JP7828598B2 - A negative electrode active material, a negative electrode material, an alkaline ion battery comprising the negative electrode material, and a method for manufacturing the negative electrode active material. - Google Patents

Description

This invention relates to a negative electrode active material, a negative electrode material, an alkaline ion battery comprising the negative electrode material, and a method for producing the negative electrode active material.

Metal sulfides possess excellent redox reaction characteristics, thermal stability, and mechanical properties, making them promising compounds for use as negative electrode active materials in alkaline-ion batteries, such as high-performance sodium-ion batteries (SIBs).

For example, Non-Patent Document 1 proposes using cobalt sulfide, immobilized on a sponge-like carbon matrix synthesized by freeze-drying and hydrothermal processes, as a negative electrode material for sodium-ion batteries. Such a negative electrode material is said to significantly improve the storage performance of sodium-ion batteries.

Chemical Engineering Journal 332 (2018) 370-376

However, in recent years, there has been a strong demand for further improvements in cycle characteristics in the battery field. Accordingly, the development of battery materials capable of providing superior cycle characteristics is required. From this perspective, developing anode active materials that can provide excellent cycle characteristics when applied to the anode material of alkaline-ion batteries is of extremely important importance in the battery field.

This invention has been made in view of the above, and aims to provide a negative electrode active material, a negative electrode material, and an alkaline ion battery equipped with the negative electrode material, which can provide excellent cycle characteristics to an alkaline ion battery, as well as a method for producing the negative electrode active material.

The inventors of this invention, through diligent research to achieve the above objective, discovered that this objective can be achieved by using metal sulfides containing specific transition metals, and thus completed the present invention.

In other words, the present invention encompasses, for example, the subject matter described in the following sections.
Item 1
A negative electrode active material for alkaline ion batteries,
It contains metal sulfides,
The metal sulfide is a negative electrode active material containing at least three transition metals selected from the group consisting of Co, Ni, Zn, Mo, Mn, Cu, Fe, and V.
Section 2
The negative electrode active material according to item 1, wherein the metal sulfide is in particulate form.
Section 3
A negative electrode material comprising the negative electrode active material described in item 1 or 2.
Section 4
An alkaline ion battery comprising the negative electrode material described in item 3.
Item 5
In the method for producing a negative electrode active material described in item 1 or 2,
Step 1 involves mixing a metal source containing at least three transition metals selected from the group consisting of Co, Ni, Zn, Mo, Mn, Cu, Fe, and V with a raw material containing an organic ligand to obtain a metal-organic structure.
Step 2 involves heat-treating the raw materials, including the metal-organic structure and sulfur source obtained in Step 1, to obtain a product.
A method for producing a negative electrode active material, comprising the same components.

The negative electrode active material of the present invention can provide excellent cycle characteristics to alkaline ion batteries.

(a) is an SEM image of the negative electrode active material obtained in Example 1, (b) is an SEM image of the negative electrode active material obtained in Example 2, and (c) is an SEM image of the negative electrode active material obtained in Example 3. The X-ray diffraction (XRD) measurement results of the negative electrode active materials obtained in each example are shown. The results of constant current charge-discharge tests on the batteries assembled in each example are shown.

The embodiments of the present invention will be described in detail below. In this specification, the terms "containing" and "including" encompass the concepts of "containing," "including," "substantially consisting of," and "consisting solely of."

1. Negative Electrode Active Material The negative electrode active material of the present invention is a negative electrode active material used in alkaline-ion batteries and contains a metal sulfide. The metal sulfide contains at least three transition metals selected from the group consisting of Co, Ni, Zn, Mo, Mn, Cu, Fe, and V. The negative electrode active material of the present invention can be used as a negative electrode material for constructing alkaline-ion batteries (for example, sodium-ion batteries) and can provide excellent cycle characteristics to alkaline-ion batteries.

The metal sulfide is a sulfide containing at least three of the aforementioned transition metals. The transition metals contained in the metal sulfide may be limited to three. Examples of metal sulfides include composite sulfides containing multiple transition metal elements. In these composite sulfides, each transition metal can exist within the sulfide framework; specifically, each transition metal can exist bonded to sulfur. In the composite sulfide, two or more different transition metals can be bonded to a single sulfur atom. Furthermore, in other embodiments of the negative electrode active material, the negative electrode active material may contain multiple metal sulfides, for example, a mixture of three or more different metal sulfides, or a mixture of a composite sulfide composed of two transition metals and a sulfide composed of one transition metal.

In order to provide excellent cycle characteristics to alkaline-ion batteries and to be manufactured by a simple method, metal sulfides are preferably composite sulfides, and more preferably composite sulfides composed of three or more transition metals.

The transition metals contained in the metal sulfide preferably include at least Co, Ni, and Zn. In this case, the negative electrode active material can be manufactured by a simple method, and particularly excellent cycle characteristics can be obtained for alkaline-ion batteries. The transition metals contained in the metal sulfide may consist of only Co, Ni, and Zn, and among these, a composite sulfide of Co, Ni, and Zn is preferred. However, this does not exclude metal elements that are inevitably present in the metal sulfide.

In metal sulfides, the content ratio of each transition metal is not particularly limited; as long as at least three transition metal elements are present, any content ratio can result in the desired negative electrode active material. For example, when the metal sulfide contains Co, Ni, and Zn as described above, the Co content per 100 parts by mass of Ni is preferably 10 parts by mass or more, more preferably 50 parts by mass or more, even more preferably 80 parts by mass or more, and particularly preferably 100 parts by mass or more. Furthermore, the Co content per 100 parts by mass of Ni is preferably 1000 parts by mass or less, more preferably 800 parts by mass or less, even more preferably 500 parts by mass or less, even more preferably 300 parts by mass or less, and particularly preferably 200 parts by mass or less.

Furthermore, when the metal sulfide contains Co, Ni, and Zn as described above, for example, the Zn content per 100 parts by mass of Ni is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, even more preferably 20 parts by mass or more, and particularly preferably 25 parts by mass or more. Also, the Zn content per 100 parts by mass of Ni is preferably 100 parts by mass or less, more preferably 80 parts by mass or less, even more preferably 60 parts by mass or less, even more preferably 50 parts by mass or less, and particularly preferably 40 parts by mass or less.

The shape of the metal sulfide is not particularly limited and can form various shapes. In particular, the metal sulfide is preferably particulate. In this case, the metal sulfide can take various shapes such as porous particles, hollow particles, amorphous particles, and spherical particles.

When the metal sulfide is in particulate form, its average particle size is not particularly limited and can be, for example, 500 nm to 100 μm. In this case, the negative electrode active material tends to provide excellent cycle characteristics to alkaline ion batteries. When the metal sulfide is in particulate form, its average particle size is preferably 1 to 50 μm, more preferably 2 to 80 μm, even more preferably 3 to 60 μm, and particularly preferably 5 to 30 μm. Here, the average particle size refers to the value obtained by arithmetic mean of 50 randomly selected particles obtained by direct observation of the metal sulfide using a scanning electron microscope, and measuring their equivalent circular diameters.

The negative electrode active material may contain other components in addition to the metal sulfide, as long as the effects of the present invention are not hindered, or the negative electrode active material may consist solely of the metal sulfide. The content of the metal sulfide in the negative electrode active material is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 99% by mass or more. Whether or not the metal sulfide is present in the negative electrode active material can be determined from the XRD spectrum of the negative electrode active material.

The method for producing the negative electrode active material is not particularly limited. For example, the negative electrode active material of the present invention can be produced by a production method comprising steps 1 and 2 described below.

2. Method for producing the negative electrode active material The method for producing the negative electrode active material of the present invention may comprise, for example, the following steps 1 and 2.
Step 1: A step of obtaining a metal-organic structure by mixing a metal source containing at least three transition metals selected from the group consisting of Co, Ni, Zn, Mo, Mn, Cu, Fe, and V with a raw material containing an organic ligand.
Step 2: A step of obtaining a product by heat-treating the raw materials, which include the metal-organic structure and sulfur source obtained in Step 1.

The manufacturing method comprising steps 1 and 2 described above can produce a negative electrode active material, and for example, the negative electrode active material of the present invention described above can be produced.

(Step 1)
In step 1, a metal source containing at least three transition metals selected from the group consisting of Co, Ni, Zn, Mo, Mn, Cu, Fe, and V is mixed with a raw material containing an organic ligand.

The metal source may be a transition metal element, a transition metal compound, or a mixture thereof, preferably containing at least three transition metal compounds.

The types of transition metal compounds are not particularly limited; for example, inorganic compounds of various transition metals, chlorides of various transition metals, and organic compounds of various transition metals can be cited. Examples of inorganic transition metal compounds include oxides of transition metals, compounds containing oxoanions of transition metals (metal salts), nitrates, sulfates, chlorides, chlorates, perchlorates, chloride complexes, carbonates, bicarbonates, phosphates, and hydrogen phosphates of transition metals. Among these, nitrates are preferred for the inorganic transition metal compounds. Examples of organic transition metal compounds include acetates, oxalates, formates, and succinates.

The metal source preferably contains at least three compounds selected from the group consisting of Co compounds, Ni compounds, Zn compounds, Mo compounds, Mn compounds, Cu compounds, Fe compounds, and V compounds. In particular, it is preferable to contain at least three nitrates selected from the group consisting of Co nitrate, Ni nitrate, Zn nitrate, Mo nitrate, Mn nitrate, Cu nitrate, Fe nitrate, and V nitrate. In this case, the negative electrode active material can be produced by a simpler method. For example, the metal source used in step 1 preferably contains Co nitrate, Ni nitrate, and zinc nitrate. The transition metals included in the metal source used in step 1 may be only Co, Ni, and Zn.

The metal source can be dissolved or dispersed in a solvent. Examples of such solvents include alcohol-based solvents such as water, ethanol, and isopropanol; ketone-based solvents such as acetone and methyl ethyl ketone; and others such as N,N-dimethylformamide, glycerol, ethylene glycol, N,N-diethylformamide, N-methyl-2-pyrrolidone, acetonitrile, triethylamine, and tetrahydrofuran. The solvent may also be a mixture of water and an organic solvent. By dissolving or dispersing the metal source in a solvent, the metal source containing the transition metal becomes a solution or dispersion.

The concentration of the metal source solution or dispersion is not particularly limited; for example, the total concentration of the transition metal can be 1 to 1000 g/L, preferably 5 to 500 g/L, and more preferably 10 to 100 g/L.

In the metal source, the content ratio of each transition metal is not particularly limited, and as long as it contains at least three transition metal elements, the desired anode active material can be produced with any content ratio. For example, when the transition metals contained in the metal source include Co, Ni, and Zn, for example, the Co content per 100 parts by mass of Ni is preferably 10 parts by mass or more, more preferably 50 parts by mass or more, even more preferably 80 parts by mass or more, and particularly preferably 100 parts by mass or more. Also, the Co content per 100 parts by mass of Ni is preferably 1000 parts by mass or less, more preferably 800 parts by mass or less, even more preferably 500 parts by mass or less, even more preferably 300 parts by mass or less, and particularly preferably 200 parts by mass or less. Also, the Zn content per 100 parts by mass of Ni is preferably 1 part by mass or more, preferably 5 parts by mass or more, more preferably 10 parts by mass or more, even more preferably 20 parts by mass or more, and particularly preferably 25 parts by mass or more. Furthermore, the Zn content per 100 parts by mass of Ni is preferably 100 parts by mass or less, more preferably 80 parts by mass or less, even more preferably 60 parts by mass or less, even more preferably 50 parts by mass or less, and particularly preferably 40 parts by mass or less.

In the raw materials containing the organic ligand used in step 1, the type of organic ligand is not particularly limited, and a wide range of organic ligands capable of coordinating to transition metals can be mentioned. For example, aromatic carboxylic acid compounds, imidazole compounds, amino compounds, etc., known to function as ligands, can be mentioned.

Specifically, the organic ligands include p-benzenedicarboxylic acid (H2BDC), o-benzenedicarboxylic acid, m-benzenedicarboxylic acid, 2,5-dihydroxyterephthalic acid (H4DOBDC), 1,3,5-benzenetricarboxylic acid (H3BTC), 1,4-benzenedicarboxylate, 2,4,6-tris(4-carboxyphenyl)-1,3,5-triazine (H3TATB), 2-aminoterephthalic acid (NH2BDC), 2-methylimidazole (2-MIM), 1- Examples include methylimidazole (1-MIM), 1,4-bis(imidazole-1-yl)benzene (1,4-BIB), 4-(imidazole-1-yl)phthalic acid (H2IPC), 4,4'-dimethyl-2,2'-bipyridyl, 4,4'-oxybisbenzoic acid, fumaric acid, oxalic acid, succinic acid, biphenyl-3,4',5-tricarboxylic acid (BPTC), 4,4'-biphenyl dicarboxylate (BPDC), and 2,5-dioxide terephthalate (DOT). The organic ligand may be used alone or in combination of two or more.

The resulting negative electrode active material is likely to provide excellent cycle characteristics in alkaline-ion batteries, and therefore, the organic ligand is particularly preferably p-benzenedicarboxylic acid (H2BDC).

The raw material containing the organic ligand may contain a solvent. Examples of such solvents include alcoholic solvents such as water, ethanol, and isopropanol; ketoneic solvents such as acetone and methyl ethyl ketone; and others such as N,N-dimethylformamide, glycerol, ethylene glycol, N,N-diethylformamide, N-methyl-2-pyrrolidone, acetonitrile, triethylamine, and tetrahydrofuran. The solvent may also be a mixture of water and an organic solvent. When the raw material containing the organic ligand contains a solvent, the raw material is in the form of a solution or dispersion.

When the raw material containing the organic ligand also contains a solvent, the total concentration of the organic ligand can be, for example, 0.1 to 100 g/L, preferably 1 to 50 g/L, and more preferably 3 to 30 g/L.

In step 1, if a metal source dissolved or dispersed in a solvent is used, and the raw material containing an organic ligand also contains a solvent, the solvents may be the same or different.

In step 1, the raw materials containing the metal source and organic ligand are mixed, causing a reaction between the metal source and the organic ligand to produce a metal-organic structure. Examples of metal-organic structures include various metal-organic frameworks (MOFs).

The mixing method is not particularly limited, and for example, known mixing methods can be used. The temperature during the mixing process can be 80 to 400°C, preferably 100 to 300°C, and more preferably 120 to 200°C. The mixing time can be appropriately selected according to the temperature, for example, 1 to 10 hours.

In the mixing process of step 1, the ratio of transition metal elements to organic ligands is not particularly limited. For example, it is preferable that the amount of organic ligands used be 1 to 150% by mass, more preferably 5 to 100% by mass, even more preferably 10 to 80% by mass, and particularly preferably 20 to 50% by mass, relative to the total mass of the transition metal elements.

The mixing process in step 1 yields, for example, a metal-organic structure (MOF) containing at least three transition metal elements, as a solid component. This solid component can be separated and washed by appropriate methods.

The metal-organic structure obtained in step 1 has a structure formed using transition metal elements contained in the metal source as constituent elements.

(Step 2)
In step 2, the raw materials containing the metal-organic structure (MOF) and sulfur source obtained in step 1 are heat-treated. The product obtained by this heat treatment is the metal sulfide.

The sulfur source used in step 2 may be elemental sulfur (sulfur powder or sublimated sulfur) or a sulfur-containing compound, but a sulfur-containing compound is preferable.

Examples of sulfur-containing compounds include a wide range of known sulfur compounds, such as thioacetamide ( _CH₃CSNH₂ ), thiourea (SC( _NH₂ ) _{₂ )} , cysteine ( _{C₃H₂NO₂S} ), sodium thiosulfate ( _{Na₂S₂O₃} ), _ammonium sulfide ((NH₄ _{) ₂S} ), _and sodium sulfide ( _Na₂S ) _. In addition, in sulfur-containing compounds, some of the sulfur elements may be replaced with Se and/ _or Te elements. A single sulfur source can be used, or two or more can be used in combination.

In step 2, the method for heat-treating the raw materials, including the metal-organic structure (MOF) and sulfur source obtained in step 1, is not particularly limited. For example, one method involves placing the solid metal-organic structure (MOF) and sulfur source in a reactor and heat-treating them at a predetermined temperature. This heat treatment can be carried out under an air atmosphere, an inert gas atmosphere, etc. For example, a known heating device such as a commercially available heating furnace can be used for the heat treatment.

In step 2, the heat treatment temperature is not particularly limited and can be, for example, 200 to 2000°C, preferably 250 to 1000°C, and more preferably 300 to 800°C. The heat treatment time is appropriately selected according to the temperature, for example, 1 to 10 hours.

The proportions of metal-organic frameworks (MOFs) and sulfur sources in the raw materials used in step 2 are not particularly limited. For example, in order to facilitate the formation of transition metal sulfides and obtain the desired negative electrode active material, it is preferable to use 50 to 1000 parts by mass of sulfur source per 100 parts by mass of metal-organic frameworks (MOFs), more preferably 80 to 800 parts by mass, and even more preferably 100 to 600 parts by mass.

The product obtained in step 2 is, for example, a metal sulfide containing at least three of the aforementioned transition metal elements. A purified metal sulfide can also be obtained by processing the product obtained in step 2 using an appropriate method.

The metal sulfide obtained in step 2 can be used as the negative electrode active material of the present invention, or it can be obtained by incorporating other components as needed.

The manufacturing method comprising steps 1 and 2 described above allows for the easy acquisition of the anode active material of the present invention through a simple process, resulting in a low-energy manufacturing method. Furthermore, the raw materials used in the manufacturing process are inexpensive and readily available from abundant resources.

3. Negative Electrode Material The negative electrode material of the present invention may contain other components as long as it contains the negative electrode active material described above. For example, known components used in the negative electrode material of alkaline ion batteries (e.g., sodium ion batteries) can be cited. For example, the negative electrode material of the present invention may contain a conductive additive and a binder in addition to the negative electrode active material described above.

Conductive additives can broadly include known conductive additives used, for example, to form electrode materials for various types of batteries. Examples of conductive additives include various carbon materials, such as hard carbon, soft carbon, graphene, reduced graphene oxide, natural graphite, artificial graphite, conductive carbon black, and carbon fibers. Examples of carbon fibers include carbon nanofibers and carbon nanotubes. Other conductive additives that can be used include metal powders such as copper and nickel, metal fibers, and conductive ceramic materials.

Binders, for example, can be broadly categorized into known binders used to form electrode materials for various types of batteries. Examples of binders include various resin materials, specifically polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), polyethylene terephthalate, polyacrylonitrile (PAN), polyvinyl alcohol (PVA), polyethylene oxide (PEO), polyethylene, polypropylene, and the like.

In the negative electrode material, the content ratio of the negative electrode active material is not particularly limited. For example, it is preferable that the negative electrode active material is present in an amount of 50 to 95% by mass, and more preferably 60 to 90% by mass, relative to the total mass of the negative electrode active material, conductive additive, and binder contained in the negative electrode material.

The proportion of conductive additive in the negative electrode material is not particularly limited. For example, it is preferable that the conductive additive be present in an amount of 3 to 30% by mass, and more preferably 5 to 20% by mass, relative to the total mass of the negative electrode active material, conductive additive, and binder contained in the negative electrode material.

In the negative electrode material, the binder content is not particularly limited. For example, it is preferable that the binder be present in an amount of 3 to 30% by mass, and more preferably 5 to 20% by mass, relative to the total mass of the negative electrode active material, conductive additive, and binder contained in the negative electrode material.

The negative electrode material may consist only of the negative electrode active material, conductive additive, and binder, or it may contain other components.

The method for preparing the negative electrode material is not particularly limited; for example, known methods for preparing negative electrode materials can be widely employed. For instance, the negative electrode material can be prepared by mixing a negative electrode active material, a conductive additive, and a binder in predetermined proportions using an appropriate method. When preparing the negative electrode material, a solvent can also be used to disperse the negative electrode active material, conductive additive, and binder. Examples of solvents include water and various organic solvents, such as lower alcohol compounds with 1 to 3 carbon atoms, and NMP (N-methyl-2-pyrrolidone). When the negative electrode material contains a solvent, it may take the form of a slurry or paste.

4. Alkaline Ion Battery The alkaline ion battery of the present invention is not particularly limited in its other configurations as long as it includes the negative electrode material, and can have a configuration similar to that of a known alkaline ion battery, for example. The type of alkaline ion battery is not particularly limited, and examples include sodium secondary batteries, lithium ion batteries, potassium secondary batteries, etc. The alkaline ion battery of the present invention is preferably a sodium battery, and more preferably a sodium ion secondary battery.

Alkaline-ion batteries may, for example, comprise a positive electrode, a negative electrode, an electrolyte, and a separator. The size and shape of the battery can be appropriately determined according to its application.

The positive electrode can have a structure composed of, for example, a metal foil and a positive electrode material. Examples of metals for forming the metal foil include aluminum, titanium, platinum, molybdenum, stainless steel, _and copper. A wide range of known positive electrode materials can be applied as _the positive electrode material. Examples of materials constituting the positive electrode material include sodium metal, lithium metal, _NaFePO₄ , _Na₃V₂ ( _PO₄ ) _₃ , _NaxMO₄ (M=Co _{, Mn} , _V , _{Fe )} , _LiTiS₂ , _LiCoO₂ , _{LiNiO₂ ,} LiMnO₂ _{, LiNi₀.33Mn₀.33Co₀.33O₂ , LiNi₀.8Mn₀.15Al₀.05O₂} , LiMn₂O₄, _{and LiFePO₄} . The positive electrode can be manufactured by known methods, such as coating a positive electrode material onto a metal foil.

The negative electrode can have a structure in which the negative electrode active material of the present invention is supported on a metal foil, for example. Examples of metal foils include aluminum, titanium, platinum, molybdenum, stainless steel, and copper. The negative electrode can be manufactured by known methods.

In alkaline-ion batteries, the type of electrolyte is not particularly limited; for example, any known electrolyte can be used. The electrolyte may be either a solid or liquid electrolyte.

Liquid electrolytes include solutions in which the electrolyte is dissolved in a solvent. Depending on the type of battery, various alkali salts can be used as electrolytes, such as _NaPF₂₆ , NaClO₄, _NaCF₃SO₃ , _NaFSI , NaTFSI, LiPF₂₆, _LiClO₄ , _LiBF₄ , _LiBOB , _LiAsF₂₆ , _LiCF₃SO₃ , LiTFSI, LiFSI, KPF₂₆, _KFSI , KTFSI, _{KBF₄ , etc.} Other examples include known magnesium salts, aluminum salts, zinc salts, etc. Examples of solvents include water, diglyme, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, propyl acetate, fluoroethylene carbonate, propylene carbonate, ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, etc.

Examples of solid electrolytes include inorganic materials such as sulfides and oxides, and polymer materials such as PEO (polyethylene oxide).

As the separator, known separators used in secondary batteries can be used, and examples include materials formed from polyolefin resins such as polyethylene and polypropylene; polyimide; polyvinyl alcohol; fluororesins such as terminally aminated polyethylene oxide polytetrafluoroethylene; acrylic resin; nylon; aromatic aramid; inorganic glass; ceramics, etc. The separator can be in _the form of a porous membrane, nonwoven fabric, woven fabric _, etc. Other examples of _separators include various polymer membranes and inorganic electrolytes. Examples of inorganic electrolytes include _LiLaTiO3 , _Li7La3Zr2O12 ( _{LLZO )} , _{Na3Zr2Si2PO12 , Na11Sn2PS12 , Na3PSe4 , etc.}

The present invention will be described more specifically below with reference to examples, but the present invention is not limited to the embodiments described herein.

(Example 1)
Solution 1 was obtained by dispersing a mixture consisting of 100 mg of Ni( _NO₃ ) _₂ · _6H₂O , 200 mg of Co( _NO₃ ) _₂ · _6H₂O , and 40 mg of Zn( _NO₃ ) _₂ · _6H₂O as a metal source in 15 mL of ethylene glycol. Solution 2 was obtained by dispersing 90 mg of p-benzenedicarboxylic acid (H₂BDC) in 24 mL of N,N-dimethylformamide (DMF) by sonication. Next, Solution 1 and Solution 2 were mixed and stirred at room temperature for 1 hour to obtain a mixture. This mixture was then transferred to a 50 mL Teflon-lined sealed autoclave and heat-treated at 150°C for 6 hours. The pale pink product generated by this heat treatment was collected by centrifugation, and the resulting solid was washed several times with DMF and ethanol. After that, it was dried in a vacuum oven at 60°C for 12 hours to obtain a metal-organic frame (MOF) containing Ni-Co-Zn (Step 1).

A raw material containing this MOF and thioacetamide (TAA) as a sulfur source in a mass ratio of 1:5 (MOF:sulfur source) was heat-treated at 350°C for 2 hours in an argon atmosphere (Step 2). This yielded a black product. The obtained black product (sulfide) was named "Co-Ni-Zn-S-1" and used as the negative electrode active material.

(Example 2)
A black product was obtained in the same manner as in Example 1, except that the metal source was changed to a mixture consisting of 150 mg of Ni( _NO₃ ) _₂ · _6H₂O , 150 mg of Co( _NO₃ ) _₂ · _6H₂O , and 40 mg of Zn( _NO₃ ) _₂ · _6H₂O . The obtained black product (sulfide) was named "Co-Ni-Zn-S-2" and used as the negative electrode active material.

(Example 3)
A black product was obtained in the same manner as in Example 1, except that the metal source was changed to a mixture consisting of 200 mg of Ni( _NO₃ ) _₂ · _6H₂O , 100 mg of Co( _NO₃ ) _₂ · _6H₂O , and 40 mg of Zn( _NO₃ ) _{₂ ·6H₂O} . The obtained black product (sulfide) was named "Co-Ni-Zn-S-3" and used as the negative electrode active material.

(Example 1)
A negative electrode material was prepared consisting of "Co-Ni-Zn-S-1" obtained in Example 1 as the negative electrode active material, superP (conductive carbon black) as a conductive additive, and polyvinylidene fluoride (PVDF) as a binder. In this negative electrode material, the ratio of Co-Ni-Zn-S1:superP:PVDF was set to 7.5:1.5:1 (mass ratio). N-methyl-2-pyrrolidone (NMP) was added to this negative electrode material as a solvent and stirred for 12 hours to mix uniformly. The resulting slurry was coated onto copper foil and dried in a vacuum at 120°C for 12 hours to fabricate a negative electrode. A battery was assembled using this negative electrode, a positive electrode (sodium metal with aluminum foil), a liquid electrolyte, and a separator ("Whatman GF/C glass fiber filter paper" provided by Cytiva) impregnated with this liquid electrolyte, by a known method. The electrolyte was a 1 M sodium trifluoromethanesulfonate solution, and the solvent for this solution was diglyme.

(Example 2)
The battery was assembled in the same manner as in Example 1, except that the negative electrode active material was changed from "Co-Ni-Zn-S-1" obtained in Example 1 to "Co-Ni-Zn-S2" obtained in Example 2.

(Example 3)
The battery was assembled in the same manner as in Example 1, except that the negative electrode active material was changed from "Co-Ni-Zn-S-1" obtained in Example 1 to "Co-Ni-Zn-S-3" obtained in Example 3.

(Evaluation results)
Figure 1(a) shows an SEM image of the negative electrode active material (metal sulfide) obtained in Example 1, (b) shows an SEM image of the negative electrode active material obtained in Example 2, and (c) shows an SEM image of the negative electrode active material obtained in Example 3. From these SEM images, it was found that the negative electrode active material (Ni-Co-Zn sulfide) obtained in each example is a hollow particle with a structure assembled from nanorods and possessing a microsphere structure.

Figure 2 shows the X-ray diffraction (XRD) results of the negative electrode active materials (metal sulfides) obtained in each example. X-ray diffraction measurements were performed using a Rigaku "SmartLab" system, with a Cu-Kα (λ = 1.540 Å) radiation source in the range of 2θ = 10 to 100°.

From the obtained XRD patterns, it was found that the diffraction peaks of Ni-Co-Zn-S1, Ni-Co-Zn-S2, and Ni-Co-Zn-S-3 all coincided with those of Ni ₃ S ₄ (JCPDS card No. 43-1469), Co ₃ S ₄ (JCPDS card No. 42-1448), and ZnS (JCPDS card No. 05-0566), and no other diffraction peaks were observed. From these results, it was found that the products (metal sulfides) obtained in each example are composite metal sulfides containing Ni, Co, and Zn.

Figure 3 shows the results of constant current charge-discharge tests on the batteries assembled in each fabrication example. These measurements were performed using the LAND battery test system "CT2001A" (Wuhan LAND electronics Co., Ltd. China). The measurement temperature was 30°C, and the applied voltage was 1.5–3.5V ( ^5Ag⁻¹ ).

As shown in Figure 3, after 300 cycles of testing, batteries equipped with negative electrode materials containing the negative electrode active materials of Example 1 (Ni-Co-Zn-S-1) and Example 2 (Ni-Co-Zn-S-2) were observed to provide a specific capacity of 450 ^mAhg⁻¹ at a high current density of ^2Ag⁻¹ . Furthermore, batteries equipped with negative electrode materials containing the negative electrode active material of Example 3 (Ni-Co-Zn-S-3) were observed to provide a specific capacity of 388.5 ^mAhg⁻¹ at a high current density of ^2Ag⁻¹ . In addition, batteries equipped with negative electrode materials containing the negative electrode active materials of Examples 1 to 3 all showed a Coulomb efficiency of over 90% after 300 cycles. On the other hand, conventionally known metal sulfides consisting solely of zinc, and metal sulfides consisting solely of zinc and cobalt, had specific capacities of 173.6 and 37.6 ^mAhg⁻¹ at a high current density of ^2Ag⁻¹ after 300 cycles of testing, respectively.

From the above, it was revealed that the negative electrode active materials containing metal sulfides obtained in Examples 1 to 3 provide excellent cycle characteristics to alkaline-ion batteries. Furthermore, it is presumed that as long as the negative electrode active material contains metal sulfides containing at least three transition metals selected from the group consisting of Co, Ni, Zn, Mo, Mn, Cu, Fe, and V, it can provide excellent cycle characteristics to alkaline-ion batteries.

Claims (5)

A negative electrode active material for alkaline ion batteries,
It consists only of metal sulfides,
The transition metals contained in the metal sulfide consist of only three types: Co, Ni, and Zn, and the Zn content relative to 100 parts by mass of Ni is between 10 parts by mass and 40 parts by mass.
The diffraction peaks obtained from X-ray diffraction (XRD) measurements of the aforementioned metal sulfide are _Ni₃S₄ , _Co₃S₄ , and _ZnS , with no other diffraction peaks observed; this is the negative _electrode active material. The negative electrode active material according to claim 1, wherein the metal sulfide is in particulate form. A negative electrode material comprising the negative electrode active material described in claim 1 or 2. An alkaline ion battery comprising the negative electrode material described in claim 3. In the method for producing a negative electrode active material according to claim 1 or 2,
Step 1 involves mixing a metal source consisting only of Co, Ni, and Zn with a raw material containing an organic ligand to obtain a metal-organic structure.
Step 2 involves heat-treating the raw materials, including the metal-organic structure and sulfur source obtained in Step 1, to obtain a product.
A method for producing a negative electrode active material, comprising the same components.