Alkaline metal rechargeable battery

Abstract

To provide alkaline metal secondary batteries with improved charge / discharge efficiency and energy density. [Solution] An alkali metal secondary battery comprising an electrode containing an active material capable of intercepting and releasing alkali metal ions, a counter electrode containing a carbon nanotube layer with a thickness of 12 μm to 70 μm, and an electrolyte, wherein the amount of alkali metal X equivalent to the theoretical capacity of the active material contained in the electrode, and the sum of the amounts of alkali metal Y contained in the electrode and the counter electrode satisfy 0.7 ≤ X / Y ≤ 1.1.

Description

Technical Field

[0001] The present disclosure relates to an alkali metal secondary battery.

Background Art

[0002] In recent years, the demand for electric vehicles, hybrid vehicles, unmanned aerial vehicles, etc. has been increasing, and accordingly, various secondary batteries have been developed as power storage devices capable of storing more electrical energy.

[0003] Among secondary batteries that perform charge and discharge by the movement of metal ions between a positive electrode and a negative electrode, an alkali metal secondary battery, which is a secondary battery having an alkali metal in a metallic state at the negative electrode, has attracted attention because it can achieve a high energy density.

[0004] For example, Patent Document 1 proposes a negative electrode sheet of a sodium ion battery including a negative electrode current collector and a carbon material coating formed on at least a part of the surface of the negative electrode current collector, the thickness of the carbon material coating being 10 μm or less, and the carbon material coating including a carbon material and a polymer adhesive. Further, Patent Document 2 describes a device including an anode, a counter electrode or a cathode having a metal lithium layer and a support layer with a specific thickness, and an electrolyte, and it is described that the support layer may include a carbon nanotube-based layer.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] In alkali metal secondary batteries, there is a need to improve both charge / discharge efficiency and energy density.

[0007] According to the negative electrode piece for a sodium battery described in Patent Document 1, metallic sodium is deposited on the surface of the negative electrode current collector during the initial charge, causing the deposited metallic sodium to adhere to the carbon material coating on the surface of the negative electrode current collector. Therefore, the technology described in Patent Document 1 is not yet sufficient in terms of suppressing the decrease in charge and discharge efficiency caused by the deposition of metallic sodium on the surface of the negative electrode current collector and improving charge and discharge efficiency. Furthermore, the apparatus described in Patent Document 2 forms an anode consisting of a metallic lithium layer and a carbon nanotube-based support layer before the initial charge. Therefore, in terms of suppressing the decrease in energy density originating from the metallic lithium layer contained in the electrode before the initial charge and improving the energy density, there is still room for improvement in the technology described in Patent Document 2.

[0008] One embodiment of this disclosure aims to solve the problem of providing an alkali metal secondary battery with improved charge-discharge efficiency and energy density. [Means for solving the problem]

[0009] This disclosure includes the following aspects: <1> An alkali metal secondary battery comprising an electrode containing an active material capable of intercepting and releasing alkali metal ions, a counter electrode containing a carbon nanotube layer with a thickness of 12 μm to 70 μm, and an electrolyte, wherein the amount of alkali metal X equivalent to the theoretical capacity of the active material contained in the electrode, and the sum of the amounts of alkali metal Y contained in the electrode and the counter electrode satisfy 0.7 ≤ X / Y ≤ 1.1.

[0010] <2> The G / D intensity obtained by Raman measurement of the above carbon nanotube layer is 2.5 or higher. <1> Alkaline metal rechargeable batteries as described above. <3> The alkali metal mentioned above is lithium or sodium. <1> or <2> Alkaline metal rechargeable batteries as described above. [Effects of the Invention]

[0011] According to one embodiment of the present disclosure, it is possible to provide an alkali metal secondary battery with improved charge-discharge efficiency and energy density. [Brief explanation of the drawing]

[0012] [Figure 1] This is a schematic diagram showing one aspect of the layer configuration of the alkali metal secondary battery of the present disclosure before the first charge. [Modes for carrying out the invention]

[0013] The following describes in detail an example of an embodiment relating to this disclosure. The following description may be based on a typical embodiment of this disclosure, but this disclosure is not limited to such embodiments and can be implemented with appropriate modifications within the scope of the purpose of this disclosure.

[0014] In this disclosure, a numerical range indicated using "~" means a range that includes the numbers before and after "~" as the minimum and maximum values, respectively. In the numerical ranges described in stages in this disclosure, the upper or lower limit stated in one numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Furthermore, in the numerical ranges described in this disclosure, the upper or lower limit stated in one numerical range may be replaced with the values ​​shown in the examples. In this disclosure, a combination of two or more preferred embodiments is a more preferred embodiment. In this disclosure, unless otherwise specified, the amount of each component refers to the total amount of multiple substances if there are multiple substances corresponding to each component. In this specification, the term "process" includes not only independent processes but also processes that cannot be clearly distinguished from other processes, as long as their intended purpose is achieved.

[0015] [Alkali metal secondary battery] The alkali metal secondary battery according to this disclosure comprises an electrode containing an active material capable of intercalating and releasing alkali metal ions, a counter electrode containing a carbon nanotube layer with a thickness of 12 μm to 70 μm, and an electrolyte, wherein the amount of alkali metal X equivalent to the theoretical capacity of the active material contained in the electrode, and the sum Y of the amounts of alkali metal contained in the electrode and the counter electrode satisfy 0.7 ≤ X / Y ≤ 1.1. In the following text, the terms "carbon nanotube" and "carbon nanotube layer" may be abbreviated as "CNT" and "CNT layer," respectively.

[0016] The alkali metal secondary battery described herein is a type of so-called anode-free battery. An anode-free battery is a battery that does not contain negative electrode active material before the battery assembly process of an alkali metal secondary battery. It has an electrode containing an active material that can absorb and release alkali metal ions as a positive electrode, and a counter electrode. During charging, alkali metal ions in the positive electrode are deposited on the negative electrode material as metallic alkali metal, and during discharging, the metallic alkali metal decomposes into alkali metal ions, thereby performing charging and discharging. Because an anode-free battery does not have a layer containing negative electrode active material before the battery assembly process, the volume and mass of the secondary battery can be kept lower than conventional batteries, and the energy density per unit volume and unit mass is improved.

[0017] Figure 1 is a schematic diagram showing one aspect of the layer configuration of the alkali metal secondary battery of this disclosure before the first charge. In the embodiment of the alkali metal secondary battery 10 shown in Figure 1, an electrode (positive electrode) 12 containing an active material 12A capable of intercalating and releasing alkali metal ions, a separator 14, and a counter electrode 16 containing a CNT layer are stacked, and there is no negative electrode layer containing an active material.

[0018] According to the configuration of the alkali metal secondary battery described herein, an alkali metal secondary battery with improved charge-discharge efficiency and energy density is provided. Regarding the improvement of energy density, it is presumed that the reason why the alkali metal secondary battery according to this disclosure can achieve an improvement in energy density is that the battery structure is an anode-free battery structure, which does not contain negative electrode active material, before the battery assembly process of the alkali metal secondary battery. Regarding the improvement of charge and discharge efficiency, the reason why the alkali metal secondary battery according to this disclosure can achieve improved charge and discharge efficiency is not clear, but the inventors speculate as follows. However, the following speculation is not intended to be a restrictive interpretation of the alkali metal secondary battery according to this disclosure, but is explained as an example.

[0019] The alkali metal secondary battery described herein was developed based on the discovery that there is a correlation between exhibiting good charge-discharge efficiency in an alkali metal secondary battery having an anode-free battery structure and the counter electrode 16, which includes the CNT layer shown in Figure 1, having a specific thickness. The alkali metal secondary battery according to this disclosure has a CNT layer of a specific thickness as shown in Figure 1. Therefore, it is presumed that during charging, alkali metal ions in the electrode 12 shown in Figure 1 are deposited inside the voids of the CNT layer, and during discharge, they are decomposed into alkali metal ions inside the voids of the CNT layer, thereby improving the charge and discharge efficiency.

[0020] (An electrode containing an active material capable of intercepting and releasing alkali metal ions: positive electrode) The alkali metal secondary battery according to this disclosure satisfies the following conditions: X is the amount of alkali metal equivalent to the theoretical capacity of the active material contained in the electrode, and Y is the sum of the amounts of alkali metal contained in the electrode and the counter electrode, and 0.7 ≤ X / Y ≤ 1.1. The energy density of an alkali metal secondary battery increases when the relationship between X and Y satisfies 0.7 ≤ X / Y ≤ 1.1. In the above formula, X / Y is 0.7 or greater, preferably 0.8 or greater, more preferably 0.9 or greater, and even more preferably 0.95 or greater. When X / Y is 0.7 or higher, the amount of alkali metal that does not contribute to charging and discharging is reduced, which is thought to result in a higher energy density for the secondary battery. The upper limit for X / Y can be 1.1, and it is preferable to have it be 1.0 or lower.

[0021] In this disclosure, X / Y are determined by the following method. First, the mass of the active material in the electrode (positive electrode) containing an active material capable of intercepting and releasing alkali metal ions is measured. Next, the molar mass of the active material is calculated from its chemical formula. Then, the amount of alkali metal contained in the active material that can theoretically intercept and release through charging and discharging is calculated as the amount of substance X. Next, the amount of alkali metal contained in the electrode (positive electrode) and counter electrode, which contain an active material capable of intercalating and releasing alkali metal ions, is measured, and the total amount Y of alkali metal contained in the electrode and counter electrode is calculated. Here, alkali metal ion components originating from the electrolyte are removed from the electrode and counter electrode. When removing alkali metal ion components originating from the electrolyte, dimethyl carbonate, for example, can be suitably used as a cleaning solution. Furthermore, as a method for measuring the amount of alkali metal ions, a method of performing elemental quantitative analysis using an ICP (Inductively Coupled Plasma) mass spectrometer can be suitably used. Suitable pre-measurement processing steps include low-temperature ashing and acid decomposition using a mixed acid solution of nitric acid and hydrochloric acid.

[0022] The alkali metal is preferably lithium or sodium, and more preferably sodium. The alkali metal secondary battery relating to this disclosure may be a lithium metal secondary battery or a sodium metal secondary battery. The structure of lithium metal secondary batteries and sodium metal secondary batteries, as well as the positive electrode and positive electrode active material, are described below.

[0023] (Lithium metal secondary battery) A lithium metal secondary battery, which is one embodiment of the alkali metal secondary battery according to this disclosure, has a positive electrode, a counter electrode including a CNT layer with a thickness of 12 μm to 70 μm, and an electrolyte disposed between the positive electrode and the counter electrode. A separator may be included between the positive electrode and the counter electrode. Counter electrodes containing CNT layers with a thickness of 12 μm to 70 μm will be discussed later.

[0024] (positive electrode) The positive electrode of a lithium metal secondary battery can be manufactured by preparing a positive electrode composite material containing a positive electrode active material and a binder, and then supporting the positive electrode composite material on a positive electrode current collector. A conductive material may be further added to the positive electrode composite material.

[0025] (Cathode active material) The positive electrode active material can consist of a lithium-containing compound or another metal compound. Examples of lithium-containing compounds include lithium cobalt composite oxide having a layered structure, lithium nickel composite oxide having a layered structure, lithium manganese composite oxide having a spinel structure, and lithium iron phosphate having an olivine-type structure. Examples of other metal compounds include oxides such as titanium oxide, vanadium oxide, and manganese dioxide, and sulfides such as titanium sulfide and molybdenum sulfide.

[0026] (Conductive material) Carbon materials can be used as the conductive material for the positive electrode. Examples of carbon materials include graphite powder, carbon black (e.g., acetylene black), and fibrous carbon materials (carbon nanotubes, carbon nanofibers, vapor-grown carbon fibers, etc.). Carbon black (e.g., acetylene black) is fine-grained and has a large surface area, and by adding a small amount to the electrode mixture, it is possible to improve the conductivity inside the resulting electrode, thereby improving the charge-discharge efficiency and high-current discharge characteristics.

[0027] The proportion of conductive material in the positive electrode composite is preferably 1 to 20 parts by mass per 100 parts by mass of positive electrode active material.

[0028] (binder) Examples of binders that can be used in cathode composites include thermoplastic resins. Specifically, examples of thermoplastic resins include polyvinylidene fluoride (hereinafter sometimes referred to as PVDF), polytetrafluoroethylene (hereinafter sometimes referred to as PTFE), fluororesins such as tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymers, hexafluoropropylene-vinylidene fluoride copolymers, and tetrafluoroethylene-perfluorovinyl ether copolymers; and polyolefin resins such as polyethylene and polypropylene. Two or more of these may be used.

[0029] The proportion of binder in the positive electrode composite material is preferably 1 to 20 parts by mass per 100 parts by mass of positive electrode active material.

[0030] (Positive electrode current collector) Examples of positive electrode current collectors include Al, Ni, and stainless steel. Al is preferred because it is easy to process into a thin film and is inexpensive. Examples of positive electrode current collector shapes include foil, flat plate, mesh, net, lath, and punched shapes, as well as combinations thereof (e.g., mesh-shaped flat plate). The surface of the positive electrode current collector may also have irregularities formed by etching or embossing.

[0031] One method for supporting the positive electrode composite material on the positive electrode current collector is to fix the positive electrode composite material to the current collector by pressure molding. Alternatively, an organic solvent can be added to the positive electrode mixture to form a positive electrode mixture paste, and this paste can be applied to the positive electrode current collector and dried to fix the positive electrode mixture to the positive electrode current collector. In this method, the sheet obtained by fixing the positive electrode mixture to the positive electrode current collector may be pressed to firmly fix the positive electrode mixture to the positive electrode current collector.

[0032] Examples of organic solvents used in positive electrode composite pastes include amine solvents such as N,N-dimethylaminopropylamine and diethylenetriamine; ether solvents such as tetrahydrofuran; ketone solvents such as methyl ethyl ketone; ester solvents such as methyl acetate; and amide solvents such as dimethylacetamide and N-methyl-2-pyrrolidone (hereinafter sometimes referred to as NMP).

[0033] The mixing method used in the preparation of the cathode composite paste is not particularly limited, but a mixer with high shear force is preferred for mixing. Specifically, examples include planetary mixers, kneaders, extrusion mixers, and thin-film swirling high-speed agitators.

[0034] In terms of the mixing order, the positive electrode active material, conductive material, binder, and solvent may be mixed together at once, or the binder, positive electrode active material, and conductive material may be mixed sequentially into the solvent. This order is not particularly limited, and the mixture of positive electrode active material and conductive material may be added gradually. Alternatively, the solvent and binder may be mixed and dissolved in advance.

[0035] Methods for applying the positive electrode composite paste to the positive electrode current collector include, for example, slit die coating, screen coating, curtain coating, knife coating, gravure coating, and electrostatic spraying. The positive electrode of the lithium metal secondary battery according to this disclosure can be manufactured by the methods described above.

[0036] Examples of electrode composite pastes for lithium metal secondary batteries that constitute the positive electrode include pastes containing a positive electrode active material that can be doped and dedoped with lithium ions, a conductive material, a binder, and an organic solvent. The quality of electrode composite paste for lithium metal secondary batteries is preferably controlled by viscosity. Here, viscosity represents the difficulty of a liquid (fluid) flowing; highly viscous substances require a large force to flow, while low-viscosity substances can flow with little force.

[0037] The viscosity of the electrode composite paste for lithium metal secondary batteries was measured using a stress rheometer for 10 seconds. -1 The viscosity is measured by reading the equilibrium value when a shear rate is applied (steady flow measurement). As paste viscosity increases, fluidity decreases, making it difficult to control the smoothness and thickness distribution of the electrode surface during the coating process. On the other hand, as paste viscosity decreases, fluidity increases, making it easier to obtain electrodes with high surface smoothness and uniform thickness distribution. However, if the paste viscosity becomes too low, the degree of mixing of the components in the paste decreases, making it easier for the solid and liquid layers to separate. Therefore, the paste viscosity is preferably between 1000 mPa·s and 100000 mPa·s, and more preferably between 3000 mPa·s and 15000 mPa·s. It is preferable that the viscosity of the lithium metal secondary battery electrode composite paste before coating is always within this range.

[0038] In the electrode composite paste for lithium metal secondary batteries, the total mass ratio of positive electrode active material, conductive material, and binder is preferably 30% by mass or more, more preferably 35% by mass or more, and even more preferably 45% by mass or more, from the viewpoint of reducing the energy required for drying after application. Furthermore, from the viewpoint of improving fluidity, it is preferably 75% by mass or less, more preferably 70% by mass or less, and even more preferably 65% ​​by mass or less.

[0039] - Acids with a valency of 2 or higher - The electrode composite paste for lithium metal secondary batteries may contain an acid with a valency of 2 or higher. An acid with a valency of 2 or higher is an acid that can release two or more protons when reacting with a base, and specifically, it is an acid that undergoes the following reactions: H x A + (x / y)B(OH) y → xH2O+B( x / y )A (In the formula, x is an integer greater than or equal to 2, and y is an integer greater than or equal to 1. H xA represents an acid, and B(OH)y represents a base. It is preferable that the above reaction has an ionization equilibrium of two or more steps. The acid is preferably a divalent or trivalent acid. The acid may be either an organic acid or an inorganic acid. Two or more divalent or trivalent acids may be mixed and used.

[0040] Examples of organic acids include carboxylic acids, phosphonic acids, sulfonic acids, etc. Among these, carboxylic acids such as oxalic acid, malic acid, succinic acid, maleic acid, etc. are preferable. Two or more of these may be mixed and used. Preferably, anhydrides are used.

[0041] [[ID=,8]]Examples of inorganic acids include phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, boric acid, sodium hydrogen borate, sodium dihydrogen borate, sodium trihydrogen borate, sulfuric acid, etc. Two or more of these may be mixed and used.

[0042] -Cathode active material that can be doped and undoped with lithium ions- In a lithium metal secondary battery, the cathode active material can dope and undope lithium ions and is preferably composed of a lithium-containing transition metal compound. The cathode active material is as described above.

[0043] As the cathode active material, it is preferable to use a composite metal oxide represented by the following formula (A). By using the composite metal oxide represented by the following formula (A) as the cathode active material, the charge-discharge capacity of the battery can be improved. Li a M 1 b M 2 O2(A) (Here, M 1 represents one or more elements selected from the group consisting of Mg, Ca, Sr, and Ba, and M 2(where a represents one or more elements selected from the group consisting of Mn, Fe, Co, Cr, V, Ti, Cu, and Ni, where a is a value in the range of 0.5 to 1.05, b is a value in the range of 0 to 0.5, and a+b is a value in the range of 0.5 to 1.10.)

[0044] (Method of manufacturing lithium metal secondary batteries) A lithium metal secondary battery has a positive electrode as described above, a counter electrode including a CNT layer as described later, and a non-aqueous electrolyte. There are no particular restrictions on the shape of lithium metal secondary batteries; a known and appropriate shape should be selected depending on the intended use. Lithium metal secondary batteries can be manufactured by known methods, depending on their shape and intended use.

[0045] The following describes separators, non-aqueous electrolytes, and other components that can be used in lithium metal secondary batteries.

[0046] (Separator) Examples of materials used to form the separator include polyethylene, polyolefin resins such as polypropylene, fluororesins, and nitrogen-containing aromatic polymers. Alternatively, a single-layer or laminated separator may be formed using two or more of these materials.

[0047] Examples of separators include those described in Japanese Patent Publication No. 2000-30686 and Japanese Patent Publication No. 10-324758.

[0048] The thickness of the separator is preferable as thin as possible while maintaining mechanical strength, as this increases the volumetric energy density of the battery and reduces internal resistance. Generally, the thickness of the separator is preferably 5 μm to 200 μm, and more preferably 5 μm to 40 μm.

[0049] The separator preferably has a porous film containing a thermoplastic resin. In a lithium secondary battery, it is preferable to interrupt the current and prevent excessive current from flowing (shut down) when an abnormal current flows in the battery due to a short circuit between the positive and negative electrodes or the like.

[0050] When the separator has a porous film containing a thermoplastic resin, shutdown occurs when a short circuit causes the separator at the short-circuit location to heat up, exceeding a predetermined (normal) operating temperature. This causes the porous film in the separator to soften or melt, blocking the micropores. It is preferable that the separator has high enough heat resistance to maintain the shut-down state without rupturing even if the temperature inside the battery rises to a certain level after shutdown.

[0051] By using a separator consisting of a laminated porous film in which a heat-resistant porous layer containing a heat-resistant resin and a porous film containing a thermoplastic resin are laminated, it becomes possible to further prevent thermal film rupture. Here, the heat-resistant porous layer may be laminated on both sides of the porous film.

[0052] (Non-aqueous electrolytes) Non-aqueous electrolytes that can be used in lithium metal secondary batteries according to this disclosure include LiClO4, LiPF6, LiAsF6, LiSbF6, LiBF4, LiCF3SO3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(COCF3), Li(C4F9SO3), LiC(SO2CF3)3, and Li2B 10 Cl 10 Examples include lithium salts such as LiBOB (where BOB stands for bis(oxalato)borate), LiFSI (where FSI stands for bis(fluorosulfonyl)imide), lithium salts of lower aliphatic carboxylates, and LiAlCl4. These may be used individually or as a mixture of two or more types. In particular, the electrolyte preferably contains at least one fluorine-containing lithium salt selected from the group consisting of LiPF6, LiAsF6, LiSbF6, LiBF4, LiCF3SO3, LiN(SO2CF3)2, and LiC(SO2CF3)3, which contain fluorine. Furthermore, the above-mentioned non-aqueous electrolyte can be dissolved in an organic solvent and used as a non-aqueous electrolyte solution.

[0053] Examples of organic solvents that can be used in non-aqueous electrolytes include, Carbonates such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, isopropyl methyl carbonate, vinylene carbonate, and 1,2-di(methoxycarbonyloxy)ethane; Ethers such as ethylene glycol dimethyl ether (1,2-dimethoxyethane), ethylene glycol dibutyl ether (1,2-dibutoxyethane), diethylene glycol diethyl ether (diglym), triethylene glycol dimethyl ether (trigym), tetraethylene glycol dimethyl ether (tetragym), diethyl ether, dipropyl ether, hexaethylene glycol dimethyl ether, 1,3-dimethoxypropane, 1,2-dimethoxypropane, 1,2-dimethoxybutane, tetrahydrofuran, and 2-methyltetrahydrofuran; Esters such as methyl formate, methyl acetate, and γ-butyrolactone; Nitriles such as acetonitrile and butyronitrile; amides such as N,N-dimethylformamide and N,N-dimethylacetamide; Carbamates such as 3-methyl-2-oxazolidone; Sulfur-containing compounds such as sulfolanes, dimethyl sulfoxides, and 1,3-propanesultones; A fluorine substituent-containing organic solvent can be used, which is obtained by introducing a fluorine substituent to the above organic solvent.

[0054] It is preferable that a portion of the organic solvent in the non-aqueous electrolyte contains an organic solvent having a fluorine substituent.

[0055] Examples of organic solvents having a fluorine substituent include 4-fluoro-1,3-dioxolan-2-one (hereinafter sometimes referred to as FEC or fluoroethylene carbonate), trans or cis-4,5-difluoro-1,3-dioxolan-2-one (hereinafter sometimes referred to as DFEC or difluoroethylene carbonate), 2,2,3,3,3-pentafluoropropyl methyl ether, 2,2,3,3-tetrafluoropropyl difluoromethyl ether, and 1,1,2,2-tetrafluoroethylene Examples include 2,2,3,3-tetrafluoropropyl ether, bis(2,2,2-trifluoroethyl) ether, hydrofluoroether, 1,1,1,3,3,3-hexafluoro-2-methoxypropane, hexafluoroisopropyl methyl ether, 2,2,3,3-tetrafluoro-1,4-dimethoxybutane, 1,1,1-trifluoro-2,3-dimethoxypropane, bis(2-fluoroethyl) ether, and 2,2,3,3-tetrafluoro-1,4-dimethoxybutane.

[0056] The organic solvent having a fluorine substituent is preferably 4-fluoro-1,3-dioxolan-2-one.

[0057] The electrolyte may contain additives such as tris(trimethylsilyl) phosphate and tris(trimethylsilyl) borate.

[0058] A solid electrolyte may be used instead of the non-aqueous electrolyte mentioned above. As a solid electrolyte, for example, an organic polymer electrolyte such as a polyethylene oxide-based polymer compound, a polymer compound containing at least one of a polyorganosiloxane chain or a polyoxyalkylene chain can be used. Alternatively, a so-called gel-type electrolyte, in which the non-aqueous electrolyte is held within the polymer compound, can also be used. Furthermore, inorganic solid electrolytes containing sulfides such as Li2S-SiS2, Li2S-GeS2, Li2S-P2S5, Li2S-B2S3, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li2SO4, and Li2S-GeS2-P2S5 can be used, and mixtures of two or more of these may be used. Using these solid electrolytes can sometimes further enhance the safety of lithium secondary batteries.

[0059] Furthermore, in the lithium secondary battery of this embodiment, when a solid electrolyte is used, the solid electrolyte may also act as a separator, in which case a separator may not be necessary.

[0060] (Sodium metal secondary battery) A sodium metal secondary battery, which is one embodiment of the alkali metal secondary battery of the present disclosure, has a positive electrode and a counter electrode containing a CNT layer with a thickness of 12 μm to 70 μm, and an electrolyte disposed between the positive electrode and the counter electrode. The sodium metal secondary battery may further have a separator between the positive electrode and the counter electrode.

[0061] (positive electrode) A positive electrode for a sodium metal secondary battery can be manufactured by supporting a positive electrode composite material, which includes a positive electrode active material and a binder, on a positive electrode current collector. A conductive material may be further added to the positive electrode composite material.

[0062] Carbon materials can be used as the conductive material for the positive electrode. Examples of carbon materials include graphite powder, carbon black (e.g., acetylene black), and fibrous carbon materials (carbon nanotubes, carbon nanofibers, vapor-grown carbon fibers, etc.). Carbon black (e.g., acetylene black) is fine-grained and has a large surface area, and by adding a small amount to the electrode mixture, it is possible to improve the conductivity inside the resulting electrode, thereby improving the charge-discharge efficiency and high-current discharge characteristics.

[0063] Examples of binders include thermoplastic resins. Specifically, examples of thermoplastic resins include polyvinylidene fluoride (hereinafter sometimes referred to as PVDF), polytetrafluoroethylene (hereinafter sometimes referred to as PTFE), fluororesins such as tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymers, hexafluoropropylene-vinylidene fluoride copolymers, and tetrafluoroethylene-perfluorovinyl ether copolymers; and polyolefin resins such as polyethylene and polypropylene. One type of binder may be used, or two or more types may be used.

[0064] The positive electrode current collector is the same as the one described earlier for lithium metal secondary batteries, and the preferred example is also the same. Furthermore, examples of methods for supporting the positive electrode composite material on the positive electrode current collector include a method similar to lithium metal secondary batteries, where the positive electrode composite material is fixed to the current collector by pressure molding, and a method in which an organic solvent is added to the positive electrode composite material to form a positive electrode composite paste, which is then applied to the positive electrode current collector and dried to fix the positive electrode composite material to the current collector. In this method, the positive electrode composite material may be firmly fixed to the positive electrode current collector by pressing the sheet obtained by fixing the positive electrode composite material to the positive electrode current collector.

[0065] The type of organic solvent used in the positive electrode composite paste, the mixing method, and other methods for preparing the positive electrode composite paste are the same as those for lithium metal secondary batteries.

[0066] Methods for applying the obtained positive electrode composite paste to the positive electrode current collector include, for example, slit die coating, screen coating, curtain coating, knife coating, gravure coating, and electrostatic spraying. In this way, the positive electrode of the sodium metal secondary battery according to this disclosure can be manufactured.

[0067] Examples of electrode composite pastes for sodium metal secondary batteries that constitute the positive electrode include a positive electrode active material that can be doped and dedoped with sodium ions, a conductive material, a binder, and an organic solvent.

[0068] The quality of electrode composite paste for sodium metal secondary batteries is preferably controlled by viscosity. Here, viscosity represents the difficulty of a liquid (fluid) flowing; highly viscous substances require a large force to flow, while low-viscosity substances can flow with little force.

[0069] The viscosity of the electrode composite paste for sodium metal secondary batteries was measured using a stress rheometer for 10 seconds. -1 The measurement is performed by reading the equilibrium value when a shear rate is applied (steady flow measurement). As paste viscosity increases, fluidity decreases, making it difficult to control the smoothness and thickness distribution of the electrode surface during the coating process. On the other hand, as paste viscosity decreases, fluidity increases, making it easier to obtain electrodes with high surface smoothness and uniform thickness distribution. However, if the paste viscosity becomes too low, the degree of mixing of the components in the paste decreases. This makes it easier to separate the solid and liquid layers. Therefore, the paste viscosity is preferably 1000 mPa·s to 100000 mPa·s, and more preferably 3000 mPa·s to 15000 mPa·s. It is preferable that the viscosity of the sodium metal secondary battery electrode composite paste before application is always within this range.

[0070] In the electrode composite paste for sodium metal secondary batteries, the total mass ratio of the positive electrode active material, conductive material, and binder is preferably 30% by mass or more, more preferably 35% by mass or more, and even more preferably 45% by mass or more, from the viewpoint of reducing the energy required for drying after application. Furthermore, from the viewpoint of improving fluidity, it is preferably 75% by mass or less, more preferably 70% by mass or less, and even more preferably 65% ​​by mass or less.

[0071] - Acids with a valency of 2 or higher - The electrode composite paste for sodium metal secondary batteries may contain an acid with a valency of 2 or higher. An acid with a valency of 2 or higher is an acid that can release two or more protons when reacting with a base, and specifically, it is an acid that undergoes the following reactions: H x A + (x / y)B(OH) y → xH2O+B( x / y )A (In the formula, x is an integer greater than or equal to 2, and y is an integer greater than or equal to 1. H x A represents an acid, and B(OH)y represents a base. The above reaction preferably has two or more stages of ionization equilibrium. The acid is preferably a divalent or trivalent acid. The acid may be either an organic acid or an inorganic acid. Two or more divalent or trivalent acids may be mixed and used.

[0072] Examples of organic acids include carboxylic acids, phosphonic acids, and sulfonic acids. Among these, carboxylic acids such as oxalic acid, malic acid, succinic acid, and maleic acid are preferred. Two or more of these may be used in mixture form. Anhydrous forms are preferably used.

[0073] Examples of inorganic acids include phosphoric acid, sodium dihydrogen phosphate, sodium monohydrogen phosphate, trisodium phosphate, boric acid, sodium monohydrogen borate, sodium dihydrogen borate, sodium trihydrogen borate, and sulfuric acid. Two or more of these may be used in mixture form.

[0074] -A cathode active material that can be doped and dedoped with sodium ions- In a sodium metal secondary battery, the positive electrode active material is preferably a sodium-containing transition metal compound that can be doped and dedoped with sodium ions.

[0075] Examples of sodium-containing transition metal compounds include the following compounds: That is, NaFeO2, NaMnO2, NaNiO2, NaCoO2, NaMn 05 Ni 0.5 O2, NaFe 0.5 Ni 0.5 O2, NaFe 1 / 3 Mn 1 / 3 Ni 1 / 3 O2 and NaFe 0.4 Mn 0.3 Ni 0.3 NaM such as O2 3 a1 Oxides represented by O2, Na 0.44 Mn 1-a2 M 3 a2 Oxides represented by O2, Na 0.7 Mn 05 Ni 0.5 O2 and Na 0.7 Mn 2 / 3 Ni 1 / 3 Na such as O2 0.7 Mn 1-a2 M 3 a2 O 2.05 The oxide represented by (M 3 represents one or more transition metal elements. <a1<1、0≦a2<1); Na6Fe2Si 12 O 30 and Na2Fe5Si 12 O 30 Na etc. b1 M 4 c Si 12 O 30 The oxide represented by (M 4 represents one or more transition metal elements. (2 ≤ b1 ≤ 6, 2 ≤ c ≤ 5) Na2Fe2Si6O 18 and Na2MnFeSi6O 18 NadM 5e Si6O 18 The oxide represented by (M 5 represents one or more transition metal elements. (2≦d≦6, 1≦e≦2) Na2FeSiO6 etc. f M 6 The oxide represented by gSi2O6 (M 6 represents a transition metal element or one or more elements selected from the group consisting of Mg and Al. (1 ≤ f ≤ 2, 1 ≤ g ≤ 2) NaFePO4, NaMnPO4, Na3Fe2(PO4)3, Na3V2(PO4)2F3, Na 1.5 VOPO4F 0.5 Phosphates such as Na4Fe3(PO4)2P2O7, Na4Mn3(PO4)2P2O7, Na4Ni3(PO4)2P2O7, and Na4Co3(PO4)2P2O7; Na2FePO4F, Na2VPO4F, Na2MnPO4F, Na2CoPO 4 Fluorophosphates such as F, Na2NiPO4F; Fluorinated sulfates such as NaFeSO4F, NaMnSO4F, NaCoSO4F, and NaFeSO4F; Borates such as NaFeBO4 and Na3Fe2(BO4)3; NahM such as Na3FeF6 and Na2MnF6 7 Fluoride represented by F6 (M 7 represents one or more transition metal elements. Examples include 2≦h≦3); and the above M 3 From M 6 As such, at least one of Fe, Co, Cr, V, Ti, Cu, and Ni is preferred.

[0076] It is preferable to use a composite metal oxide represented by the following formula (A) as the positive electrode active material. By using a composite metal oxide represented by the following formula (A) as the positive electrode active material, the charge and discharge capacity of the battery can be improved. Na a M 1 b M 2 O2(A) (Here, M 1This represents one or more elements selected from the group consisting of Mg, Ca, Sr, and Ba, and M 2 (where a represents one or more elements selected from the group consisting of Mn, Fe, Co, Cr, V, Ti, Cu, and Ni, where a is a value in the range of 0.5 to 1.05, b is a value in the range of 0 to 0.5, and a+b is a value in the range of 0.5 to 1.10.)

[0077] (Method of manufacturing sodium metal secondary batteries) A sodium metal secondary battery has a positive electrode as described above, a counter electrode containing a CNT layer as described later, and a non-aqueous electrolyte. There are no particular restrictions on the shape of sodium metal secondary batteries; a known and appropriate shape should be selected depending on the intended use. Sodium metal secondary batteries can be manufactured by known methods, depending on their shape and intended use.

[0078] The following explanation of the separator and other components in sodium metal secondary batteries is omitted, as they are the same as those mentioned in the section on lithium metal secondary batteries.

[0079] (Non-aqueous electrolytes) Non-aqueous electrolytes that can be used in the sodium metal secondary battery of this disclosure include NaClO4, NaPF6, NaAsF6, NaSbF6, NaBF4, NaCF3SO3, NaN(SO2CF3)2, lower aliphatic carboxylate sodium salts, and NaAlCl4. These may be used individually or as a mixture of two or more. The electrolyte preferably contains at least one fluorine-containing sodium salt selected from the group consisting of NaPF6, NaAsF6, NaSbF6, NaBF4, NaCF3SO3, and NaN(SO2CF3)2. Furthermore, the above-mentioned non-aqueous electrolyte can be dissolved in an organic solvent and used as a non-aqueous electrolyte solution.

[0080] Examples of organic solvents in non-aqueous electrolytes include, Carbonates such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, isopropyl methyl carbonate, vinylene carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one, and 1,2-di(methoxycarbonyloxy)ethane; Ethers such as ethylene glycol dimethyl ether (1,2-dimethoxyethane), ethylene glycol dibutyl ether (1,2-dibutoxyethane), diethylene glycol diethyl ether (diglym), triethylene glycol dimethyl ether (trigym), tetraethylene glycol dimethyl ether (tetragym), diethyl ether, dipropyl ether, hexaethylene glycol dimethyl ether, 1,3-dimethoxypropane, 1,2-dimethoxypropane, 1,2-dimethoxybutane, tetrahydrofuran, and 2-methyltetrahydrofuran; Esters such as methyl formate, methyl acetate, and γ-butyrolactone; Nitriles such as acetonitrile and butyronitrile; amides such as N,N-dimethylformamide and N,N-dimethylacetamide; Carbamates such as 3-methyl-2-oxazolidone; Sulfur-containing compounds such as sulfolanes, dimethyl sulfoxides, and 1,3-propanesultones; A fluorine substituent-containing organic solvent can be used, which is obtained by introducing a fluorine substituent to the above organic solvent.

[0081] In non-aqueous electrolytes, the organic solvent is preferably an ether.

[0082] Preferred ethers include ethylene glycol dimethyl ether (1,2-dimethoxyethane), ethylene glycol dibutyl ether (1,2-dibutoxyethane), diethylene glycol diethyl ether (diglym), triethylene glycol dimethyl ether (triglym), and tetraethylene glycol dimethyl ether (tetraglym), with diethylene glycol diethyl ether (diglym) being more preferred.

[0083] Some of the organic solvents in the non-aqueous electrolyte may include organic solvents having fluorine substituents.

[0084] Examples of organic solvents having a fluorine substituent include 4-fluoro-1,3-dioxolan-2-one (hereinafter sometimes referred to as FEC or fluoroethylene carbonate), trans or cis-4,5-difluoro-1,3-dioxolan-2-one (hereinafter sometimes referred to as DFEC or difluoroethylene carbonate), 2,2,3,3,3-pentafluoropropyl methyl ether, 2,2,3,3-tetrafluoropropyl difluoromethyl ether, and 1,1,2,2-tetrafluoroethylene Examples include 2,2,3,3-tetrafluoropropyl ether, bis(2,2,2-trifluoroethyl) ether, hydrofluoroether, 1,1,1,3,3,3-hexafluoro-2-methoxypropane, hexafluoroisopropyl methyl ether, 2,2,3,3-tetrafluoro-1,4-dimethoxybutane, 1,1,1-trifluoro-2,3-dimethoxypropane, bis(2-fluoroethyl) ether, and 2,2,3,3-tetrafluoro-1,4-dimethoxybutane.

[0085] The non-aqueous electrolyte described above can also be used in a state in which the non-aqueous electrolyte is held by a polymer compound, that is, as a gel-like electrolyte.

[0086] Furthermore, a solid electrolyte can also be used as the non-aqueous electrolyte in the sodium secondary battery of this embodiment.

[0087] As the solid electrolyte, polymer electrolytes such as polyethylene oxide-based polymer compounds, polymer compounds containing at least one of polyorganosiloxane chains or polyoxyalkylene chains can be used. Furthermore, safety can be further enhanced by using sulfide electrolytes such as Na2S-SiS2, Na2S-GeS2, Na2S-P2S5, and Na2S-B2S3, or inorganic compound electrolytes containing sulfides such as Na2S-SiS2-Na3PO4 and Na2S-SiS2-Na2SO4, or NASICON-type electrolytes such as NaZr2(PO4)3 as the solid electrolyte.

[0088] Furthermore, in the sodium secondary battery of this embodiment, when a solid electrolyte is used, the solid electrolyte may also serve as a separator, as described later. In such cases, a separator may not be necessary.

[0089] (Counter electrode containing a carbon nanotube layer with a thickness of 12 μm to 70 μm) The counter electrode, which is the negative electrode of the alkali metal secondary battery according to this disclosure, includes a CNT layer with a thickness of 12 μm to 70 μm. The CNT layer contained in the counter electrode is an aggregate of CNTs, and the presence of fine voids between each CNT that make up the CNT layer makes it easier for alkali metal ions, which are electrolytes, to precipitate as alkali metals. In this respect, the charging efficiency is better compared to the negative electrode piece described in Patent Document 1, which has a carbon material coating layer with a thickness of 10 μm or less, including a carbon material and an adhesive. The thickness of the CNT layer ranges from 12 μm to 70 μm. The thickness of the CNT layer is preferably 15 μm or more. Furthermore, the thickness is preferably 50 μm or less, more preferably 40 μm or less, and even more preferably 20 μm or less. If the thickness of the CNT layer is less than 12 μm, alkali metal precipitates may not be able to fit within the voids in the CNT layer, which can lead to problems such as electrode expansion and alkali metal deposition on the electrode surface, and can degrade the cycle characteristics. Furthermore, if the thickness of the CNT layer exceeds 70 μm, the occupancy rate of the voids used for alkali metal deposition decreases, which may lead to a decrease in energy density.

[0090] The thickness of the CNT layer can be determined by methods such as directly measuring the thickness with a micrometer, or by measuring it from optical images such as cross-sectional images obtained with a scanning electron microscope. In the case of a multilayer film containing a CNT layer, the thickness can be determined by taking the difference between the thickness of the portion containing the CNT layer and the thickness of the portion not containing the CNT layer.

[0091] There are no particular restrictions on the CNTs used to form the CNT layer; known CNTs can be used. The CNTs may be single-walled or multi-walled. The length of the CNTs included in the CNT layer is also arbitrary, and the CNTs may take the form of fibers. The CNT layer may include, for example, multi-walled carbon nanotubes with a maximum length of 10 μm to 30,000 μm (preferably 1,000 μm to 30,000 μm).

[0092] In the detailed descriptions of "multiwalled carbon nanotubes" and "methods for manufacturing multiwalled carbon nanotubes" below, the terms "single-walled carbon nanotube" and "multiwalled carbon nanotube" may be abbreviated as "SWCNT" and "MWCNT," respectively.

[0093] Here, the term "fiber" is generally used to refer to a structure in which one dimension is larger than the other two dimensions. The fiber may be a thread-like fiber with a circular cross-section, a ribbon-like fiber with a rectangular cross-section, hollow, or have other shapes. From the viewpoint of improving conductivity, the cross-section of the CNT is preferably circular and preferably hollow.

[0094] Because of their fibrous structure, carbon nanotubes (CNTs) have a tendency to intertwine with each other. Hereafter, aggregates containing CNTs may be abbreviated as "CNT aggregates."

[0095] The aggregate of CNTs may also be an aggregate of intertwined three-dimensional structures. The configuration of the CNT aggregate can be confirmed by scanning electron microscopy (SEM) observation.

[0096] The length of a carbon nanotube (CNT) can be measured by focusing on a single CNT and observing multiple SEM images of adjacent fields of view. Here, "the length of the CNT" and This refers to the measured length of the CNT in the longitudinal direction, and the maximum value among the length measurements is defined as the "maximum length".

[0097] The diameter of a carbon nanotube (CNT) can be measured by observing a scanning electron microscope (SEM) or transmission electron microscope (TEM) image. Here, diameter refers to the length in the direction perpendicular to the longitudinal direction of the CNT. The diameter is measured at 10 different points on a single CNT, and the average value is taken as the diameter of that CNT.

[0098] The length of the CNTs is in the range of 10 μm to 30,000 μm, preferably in the range of 100 μm to 30,000 μm, and more preferably in the range of 100 μm to 30,000 μm. The diameter of the CNTs is preferably in the range of 1 nm to 100 nm, more preferably in the range of 2 nm to 80 nm, even more preferably in the range of 3 nm to 50 nm, and particularly preferably in the range of 5 nm to 30 nm.

[0099] The length-to-diameter ratio of the CNT, or aspect ratio, is preferably 10 or greater, more preferably 100 or greater, and even more preferably 1000 or greater. The aspect ratio can be calculated from the ratio of the maximum length to the diameter of a single carbon nanotube (CNT). For accuracy, it is preferable to use the average value of measurements from 20 or more CNTs.

[0100] Furthermore, from the viewpoint of dispersibility, the specific gravity of the CNT aggregate is preferably in the range of 1.5 to 2.5, more preferably in the range of 1.7 to 2.4, and even more preferably in the range of 1.8 to 2.2. The specific gravity of a CNT aggregate can be measured by the method described in JIS Z8807:2012 "Method for measuring the density and specific gravity of solids".

[0101] The purity of CNTs in a CNT aggregate can be measured by thermomass spectrometry. For example, a thermomass (TG) curve and differential thermal analysis (DTA) curve of the CNT aggregate are obtained using a thermal analyzer (Shimadzu Corporation, DTG-60). In the DTA curve, where the peak top appears around 650°C to 750°C, the largest exothermic peak is considered to be the combustion of MWCNTs, and other exothermic peaks are considered to be the combustion of substances other than CNTs. The purity of the CNTs is determined from the mass loss rate of the TG curve. From the viewpoint of the resulting conductivity, the purity of the CNT aggregate is preferably 50% by mass or more, more preferably 65% ​​by mass or more, even more preferably 80% by mass or more, and particularly preferably 95% by mass or more.

[0102] CNTs may contain 0.01% to 50% by mass of Fe atoms derived from iron, which is used as a catalyst during manufacturing. CNTs may contain Fe atoms in the following forms: for example, adsorbed on the surface of the CNTs, or incorporated into the fibrous CNTs formed during manufacturing.

[0103] The resulting fibrous CNTs are preferably flexible and strong. Furthermore, the conductivity of the CNT itself is 5000 ohms. -1 ·m -1 Preferably, it is 10,000 ohms or more. -1 ·m -1 The above is more preferable. The conductivity of the CNT itself is typically 1,000,000 ohms. -1 ·m -1 The following applies:

[0104] Any of the above-mentioned CNTs can be used in the alkali metal secondary battery according to this disclosure. Preferably, the CNT layer comprising the CNTs has a G / D intensity of 2.5 or higher obtained by Raman measurement of the CNT layer. In the Raman spectrum, the peak intensity ratio G / D, which is the ratio of the peak intensity G of the G band to the peak intensity D of the D band, indicates that the G band represents the crystallinity and lattice vibrations of graphene contained in carbon nanotube aggregates, at 1590 cm⁻¹. -1 A peak intensity G appears nearby. The D band is due to structural disturbances and defects in the carbon nanotubes, at 1350 cm. -1 A peak intensity D appears in the vicinity. The ratio of the peak intensity G in the G band to the peak intensity D in the D band of a CNT is called the G / D intensity. A G / D strength of 2.5 or higher indicates a low amount of defects in the CNT layer that cause irreversible alkali metal adsorption, which tends to improve charge-discharge efficiency. The G / D strength is preferably 2.5 or higher, more preferably 2.7 or higher, and even more preferably 3.0 or higher. There is no particular upper limit to the G / D strength, but it can be set to 20.0 or less. A G / D strength of 20.0 or less may suppress the aggregation of unwanted CNTs within the CNT layer due to a decrease in the uniformity of the CNT aggregate shape.

[0105] In this disclosure, the G / D intensity, which is the ratio of the peak intensity G of the G band to the peak intensity D of the D band in the Raman spectrum of the CNT layer, can be determined by the following method. Using a Raman spectrometer, the Raman spectrum of the CNT layer is obtained under the following measurement conditions. The peak intensity G of the G band and the peak intensity D of the D band are read from the obtained Raman spectrum, and the peak intensity ratio G / D is determined. As a Raman spectrometer, for example, the RAMAN-11 (trade name), a Raman spectrometer manufactured by Nanophoto Inc., can be suitably used. However, the Raman spectrometer is not limited to this.

[0106] <Measurement conditions> Excitation laser wavelength: 532nm Grating: 600 grooves / mm Objective lens: 20x magnification, numerical aperture (NA) 0.45

[0107] In peak analysis, by using peak fitting, it is possible to avoid the influence of peak overlap and improve the resolution, thereby calculating the peak intensity more accurately. As the function used for fitting, since the fitting property is high, it is preferable to use a Lorentz function. Depending on the peak shape, using a Gaussian function may be more suitable and preferable in some cases.

[0108] In the present disclosure, the peak intensity G of the G band in the Raman spectrum means the maximum value of the scattering intensity at a Raman shift of 1550 cm -1 ~1600 cm -1 Further, in the present disclosure, the peak intensity D of the D band in the Raman spectrum means the maximum value of the scattering intensity at a Raman shift of 1300 cm -1 ~1400 cm -1

[0109] The G / D intensity, which is the ratio of the peak intensity G of the G band to the peak intensity D of the D band in the Raman spectrum of the CNT layer, can be controlled by synthesis conditions such as the firing temperature, gas flow rate, and gas introduction method when synthesizing the CNTs contained in the CNT layer. The above G / D intensity can be increased, for example, by increasing the firing temperature when synthesizing the CNTs contained in the CNT layer, and can be decreased by decreasing the firing temperature when synthesizing the CNTs.

[0110] The method for manufacturing the CNTs contained in the CNT layer in the alkali metal secondary battery according to the present disclosure is not particularly limited.

[0111] As the method for manufacturing MWCNTs in the present disclosure, methods such as a conventionally known chemical vapor deposition (CVD) method and a method of reacting a gaseous reactant containing a carbon source in the presence of a catalyst can be applied.

[0112] The MWCNTs in the present disclosure can be manufactured, for example, by referring to the manufacturing method described in JP-A-2016-XXXXXX. ​The following describes an example of a method for manufacturing MWCNTs in this disclosure. However, the method for manufacturing MWCNTs in this disclosure is not limited to the following example.

[0113] An example of a method for producing MWCNTs in this disclosure is the method described in Japanese Patent Application Publication No. 2016-102047, which includes the steps of: passing a gaseous reactant containing one or more carbon sources through a reactor; reacting one or more gaseous reactants in the reaction region of the reactor in the presence of a catalyst to form carbon-containing product particles; agglomerating the product particles into aggregates; and applying force to the aggregates to continuously move the aggregates out of the reaction region (hereinafter also referred to as "production method X"). According to manufacturing method X, MWCNTs can be obtained in the form of easily handled fibrous aggregates or other aggregates.

[0114] In manufacturing method X, the force applied to the product particles may be a mechanical force. If the aggregates are fibrous MWCNTs, the mechanical force applied to the product particles can be applied by the rotating spindle around which the aggregates are wound. The fibrous CNTs may be collected on the spindle, or they may be accumulated elsewhere after being rotated around the spindle once or more times, as the spindle is continuously unwound.

[0115] Preferably, the spindle axis is positioned perpendicular or parallel to the flow direction of one or more gaseous reactants, but it may be positioned in other orientations. For example, a spindle with its axis positioned at a 25° angle to the flow direction of the gaseous reactants can also be suitably used to impart mechanical force to product particles.

[0116] The spindle can rotate around two axes (for example, two vertical axes). In particular, the spindle can rotate around axes perpendicular and parallel to the flow direction of the gaseous reactant. Such a spindle allows for the pulling and twisting of aggregates, which are fibrous carbon nanotubes, to control the number of twists and length.

[0117] The spindle may be made of metal, ceramic, or resin. The spindle can take on different suitable shapes depending on the material properties and the intended use of the MWCNT. The spindle can be used as a mold for producing carbon products, for example, by a spin-coating process. Preferred spindle shapes are rod-shaped or box-shaped.

[0118] The fibrous MWCNTs are accumulated on the spindle or elsewhere, and the coating thickness and orientation can be controlled by controlling the reaction time and reaction conditions, or by applying an electric field or other field to the carbon product. The coating thickness and orientation of the carbon product can be controlled, for example, by the fluidity of the gas.

[0119] The spindle rotation speed is preferably 0.01 rpm (revolutions / minute; the same applies hereafter) to 10,000 rpm, and more preferably 0.1 rpm to 100 rpm. The spinning speed (i.e., the spindle rotation speed) may be adjusted so that the material is recovered at the same rate as it is produced. The spindle rotation speed may also control the thickness of the accumulated fibrous MWCNTs. In one preferred embodiment, as the spindle rotates, the fibrous CNTs are processed in the axial direction of the spindle. In this processing, the fibrous MWCNTs are wound evenly along the spindle, rather than being wound at only one specific point on the spindle.

[0120] Fibrous MWCNTs may be recovered onto the reactor wall, for example, by a substrate placed in the reactor. The substrate may be a fixed substrate or a rotating guide used to apply a strong and equal force to the fibrous MWCNTs when they are recovered. A suitable substrate arrangement used in the fibrous technology is a substrate consisting of two guides positioned orthogonally to each other.

[0121] In manufacturing method X, the mechanical force applied to the product particles may be a force applied by an accelerating gas flow. The accelerating gas flow can be generated by passing the product particles through a reactor having a narrow diameter or through a capillary located downstream of the reaction region. A vacuum may be applied to the product particles.

[0122] Other forces applied to the product particles include electrostatic forces appropriately applied by a charged plate. When using electrostatic forces, the product particles must be charged. Using a charged plate allows MWCNTs to be generated on the plate in the form of an intertwined mat.

[0123] Furthermore, other forces applied to the generated particles may include magnetic force or photon pressure applied by a light source.

[0124] The raw material for CNTs may be injected in the form of a liquid containing a carbon source, instead of a gaseous reactant containing a carbon source. When a liquid is used as the raw material for CNTs, it can be injected through a single inlet or multiple inlets, for example, in a showerhead configuration.

[0125] One or more gaseous reactants are preferably reacted at 500°C to 1600°C, and more preferably at 1000°C to 1500°C or 1600°C. The temperature gradient is maintained within the reactor, and the reaction region is preferably kept at a higher temperature than the product region of the reactor.

[0126] The gaseous reactant may be used in combination with one or more gases that act as diluents. The gaseous reactant may also be used in combination with gases that do not play a direct role in the reaction but play an auxiliary role. If amorphous carbon is produced as a byproduct, it is also preferable to use a gas as a diluent that can react with amorphous carbon to maintain the reaction sites on the catalyst and produce nanotubes.

[0127] Gases that can be used as diluents include argon or other inert gases, hydrogen, nitrogen, ammonia, carbon dioxide, and helium. Among these, hydrogen is particularly preferred as a diluent. The flow rate of the gas used as a diluent is preferably in the range of 0.1 to 10,000 times the amount of the gaseous reactant, more preferably in the range of 1 to 1,000 times, and even more preferably in the range of 10 to 100 times.

[0128] The gas pressure of the gaseous reactants and any diluents is preferably 0.1 bar to 50 bar, more preferably 0.5 bar to 5 bar, and even more preferably 1 bar to 2 bar. If there is gas leakage from the furnace, the leaked gas can be recycled with or without cleaning.

[0129] The composition of the product particles can be controlled by monitoring the aggregates and changing the reaction conditions based on the information obtained. For example, aggregates can be monitored by online Raman spectroscopy. Online Raman spectroscopy provides data indicating whether the carbon nanotubes (CNTs) are single-walled or multi-walled. It also provides data indicating the diameter and crystallinity of the CNTs. Aggregates can also be monitored by online conductivity measurement, gas analysis, measurement of the opacity of the reaction region, and / or measurement of the winding force.

[0130] When removing aggregates from the reactor, it is preferable to prevent air from entering the reactor. Preventing air inflow is particularly important, for example, when the diluent gas contains hydrogen, as it helps to prevent the formation of an explosive mixture of hydrogen and air in the reactor.

[0131] The product particles produced by manufacturing method X contain MWCNTs. Depending on the manufacturing conditions, SWCNTs may also be present in addition to MWCNTs.

[0132] The product particles may be generated by chemical vapor deposition. When the product particles are generated by chemical vapor deposition, the carbon source, which is a gaseous reactant, reacts in the presence of a catalyst.

[0133] Suitable carbon-containing compounds as carbon sources include carbon monoxide, carbon dioxide, aromatic hydrocarbons (e.g., benzene, toluene, xylene, cumene, ethylbenzene, naphthalene, or mesitylene), non-aromatic hydrocarbons (e.g., methane, ethane, propane, butane, pentane, hexane, cyclohexane, ethylene, propylene, or acetylene), and oxygen-containing hydrocarbons (e.g., formaldehyde, acetaldehyde, acetone, methanol, ethanol, diethyl ether, polyethylene glycol, 1-propanol, ethyl formate, and hydrocarbons containing two or more of these). Preferred carbon-containing compounds include carbon monoxide, methane, ethylene, or acetylene.

[0134] The gaseous reactant, which is the carbon source, is preferably injected at a partial pressure of 0.0001 to 0.9 times the total pressure in the reactor, more preferably at a partial pressure of 0.001 to 0.5 times, and even more preferably at a partial pressure of 0.01 to 0.1 times.

[0135] As catalysts, transition metals are preferred, particularly those belonging to group VIB chromium (Cr), molybdenum (Mo), tungsten (W), or the VIIIB transition metal group. Specifically, examples of catalysts include iron (Fe), cobalt (Co), nickel (Ni), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and p Platinum (Pt) or manganese (Mn), or mixtures thereof, are preferred. Metals from the lanthanide and actinide series (e.g., yttrium (Y)) can also be used as catalysts. Fe, Ni, Co, Mo, and mixtures thereof are preferred. Any of these transition metals can be used alone or in combination with any of the other transition metals listed, and can function as a catalyst for the growth of carbon nanotubes (CNTs).

[0136] The catalyst is preferably formed by the decomposition of a precursor. The precursor is preferably a thermal, photocatalytic, or plasma-degradable compound of one or more of the above-mentioned metals, such as a carbonyl or cyclopentadienyl organometallic compound. Ferrocene, iron pentacarbonyl, nickerocene, and cobaltocene are particularly preferred as precursors. Preferably, at least 0.01% by mass of the precursor is contained in the carbon source, and preferably 0.2% to 2.5% by mass of the precursor is contained in the carbon source. In some embodiments, 0.23% to 2.3% by mass of the precursor is contained in the carbon source. The catalyst may also be used supported on a carrier. Preferred carriers include silica and magnesium oxide.

[0137] The carbon source is preferably reacted in the presence of an accelerator. Suitable accelerators are one or more of sulfur, phosphorus, molybdenum, and organic compounds of these elements. Thiophene is also a preferred accelerator. Preferably, the accelerator is contained in the carbon source at a concentration of up to 10% by mass. Preferably, the accelerator is contained in the carbon source at a concentration of 0.2% to 6% by mass. When high or low concentrations of thiophene are used as the accelerator, MWCNTs are formed.

[0138] According to manufacturing method X, fibrous CNTs having a length of at least 500 μm, for example, at least 1 mm, can be obtained. The fibrous CNTs can take the form of threads or mats. The length of fibrous CNTs can be controlled, for example, by the winding capacity of the spindle used when manufacturing the fibrous CNTs.

[0139] The manufacturing method X preferably includes the steps of generating CNTs by reacting a carbon source in the reaction region of a reactor, and agglomerating the CNTs into aggregates by applying force to them. According to this manufacturing method, fibrous CNTs can be easily produced.

[0140] In other embodiments, the method may include generating CNTs in the reaction region by the above method, then condensing them to form CNTs, and continuously extracting MWCNTs from near the reaction region. In addition, other embodiments may include means of generating CNTs in the reaction region, continuously electrostatically attracting CNTs from the reaction region, and recovering the CNTs.

[0141] The above manufacturing method X is just one example, and the manufacturing method of CNTs is not limited to that described above.

[0142] There are no particular restrictions on the method for forming the CNT layer. The fibrous aggregates obtained by manufacturing method X may be used as is, a single CNT film formed by coating with a CNT dispersion may be used as the CNT layer, or a multilayer CNT layer may be formed by bonding an auxiliary negative electrode current collector to the CNT film and providing the CNT layer on the auxiliary negative electrode current collector.

[0143] The CNT layer can be formed by first preparing a dispersion containing CNTs and a dispersion medium, and then applying it to the surface.

[0144] <Dispersion medium> A hydrophilic solvent or an organic solvent can be used as the dispersion medium for the CNT dispersion. In the case of hydrophilic solvents, it is more preferable that water is the main component. Examples of organic solvents include cyclic aliphatic hydrocarbons, aromatic hydrocarbons, ketones, chlorinated aliphatic hydrocarbons, esters, acylonitriles, ethers, alcohols, and amides. Among these, N-methyl-2-methylpyrrolidone (NMP) is preferred. The solvent may be used alone, or two or more solvents may be mixed together to form a mixed solvent. "Containing water as the main component" means that the proportion of water in the dispersion medium is more than 50% by mass. Preferably, the proportion of water in the dispersion medium is 90% by mass or more, more preferably 95% by mass or more, even more preferably 99% by mass or more, and may be, for example, 100% by mass.

[0145] The water is not particularly limited, but it is preferable to use distilled water, deionized water, or pure water, for example, because it contains fewer impurities.

[0146] The dispersion medium may be a mixture of water and a hydrophilic solvent. Examples of hydrophilic solvents include carbonate compounds such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, and butylene carbonate; ether compounds such as tetrahydrofuran; ketone compounds such as acetone; lower alcohol compounds such as methanol and ethanol; and solvents such as acetonitrile. When the dispersion medium contains a hydrophilic solvent, the proportion of the hydrophilic solvent in the dispersion medium is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 1% by mass or less.

[0147] The dispersion may further contain, in addition to CNTs and the dispersion medium, other components that can be used in the dispersion. Other components may include dispersants, defoamers, and antistatic agents. Furthermore, trace amounts of impurities, so-called unavoidable impurities, may also be included. [Examples]

[0148] The alkali metal secondary battery according to this disclosure will be described in more detail below with reference to examples. The alkali metal secondary battery according to this disclosure is not limited to the following examples, unless it exceeds the spirit of the disclosure.

[0149] <Example 1: Sodium Metal Secondary Battery> [Fabrication of the negative electrode current collector 1] The negative electrode current collector 1 of Example 1 was fabricated by cutting out a sheet-like CNT aggregate produced by a floating catalyst method (FCCVD method) that directly interacts with the self-assembly of CNT bundles in the gas phase. First, methane (a carbon source), ferrocene (a metal catalyst precursor containing Fe atoms), and thiophene (an accelerator) were introduced into a continuous flow of carrier gas in a once-through reactor, which was temperature-controlled to 400°C to 700°C. Hydrogen was used as the carrier gas, and the carrier gas flow rate was set to 30,000 sccm (standard cubic centimeter per minute). By maintaining the temperature within the once-through reactor within the above range, the metal catalyst precursor was generated as a particulate metal catalyst. The metal catalyst and carbon source were supplied to a temperature-controlled zone at 1400°C. This temperature zone was maintained at a temperature sufficient to generate carbon nanotube aggregates. The generated aggregates were continuously discharged through the outlet of a flow-through reactor, which was temperature-controlled between 100°C and 500°C, and collected as sheet-like CNT aggregates. The obtained sheet-like CNT aggregates 1 were cut into φ15 mm discs to obtain a negative electrode current collector 1 containing the CNT layer.

[0150] [Method for measuring the thickness of the CNT layer] A negative electrode current collector was cut to a length of 100 mm and a width of 10 mm to obtain a test piece for thickness measurement. The test piece used was free from defects such as wrinkles or dirt. A center line was identified to divide the test specimen in two in the width direction. Ten measurement points were set along the center line at 10 mm intervals along the length direction. At each measurement point on the test specimen, the thickness was measured using a micrometer, and the average of the thicknesses obtained at the ten measurement points was calculated to determine the thickness of the negative electrode current collector. The micrometer used for thickness measurement had a circular measuring surface with a diameter of 5 mm. The measuring force applied to the test specimen was 0.1 N. The micrometer's zero point was confirmed before starting the measurement and reconfirmed that there was no shift after the series of measurements. When the negative electrode current collector consists of only a CNT layer, the thickness of the negative electrode current collector obtained by the above method was taken as the target thickness of the CNT layer.

[0151] Furthermore, when the negative electrode current collector contains a Cu foil in addition to a CNT layer, the thickness of the CNT layer was calculated using the method described below. The thickness T1 of the negative electrode current collector was determined by the method described above. Next, a piece of Cu foil identical to that contained in the negative electrode current collector was cut to a length of 100 mm and a width of 10 mm. Using this as a test piece, the thickness T2 of the Cu foil was determined by the same thickness measurement method as described above. Subtracting T2 from T1, the resulting value was taken as the target thickness of the CNT layer.

[0152] [Calculation of X / Y] First, the mass of the active material in the electrode (positive electrode) containing an active material capable of intercalating and releasing alkali metal ions was measured. Next, the chemical formula (chemical structure) of the active material was identified by analyzing it using a powder X-ray diffraction analyzer, RINT2500TTR model, manufactured by Rigaku Corporation, under the following conditions. conditions: X-ray:CuKα Voltage-Current: 40kV-140mA Measurement angle range: 2θ = 10 to 90° Step: 0.02° Scan speed: 4° / min The molar mass of the active material was calculated from its chemical formula, and the amount of alkali metal that can be theoretically absorbed and released by charging and discharging was calculated as the amount of substance X. Next, the amount of alkali metal ions contained in the electrode (positive electrode) and the counter electrode, which both contain an active material capable of intercepting and releasing alkali metal ions, was measured. Here, the electrode and counter electrode, which do not contain alkali metal ion components derived from the electrolyte, were subjected to low-temperature ashing / acid dissolution as a pretreatment, followed by elemental quantitative analysis using an ICP (ICP-emission spectrometer, PerkinElmer, NexION 2000C) instrument, and the total amount of alkali metals Y contained in the electrode and counter electrode was calculated. As a pretreatment step, the electrode underwent acid decomposition treatment using a mixed acid solution of nitric acid and hydrochloric acid. The counter electrode underwent low-temperature ashing and acid decomposition treatment using a mixed acid solution of nitric acid and hydrochloric acid.

[0153] [Raman G / D intensity measurement method] The G / D intensity, which is the ratio of the peak intensity G of the G band to the peak intensity D of the D band in the Raman spectrum of the CNT layer, was determined by the following method. Using a Raman spectrometer, Raman spectra of the CNT layer surface were measured at 10 locations under the following measurement conditions. The peak intensity G of the G band and the peak intensity D of the D band were read from the Raman spectrum obtained by averaging the data from the 10 points, and the G / D intensity was determined. <Measurement conditions> Equipment used: RAMAN-11 (manufactured by Nanophoton Co., Ltd.) Excitation laser wavelength: 532nm Grating: 600 grooves / mm Objective lens: 20x magnification, numerical aperture (NA) 0.45

[0154] [Fabrication of positive electrode 1] (Manufacturing of electrode active material (composite metal oxide A1)) In a polypropylene beaker, 300 mL of distilled water and 44.88 g of potassium hydroxide were added and stirred to dissolve the potassium hydroxide completely, preparing an aqueous potassium hydroxide solution (precipitant). In another polypropylene beaker, 21.21 g of iron(II) chloride tetrahydrate, 19.02 g of nickel(II) chloride hexahydrate, and 15.83 g of manganese(II) chloride tetrahydrate were added to 300 mL of distilled water and dissolved by stirring to obtain an iron-nickel-manganese-containing aqueous solution. While stirring the aforementioned precipitating agent, the iron-nickel-manganese-containing aqueous solution was added dropwise to obtain a slurry in which a precipitate was formed. Next, the slurry was filtered and washed with distilled water, and dried at 100°C to obtain a precipitate. The precipitate, sodium carbonate, and calcium hydroxide were weighed in a molar ratio of Fe:Na:Ca = 0.4:0.99:0.01, and then dry-mixed using an agate mortar to obtain a mixture. Next, the mixture was placed in an alumina firing container and fired in an electric furnace at 850°C in an air atmosphere for 6 hours. After cooling to room temperature (25°C; the same applies hereafter), composite metal oxide A1 was obtained.

[0155] Powder X-ray diffraction analysis of composite metal oxide A1 revealed that it belongs to the α-NaFeO2 type crystal structure. A RIGAK RINT2500TTR powder X-ray diffractometer was used for the analysis under the following conditions. <Condition> X-ray:CuKα Voltage-Current: 40kV-140mA Measurement angle range: 2θ = 10 to 90° Step: 0.02° Scan speed: 4° / min

[0156] Using ICP-AES (IPC-Atomic Emission Spectroscopy, PerkinElmer, NexION 2000C), the composition of composite metal oxide A1 was analyzed after acid dissolution as a pretreatment. The molar ratio of Na:Ca:Fe:Ni:Mn was found to be 0.96:0.02:0.4:0.3:0.3.

[0157] (Manufacturing of electrode composite paste P1) For the manufacture of electrode composite paste P1, composite metal oxide A1 was used as the electrode active material, acetylene black (HS100, manufactured by Denka Co., Ltd.) as the conductive material, and PVdF (manufactured by Kureha Corporation) as the binder. The mixture was weighed and stirred so that A1:HS100:PVdF = 90:5:5 (mass ratio). The stirring was performed using a TK Robomix Filmix as the main unit with a TK Filmix (manufactured by Primix Co., Ltd.) attached to the stirring section, and the mixture was stirred and mixed at a rotation condition of 5000 rpm for 3 minutes to obtain electrode composite paste P1.

[0158] Electrode composite paste P1 was applied to 20 μm thick aluminum foil using a doctor blade (gap size: 221 μm), air-dried at 60°C for 1 hour, and then rolled using a roll press at a pressure of 0.5 MPa. Electrode DP1 was then obtained by vacuum drying at 150°C for 8 hours. The positive electrode basis weight was 12 mg / cm². 2 The electrode DP1 was cut into a φ13mm disc shape to manufacture the positive electrode 1.

[0159] [Preparation of Electrolyte Solution 1] A mixture of NaPF6 and NaBF4 as sodium salts in a molar ratio of 90:10 was dissolved in diethylene glycol dimethyl ether (G2) solvent to a concentration of 1 mol / L. Molecular sieves (4A) were then added and dehydrated to prepare electrolyte solution 1.

[0160] [Construction of a sodium-based secondary battery] A porous polyethylene separator and a glass separator were placed between the negative electrode current collector 1 and the positive electrode 1 as described above, housed in a battery case (standard 2032), and the electrolyte 1 was poured in. The battery case was then sealed to produce a coin-shaped sodium secondary battery (i.e., a coin cell) with a diameter of 20 mm and a thickness of 3.2 mm.

[0161] [Charge / Discharge Evaluation: First Charge / Discharge] The separator of the coin-type sodium rechargeable battery was allowed to fully impregnate with electrolyte by leaving it undisturbed at room temperature for 12 hours. Next, the initial charge and discharge was performed by constant current charging at 0.4 mA (0.2 C) to 3.9 V at room temperature (Na deposition on CNTs), followed by constant voltage charging at 3.9 V for 5 hours, and then constant current discharge at 0.4 mA (0.2 C) to 2.0 V (Na dissolution from CNTs). The charging capacity was measured, and the obtained value was defined as the "initial charging capacity" (mAh), i.e., the charging capacity for the first cycle. Similarly, the discharge capacity was measured, and the obtained value was defined as the "initial discharge capacity" (mAh), i.e., the discharge capacity for the first cycle. Using the values ​​of the initial discharge capacity and the initial charge capacity, the initial charge-discharge efficiency is calculated using the following formula. Initial charge / discharge efficiency (%) = Initial discharge capacity (mAh) ÷ Initial charge capacity (mAh) × 100

[0162] [Cycle characteristic evaluation: Number of cycles with an 80% maintenance rate] After the initial charge and discharge, the same conditions as the initial charge and discharge were used, with charging at 0.6mA and discharging at 0.6mA repeated twice. From the fourth cycle onward, constant current charging was performed for 5 hours at 1.0mA (0.5C) up to 3.9V (Na deposition on CNTs), followed by constant voltage charging at 3.9V. Then, constant current discharge was performed at 1.0mA (0.5C) up to 2.0V (Na dissolution from CNTs). The discharge capacity in the first cycle test was taken as 100%, and the life cycle count (LT80) was calculated as the number of cycles (n) in which the discharge capacity reached 80% from the second cycle onward. 80% (life cycle count) = Discharge capacity on the nth cycle / Discharge capacity on the first cycle × 100

[0163] <Example 2> [Fabrication of negative electrode current collector 2 and battery evaluation] Except for using CNT aggregate 2, which was obtained by placing CNT aggregate 1 in a firing furnace (Kurata Giken Co., Ltd. ultra-high temperature heat treatment furnace, model number: SCC-220 / 175 / 310), raising the temperature to 1800°C at 20°C / min under an argon atmosphere at atmospheric pressure, holding at 1800°C for 1 hour, and allowing it to cool naturally, the negative electrode current collector 2 was manufactured in the same manner as the negative electrode current collector 1, and a coin cell was manufactured under the same conditions as in Example 1, and the cycle characteristics and initial charge / discharge efficiency were calculated.

[0164] <Example 3> [Fabrication of negative electrode current collector 3 and battery evaluation] Except for using a CNT assembly 3 obtained by placing CNT assembly 1 in a firing furnace (Motoyama Corporation, high-speed heating electric furnace, model: SK-2030D), heating it to 500°C at 1°C / min under atmospheric pressure, and then allowing it to cool naturally, the negative electrode current collector 3 was manufactured in the same manner as the negative electrode current collector 1, and a coin cell was manufactured under the same conditions as in Example 1, and the cycle characteristics and initial charge / discharge efficiency were calculated.

[0165] <Example 4> [Fabrication of negative electrode current collector 4 and battery evaluation] Four commercially available sheet-type CNT aggregates (manufactured by Hamamatsu Carbonics Co., Ltd., model: NTS2130) were stacked, ethanol was added dropwise, and the assembly was vacuum-dried at 60°C for 2 hours to evaporate the ethanol and bring the CNT aggregates into close contact, thereby creating a CNT aggregate laminate. The obtained CNT aggregate laminate was cut into a φ15 mm disc shape to obtain a negative electrode current collector 4. A coin cell was fabricated under the same conditions as in Example 1, except that negative electrode current collector 4 was used instead of negative electrode current collector 1, and the cycle characteristics and initial charge / discharge efficiency were calculated.

[0166] <Example 5> [Fabrication of negative electrode current collector 5 and battery evaluation] Ten commercially available sheet-type CNT aggregates (manufactured by Hamamatsu Carbonics Co., Ltd., model: NTS2130) were stacked, ethanol was added dropwise, and the assembly was vacuum-dried at 60°C for 2 hours to evaporate the ethanol and bring the CNT aggregates into close contact, thereby creating a CNT aggregate laminate. The obtained CNT aggregate laminate was cut into a φ15 mm disc shape to obtain a negative electrode current collector 5. A coin cell was fabricated under the same conditions as in Example 1, except that negative electrode current collector 5 was used instead of negative electrode current collector 1, and the cycle characteristics and initial charge / discharge efficiency were calculated.

[0167] <Comparative Example 1> [Fabrication of negative electrode current collector 6 and battery evaluation] 1.1 g of CNT aggregate 1, 1.65 g of carboxymethylcellulose (product name: Carboxymethyl Cellulose Sodium Salt High Viscosity, manufactured by MP Biomedicals) as a dispersant, and 547.25 g of deionized water were mixed. The resulting mixture was subjected to a 1-hour dispersion treatment using an Ace homogenizer manufactured by Nippon Seiki Co., Ltd. as a pre-dispersion before the main dispersion to obtain dispersion A. To prevent CNT aggregate 1 from becoming entangled in the homogenizer blades, CNT aggregate 1 was cut into small pieces of approximately 1 cm square using scissors before mixing. Next, the dispersion A obtained above was subjected to the main dispersion treatment under the following conditions using a wet jet mill, an ultra-high pressure homogenizer [model number: NAGS100] manufactured by Jōkō Co., Ltd., to obtain CNT dispersion A. -conditions- Nozzle diameter: 0.22mm Pressure: 85 MPa Number of times: 8 Method: Circulation method

[0168] The obtained CNT dispersion A was coated onto a 10 μm thick Cu foil, which served as the current collector, using a doctor blade (gap size: 600 μm). The coated CNT layer was vacuum-dried at 60°C for 2 hours to evaporate the solvent. The resulting laminate of CNTs and the current collector was cut into a φ15 mm disc to obtain a negative electrode current collector 6. A coin cell was fabricated under the same conditions as in Example 1, except that negative electrode current collector 6 was used instead of negative electrode current collector 1, and the cycle characteristics and initial charge / discharge efficiency were calculated.

[0169] <Comparative Example 2> 1.1 g of commercially available CNT aggregate (product number: FT6120, multi-walled carbon nanotube powder, manufactured by C-nano), 1.65 g of carboxymethylcellulose (product name: Carboxymethyl Cellulose Sodium Salt High Viscosity, manufactured by MP Biomedicals) as a dispersant, and 547.25 g of deionized water were mixed. The resulting mixture was subjected to a 1-hour dispersion treatment using an Ace homogenizer manufactured by Nippon Seiki Co., Ltd. as a pre-dispersion before the main dispersion to obtain dispersion B. Next, the dispersion B obtained above was subjected to the same dispersion treatment as the one performed on CNT aggregate 1 in Comparative Example 1 using a wet jet mill, an ultra-high pressure homogenizer [model number: NAGS100] manufactured by Jōkō Co., Ltd., to obtain CNT dispersion B. Except for using CNT dispersion B instead of CNT dispersion A, the negative electrode current collector 7 was fabricated using the same procedure as for the negative electrode current collector 6, and a coin cell was fabricated under the same conditions as in Example 1. The cycle characteristics and initial charge-discharge efficiency were then calculated.

[0170] Table 1 shows the thickness of the CNT layer in the negative electrode current collector, the G / D intensity and X / Y values ​​obtained by Raman measurement of the negative electrode current collector, the initial charge-discharge efficiency and cycle characteristics of the sodium secondary battery, and the energy density per unit mass of the electrode and counter electrode for Examples 1 to 5 and Comparative Examples 1 to 2.

[0171] [Table 1]

[0172] As shown in Table 1, the sodium secondary batteries using the negative electrode current collector containing the CNT layer in Examples 1 to 5 had an initial charge / discharge efficiency of 70% or more and a cycle characteristic of 50 cycles or more. It has become clear that a sodium secondary battery using a negative electrode current collector containing a CNT layer as described in this disclosure can achieve high initial charge-discharge efficiency and cycle characteristics.

[0173] <Example 6: Lithium Metal Secondary Battery> [Fabrication of positive electrode 2] A LiCoO2 foil (82 μm thick: manufactured by Nippon Chemical Industrial Co., Ltd.) was cut into a φ13 mm disc shape to produce the positive electrode 2.

[0174] [Preparation of Electrolyte Solution 2] An electrolyte was prepared by dissolving LiPF6 as a lithium salt in fluoroethylene carbonate (FEC) solvent at a concentration of 1 mol / L.

[0175] [Fabrication of lithium metal secondary batteries] A glass separator was placed between the negative electrode current collector 1 and the positive electrode 2, housed in a battery case (standard 2032), and the electrolyte 2 was poured in. By sealing the battery case, a coin-shaped lithium secondary battery with a diameter of 20 mm and a thickness of 3.2 mm was fabricated.

[0176] [Charge / Discharge Evaluation: First Charge / Discharge] The separator of the coin-type lithium secondary battery was allowed to fully impregnate with electrolyte by leaving it undisturbed at room temperature for 12 hours. Next, the initial charge and discharge was performed by constant current charging at 0.8 mA (0.2 C) up to 4.2 V at room temperature (Li deposition on CNTs), followed by constant voltage charging at 4.2 V for 5 hours, and then constant current discharge at 0.8 mA (0.2 C) down to 2.2 V (Li dissolution from CNTs). The charging capacity was measured, and the obtained value was defined as the "initial charging capacity" (mAh), i.e., the charging capacity for the first cycle. Similarly, the discharge capacity was measured, and the obtained value was defined as the "initial discharge capacity" (mAh), i.e., the discharge capacity for the first cycle. The initial charge-discharge efficiency was calculated using the following formula, based on the values ​​of the initial discharge capacity and the initial charge capacity. Initial charge / discharge efficiency (%) = Initial discharge capacity (mAh) ÷ Initial charge capacity (mAh) × 100

[0177] <Comparative Example 3> [Fabrication of negative electrode current collector 8 and battery evaluation] Twelve commercially available sheet-type CNT aggregates (manufactured by Hamamatsu Carbonics Co., Ltd., model: NTS2130) were stacked, ethanol was added dropwise, and the assembly was vacuum-dried at 60°C for 2 hours to evaporate the ethanol and bring the CNT aggregates into close contact, thereby creating a CNT aggregate laminate. The obtained CNT aggregate laminate was cut into a φ15 mm disc shape to obtain a negative electrode current collector 8. A coin cell was fabricated under the same conditions as in Example 1, except that negative electrode current collector 8 was used instead of negative electrode current collector 1, and the initial charge-discharge efficiency was calculated.

[0178] <Comparative Example 4> [Fabrication of negative electrode current collector 9 and battery evaluation] Sixteen commercially available sheet-type CNT aggregates (manufactured by Hamamatsu Carbonics Co., Ltd., model: NTS2130) were stacked, ethanol was added dropwise, and the assembly was vacuum-dried at 60°C for 2 hours to evaporate the ethanol and bring the CNT aggregates into close contact, thereby creating a CNT aggregate laminate. The obtained CNT aggregate laminate was cut into a φ15 mm disc shape to obtain a negative electrode current collector 9. A coin cell was fabricated under the same conditions as in Example 1, except that negative electrode current collector 9 was used instead of negative electrode current collector 1, and the initial charge-discharge efficiency was calculated.

[0179] Table 2 shows the thickness of the CNT layer in the negative electrode current collector, the G / D intensity and X / Y values ​​obtained by Raman measurement of the negative electrode current collector, the initial charge-discharge efficiency of the lithium secondary battery, and the energy density per unit mass of the electrode and counter electrode for Example 6 and Comparative Examples 3-4.

[0180] [Table 2]

[0181] As shown in Table 2, the lithium secondary battery using the negative electrode current collector of Example 6 had an initial charge-discharge efficiency of 70% or more. It has become clear that a lithium secondary battery using a negative electrode current collector containing a CNT layer according to this disclosure can achieve high initial charge-discharge efficiency. [Explanation of symbols]

[0182] 10 Alkali metal rechargeable batteries 12. Electrode (positive electrode) containing an active material capable of intercepting and releasing alkali metal ions. 14 Separator 16 Counter electrode (negative electrode) containing a CNT layer

Claims

1. An alkali metal secondary battery comprising an electrode containing an active material capable of intercepting and releasing alkali metal ions, a counter electrode containing a carbon nanotube layer with a thickness of 12 μm to 70 μm, and an electrolyte, An alkali metal secondary battery in which the amount of alkali metal substance X equivalent to the theoretical capacity of the active material contained in the electrode, and the sum of the amounts of alkali metal substance Y contained in the electrode and the counter electrode satisfy 0.7 ≤ X / Y ≤ 1.

1.

2. The alkali metal secondary battery according to claim 1, wherein the G / D intensity obtained by Raman measurement of the carbon nanotube layer is 2.5 or higher.

3. The alkali metal secondary battery according to claim 1 or claim 2, wherein the alkali metal is lithium or sodium.

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