device includes an electrode containing the above-described carbon material as an active material and an ion conduction medium that conducts carrier ions. This energy storage device may be a lithium-ion secondary battery having an electrode containing the carbon material as an active material as the negative electrode. That is, this energy storage device may include a positive electrode having a positive electrode active material that stores and releases lithium ions, a negative electrode having the above-described carbon material that stores and releases lithium ions as the negative electrode active material, and an ion conduction medium that is interposed between the positive electrode and the negative electrode and conducts lithium ions.

[0027] 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, and the basic composition formula Li (1-x) MnO2 (0 < x < 1, etc., the same below), and Li (1-x)Lithium manganese composite oxides such as Mn2O4, with the basic composition formula Li (1-x) Lithium cobalt composite oxides such as CoO2, with the basic composition formula Li (1-x) Lithium nickel composite oxide such as NiO2, the basic composition formula is Li (1-x) Ni a Mn b O2(a+b=1) and Li (1-x) Ni a Mn b Lithium 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 / 3 O2, etc. are preferred. The "basic composition formula" means that other elements may be included. The binder, conductive material, current collector, etc. used in the positive electrode may be those described above for the electrode, as appropriate.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] The electricity storage device preferably has a higher initial charge / discharge efficiency (%), preferably 82.5% or higher, more preferably 85.0% or higher, and even more preferably 86.0% or higher. The higher the charge / discharge efficiency, the higher the energy density. The initial charge / discharge efficiency is calculated from the initial charge and discharge capacities of an uncharged electricity storage device, which is subjected to a conditioning charge / discharge process in the range of 4.1 V to 3.0 V at a current value equivalent to 0.1 C. Furthermore, the electricity storage device preferably has a lower Rct relative ratio (%), where Rct (Ω) is the reaction resistance of the electrode when graphite not coated with amorphous carbon is used as the negative electrode active material, and is preferably 120% or lower, more preferably 110% or lower, and even more preferably 100% or lower. The reaction resistance of this electrode was measured by preparing a symmetrical cell in which a pair of electrodes containing the above carbon material were opposed to each other, and after the above conditioning charge and discharge, the electrode was charged at -30°C with an amplitude voltage of 5 mV and a main wave number range of 10 5 Measurements are made in the range of 0.002 Hz to 1.002 Hz. The arc portion of the obtained impedance spectrum is then fitted using a parallel circuit of resistance R and capacitance C to calculate the reaction resistance value Rct (Ω).

[0033] 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. The 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 lithium secondary battery 20, an ion conductive 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 above-mentioned carbon material as a negative electrode active material.

[0034] [Electrode manufacturing method] The electrode manufacturing method of the present disclosure is the above-described electrode manufacturing method. This manufacturing method includes a coating step and an electrode forming step. In the coating step, a carbon material is obtained by coating the surface of graphite with amorphous carbon using a multi-purpose mixer and performing a predetermined heat treatment. The conditions for obtaining the carbon material can be empirically determined depending on the material, shape, and dimensions of the raw graphite and amorphous carbon. In this step, a mixed raw material containing graphite and amorphous carbon is first prepared as raw materials, and a mixed powder is obtained using a multi-purpose mixer. As the raw material, either graphite or amorphous carbon described above for the carbon material can be used. The graphite raw material is preferably natural graphite, and the coating raw material is preferably coal tar pitch. The blending ratio of graphite to amorphous carbon may also be appropriately selected from the above-described range. The multi-purpose mixer is capable of performing convective mixing, shear mixing, diffusive mixing, and the like, which can, for example, spheroidize natural graphite.

[0035] In the coating step, the mixed raw material may be first heated in an oxidizing atmosphere and subjected to a first heat treatment at the first temperature, and then heated to a second temperature higher than the first temperature in an inert atmosphere and subjected to a second heat treatment. The first heat treatment may be omitted and only the second heat treatment may be performed. The first heat treatment may be performed at a temperature ranging from 250°C to 350°C, more preferably from 275°C to 325°C, for a time ranging from 0.5 to 5 hours, more preferably from 1 to 2 hours. The second heat treatment may be performed at a temperature ranging from 800°C to 1100°C, more preferably from 900°C to 1000°C, for a time ranging from 0.5 to 5 hours, more preferably from 1 to 2 hours. The first and second temperature increases may be performed at a rate ranging from 1°C / min to 20°C / min, more preferably from 5°C / min to 15°C / min. The heat treatment conditions may be appropriately selected so that the real value C' of the capacitance falls within a desired range. Alternatively, the heat treatment conditions may be appropriately selected so that the mass loss rate measured by thermogravimetric analysis falls within a desired range. The oxidizing atmosphere in which the first and second heat treatments are performed is preferably air. Examples of inert atmospheres include reduced pressure, rare gas atmospheres, and nitrogen atmospheres, with an Ar atmosphere being preferred. The first and second heat treatments may be performed within the first and second temperature ranges, and it is not necessary to maintain the temperature strictly constant.

[0036] In the electrode formation process, an electrode is fabricated using a carbon material. In this process, the above-mentioned carbon material may be used as the electrode active material, and the electrode active material and a current collector may be bonded together. Alternatively, in this process, the carbon material electrode active material, 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 is then applied to the surface of a current collector, dried, and, if necessary, compressed to increase electrode density. The components and amounts used in this electrode can be appropriately adapted from those described for the electrode. A higher amount of electrode active material is preferable from the standpoint of capacity, and is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 98% by mass or more. A lower amount of binder is preferable from the standpoint of capacity, and is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 2% by mass or less.

[0037] The above-described embodiments provide an electrode, an electricity storage device, and an electrode manufacturing method that achieve both high charge / discharge efficiency and high input / output. The reason for this effect is presumed to be as follows: By appropriately coating the surface with amorphous carbon, the specific surface area of ​​graphite is reduced, thereby reducing the amount of coating formed on the graphite surface, while suppressing the reduction in the surface area contributing to lithium insertion and desorption reactions, thereby achieving both charge / discharge efficiency and input / output characteristics. The real value C' of capacitance obtained by measuring the impedance of the electrode during charging can be used as an index representing the electrode characteristics depending on the temperature and frequency. When this capacitance is within a predetermined range, an electrode can be obtained that achieves both charge / discharge efficiency and input / output characteristics. Furthermore, the mass loss rate determined by thermogravimetric analysis of a carbon material used as an electrode active material is related to its reactivity with oxygen and can be used as an index representing the surface characteristics of the electrode active material. When this mass loss rate determined by thermogravimetric analysis is within a predetermined range, an electrode can be obtained that achieves both charge / discharge efficiency and input / output characteristics.

[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, in the above-described embodiment, the carrier of the power storage device is lithium ion, but is not limited thereto, and 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 solution is a non-aqueous electrolyte solution, but may be an aqueous electrolyte solution.

[0040] 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 considered that the 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. [Example]

[0041] Specific examples of fabricating the above-described electricity storage device electrode and electricity storage device will be described below as experimental examples. Experimental Examples 5 to 9 are examples of the present disclosure, and Experimental Examples 1 to 4 and 10 to 11 are comparative examples.

[0042] (Preparation of carbon materials) Ten grams of spherical natural graphite (Nippon Graphite Industries, CGB-10) and coal tar pitch (coating material, JFE Chemical) were weighed out to a ratio of 3.5 to 5 mass% based on the graphite, and mixed in a mixer (Nippon Coke & Co., Ltd., Multi-Purpose Mixer (MPM)) or a simple mixer (Labo Mill (LBM)) to obtain a mixed powder. The median diameter D50 of the spherical natural graphite measured by laser diffraction (MS Scientific, LUMiSizer 610) was 9.8 μm. The mixed powder was then placed in an alumina crucible, heated from room temperature to 300°C (first temperature) at a rate of 10°C / min (first heating), held at 300°C for 1-2 hours (first heat treatment), heated to 700-1000°C at a rate of 10°C / min (second heating), held at that temperature (second temperature) for 1-2 hours (second heat treatment), and then allowed to cool to room temperature to coat the graphite surface with an amorphous carbon layer. The first heating was performed in an air atmosphere (air flow), and after reaching 300°C, the air was stopped, the heat treatment device was evacuated, and then switched to Ar introduction. From then on, the atmosphere was kept in Ar atmosphere (Ar flow 50 cm). 3 The heat treatment was carried out at a rate of 1 / min. Figure 2 shows the temperature profile of the heat treatment during the production of the carbon material. The treatment conditions for the graphite material produced in this study are summarized in Table 1. The state of the amorphous carbon coating on the graphite surface was changed by controlling the mixing method, heat treatment method, heat treatment temperature, time, and pitch mixing ratio.

[0043] (Experimental Example 1) In Experimental Example 1, graphite was used that was not coated with coal tar pitch as amorphous carbon.

[0044] (Experimental Examples 2 to 10) Using 5.0 mass% coal tar pitch as amorphous carbon relative to graphite, the mixed powder was mixed with MPM and subjected to a first heat treatment at 300°C for 1 hour followed by a second heat treatment at 1000°C for 1 hour to coat the graphite surface, resulting in a carbon material designated Experimental Example 2. A carbon material was obtained by carrying out the same process as Experimental Example 2, except that the second heat treatment was carried out at 1000°C for 2 hours, resulting in a carbon material designated Experimental Example 3. A carbon material was obtained by carrying out the same process as Experimental Example 2, except that the first heat treatment was omitted, resulting in a carbon material designated Experimental Example 4. A carbon material was obtained by carrying out the same process as Experimental Example 2, except that 3.5 mass% coal tar pitch was used and the first heat treatment was omitted, resulting in a carbon material designated Experimental Example 5. A carbon material was obtained by carrying out the same process as Experimental Example 2, except that 3.5 mass% coal tar pitch was used, resulting in a carbon material designated Experimental Example 6. A carbon material was obtained by carrying out the same process as Experimental Example 2, except that 3.5 mass% coal tar pitch was used and the first heat treatment was carried out at 300°C for 2 hours, resulting in a carbon material designated Experimental Example 7. The same treatment as in Experimental Example 2 was carried out except that 3.5 mass% of coal tar pitch was used and the second heat treatment was carried out at 900°C for 1 hour, and the resulting carbon material was designated Experimental Example 8. The same treatment as in Experimental Example 2 was carried out except that 3.5 mass% of coal tar pitch was used and the second heat treatment was carried out at 800°C for 1 hour, and the resulting carbon material was designated Experimental Example 9. The same treatment as in Experimental Example 2 was carried out except that 3.5 mass% of coal tar pitch was used and the second heat treatment was carried out at 700°C for 1 hour, and the resulting carbon material was designated Experimental Example 10.

[0045] (Experimental Example 11) The same treatment as in Experimental Example 6 was carried out except that a mixed powder of coal tar pitch and graphite mixed in an LBM was used. The obtained carbon material was designated as Experimental Example 11.

[0046] (Thermogravimetric TG measurement of carbon materials) 4 mg of the carbon materials obtained above in Experimental Examples 1 to 11 were placed in an alumina pan, and thermogravimetry was performed while heating from room temperature to 500°C at a temperature increase rate of 20°C / min, and from 500 to 800°C at a temperature increase rate of 2°C / min, with air flow. TG measurements were performed using a Rigaku TG8120. From the obtained thermogravimetry results, the mass loss (mass%) at 600°C and 700°C was read.

[0047] (Preparation of electrode (negative electrode)) The carbon material, carboxymethyl cellulose, and styrene-butadiene rubber were mixed together with an aqueous solvent in a mass ratio of 98:1:1 to obtain a slurry electrode mixture, which was then applied to a copper foil. The coating weight of the electrode mixture was 4 mg / cm. 2 After vacuum drying, the electrode sheet was produced by rolling using a roll press.

[0048] (Capacitance evaluation) A symmetrical laminate cell was fabricated as an evaluation cell by placing the above electrodes opposite each other via a separator in an uncharged state. The above-mentioned opposing electrodes were placed inside the laminate, and an electrolyte solution was poured into a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 3:4:3, to which LiPF6 was added to make a 1M solution. This was used to prepare an evaluation cell. Impedance measurements were performed using an impedance measuring device at 60°C and -30°C, and the real value C' of capacitance was calculated from the obtained impedance spectrum using the above-mentioned formula (1). This impedance measurement was performed at 60°C or -30°C with an amplitude voltage of 5 mV and a main wavenumber range of 10 5 Using the basis weight of the electrode composite, the real value C' per unit mass of the carbon material at -30°C and 1 Hz, and the real value C' per unit mass of the carbon material at 60°C and 0.01 Hz were calculated.

[0049] (Electricity storage device: Fabrication of lithium-ion secondary battery) LiNi as a positive electrode active material 1 / 3 Co 1 / 3 Mn 1 / 3 A positive electrode was prepared by mixing O2, carbon black (HS-100 manufactured by Denka) as a conductive material, and PVdF (#7300 manufactured by Kureha) as a binder in a mass ratio of 90:6:4 and coating the mixture on an Al foil. After placing the positive and negative electrodes opposite each other with a separator between them, a laminate cell was prepared by injecting an electrolyte solution of EC, DMC, and EMC mixed with LiPF6 to a concentration of 1M.

[0050] (Evaluation of electricity storage devices) Conditioning charge and discharge were performed in the range of 4.1 V to 3.0 V at a current value equivalent to 0.1 C, and the initial charge and discharge efficiency (%) was calculated. After several cycles of conditioning charge and discharge, impedance measurements were performed at -30°C and 3.7 V (remaining capacity SOC 50%), and the reaction resistance Rct was calculated. For the impedance measurement to calculate the reaction resistance Rct, a symmetrical cell was prepared with two electrodes facing each other, and the frequency range was 10 5 Measurements were carried out in the frequency range of 100 to 0.002 Hz. The arc portion of the obtained impedance spectrum was fitted using a parallel circuit of resistance R and capacitance C to calculate the reaction resistance Rct (Ω). Note that the reaction resistance Rct values ​​of Experimental Examples 2 to 11 were normalized by setting the measured value of Experimental Example 1 as 100.

[0051] (Results and Discussion) FIG. 3 shows the results of thermogravimetric measurements of the carbon materials of Experimental Examples 1, 3, 5, 7, and 11. FIG. 4 shows the results of capacitance evaluation at -30°C for Experimental Examples 1, 3, 5, and 7. FIG. 5 shows the results of capacitance evaluation at 60°C for Experimental Examples 1, 3, 5, and 7. FIG. 6 shows a graph illustrating the relationship between the relative resistance value Rct and the initial charge-discharge efficiency for Experimental Examples 1 to 11. FIG. 7 shows a graph illustrating the relationship between the actual capacitance value C' at -30°C and 60°C for Experimental Examples 1 to 11. FIG. 8 shows the relationship between the mass loss rates at 600°C and 700°C in thermogravimetric analysis for Experimental Examples 1 to 11. Table 1 summarizes the outline of the coating treatment for Experimental Examples 1 to 11, the thermogravimetric analysis results, the capacitance evaluation results at -30°C-1 Hz and 60°C-0.1 Hz, the initial charge-discharge efficiency (%), and the relative resistance value Rct (%).

[0052] As shown in Figure 3, thermogravimetric analysis revealed that the mass loss behavior associated with the combustion reaction with oxygen in the air varies significantly depending on the surface structure of the carbon material. To examine the surface condition in the thermogravimetric analysis, the mass loss rates at 600°C and 700°C were measured and are shown in Table 1. Furthermore, as shown in Figures 4 and 5, capacitance evaluation results indicated that the capacitance of the negative electrode was significantly affected when carbon materials with different coal tar pitch blending amounts and heat treatment conditions were used as the negative electrode. To examine the surface condition, the real value C' at 1 Hz indicated by the arrow in Figure 4 was measured at -30°C, and the real value C' at 0.01 Hz at 60°C was measured and are summarized in Table 1 and Figure 6.

[0053] Carbon materials in which the surface of graphite is coated with amorphous carbon can improve charge-discharge efficiency by coating the surface with amorphous carbon. However, this coating can also increase reaction resistance, making it necessary to achieve both. Here, the relationship between the relative reaction resistance value Rct and the initial charge-discharge efficiency in Figure 6 indicates that electrodes with lower reaction resistance and higher charge-discharge efficiency, indicated by the white arrows, have better performance. As shown in Figure 6, Experimental Examples 5 to 9 demonstrated excellent charge-discharge characteristics, with reaction resistance values ​​kept below 120% and initial charge-discharge efficiencies of 82.5% or higher, achieving both reduced reaction resistance and improved initial charge-discharge efficiencies. Among these, Experimental Examples 6 to 8 demonstrated even better charge-discharge characteristics, with reaction resistance values ​​kept below 110% and initial charge-discharge efficiencies of 85% or higher.

[0054] In the capacitance evaluation results shown in Figures 4 and 5, the real value C' is 0 at high frequencies. However, as the frequency decreases, the real value C' increases from around 100 Hz. This is because, while only spontaneous polarization of the electrolyte bulk occurs at high frequencies, an electric double layer forms at the electrode / electrolyte interface as the frequency decreases. A sufficient electric double layer is clearly formed around 1 Hz, but as the frequency decreases, the real value C' increases at a gentle slope. This behavior is thought to be due to leakage current generated at the electrode / electrolyte interface due to the Faradaic reaction. Based on the results shown in Figure 6, we investigated the relationship between the capacitance and the electrode's improved charge / discharge characteristics (Figure 7). Electrode capacitance can be used as an index of electrode performance at low and high temperatures. The battery performance shown in Figure 6 demonstrates that Experiments 5 to 9 achieved both low reaction resistance and high initial charge / discharge efficiency. As shown in Figure 7, when impedance measurements were performed on uncharged electrodes corresponding to Experimental Examples 5 to 9, the real capacitance value C' was measured. It was estimated that a real value C' of 50 mF / g or more, preferably 53.0 mF / g or more, and even more preferably 56.3 mF / g or more at -30°C and 1 Hz increased the electric double layer capacitance at the electrode / electrolyte interface, thereby promoting smooth charge transfer reactions at the interface and further reducing reaction resistance. Similarly, in Experimental Examples 5 to 9, it was also found that initial charge / discharge efficiencies were higher when the real value C' at 60°C and 0.01 Hz was 150 mF / g or less, more preferably 121.4 mF / g or less, and even more preferably 118.6 mF / g or less. This is likely due to the fact that the real value C' at 60°C, which is an index of increased Faradaic reaction, reduces the amount of coating formed on the graphite surface during initial charge / discharge due to reductive decomposition of the electrolyte.

[0055] Based on the results shown in Figure 6, we also investigated the relationship between the mass loss rate of carbon materials measured by thermogravimetric analysis and the improved charge-discharge characteristics (Figure 8). The mass loss rate of carbon materials used as electrode active materials, measured by thermogravimetric analysis due to a combustion reaction with oxygen in air, was also confirmed to be related to battery performance, suggesting that it could be used as an index of the surface properties of electrode active materials. As shown in Figure 8, thermogravimetric measurements revealed that, when heated in air at a rate of 2°C / min, the mass loss rate at 600°C was 10% by mass or more, preferably 11.8% by mass or more, and the mass loss rate at 700°C was 70% by mass or less, preferably 62.1% by mass or less, in Experimental Examples 5 to 9, reducing reaction resistance and improving initial charge-discharge efficiency. While the reasons for this are unclear, it is speculated that, for example, in carbon materials with lower crystallinity in the low-temperature range of 500 to 600°C, the more active sites for lithium ion insertion and extraction are present. On the other hand, when the mass loss rate at 700°C is high, it is presumed that the initial charge / discharge efficiency decreases because there are too many edge faces that serve as the starting points for the combustion reaction.

[0056] [Table 1]

[0057] It goes without saying that the present disclosure is not limited to the above-described embodiments, and can be embodied in various forms as long as they fall within the technical scope of the present disclosure. [Industrial Applicability]

[0058] The present invention can be used in technical fields related to secondary batteries. [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. An electrode used in an electricity storage device, The electrode contains a carbon material containing graphite as an electrode active material, and in the real value C' of capacitance obtained by performing impedance measurement of the uncharged electrode, the real value C' per unit mass of the carbon material at -30°C and 1 Hz is 50 mF / g or more, and the real value C' per unit mass of the carbon material at 60°C and 0.01 Hz is 150 mF / g or less. electrode.

2. 2. The electrode according to claim 1, wherein the real value C' of the capacitance per unit mass of the carbon material at -30°C and 1 Hz is 53.0 mF / g or more, and the real value C' per unit mass of the carbon material at 60°C and 0.01 Hz is 121.4 mF / g or less.

3. 3. The electrode according to claim 1, wherein the real value C' of the capacitance per unit mass of the carbon material at -30°C and 1 Hz is 56.3 mF / g or more, and the real value C' per unit mass of the carbon material at 60°C and 0.01 Hz is 118.6 mF / g or less.

4. The electrode according to any one of claims 1 to 3, wherein the carbon material exhibits, in thermogravimetry, a mass loss rate of 10% by mass or more at 600°C and a mass loss rate of 70% by mass or less at 700°C when heated in air at a rate of 2°C / min.

5. 5. The electrode according to claim 4, wherein the carbon material exhibits a mass loss rate of 11.8% by mass or more at 600°C and a mass loss rate of 62.1% by mass or less at 700°C in the thermogravimetric measurement.

6. The electrode according to any one of claims 1 to 5, wherein the carbon material has one or more of the characteristics (1) or (2). (1) In the carbon material, at least a portion of the surface of the graphite is coated with amorphous carbon. (2) The carbon material contains natural graphite in which the graphite has been spheroidized.

7. The electrode according to any one of claims 1 to 6, An electricity storage device comprising:

8. A method for producing an electrode according to any one of claims 1 to 6, comprising: a coating step of coating the surface of graphite with amorphous carbon using a multi-purpose mixer and then performing a predetermined heat treatment to obtain the carbon material; an electrode formation step of producing an electrode using the carbon material; A method for manufacturing an electrode comprising the steps of:

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