JP7407377B2 - Magnesium ion secondary batteries, positive electrode active materials for magnesium ion secondary batteries, and positive electrodes for magnesium ion secondary batteries - Google Patents

Description

The present disclosure relates to a magnesium ion secondary battery, a positive electrode active material for a magnesium ion secondary battery, and a positive electrode for a magnesium ion secondary battery.

In recent years, research on magnesium ion secondary batteries has attracted attention.

Non-Patent Document 1 describes a magnesium secondary battery using Mo ₆ S ₈ having a Chevrel phase as a positive electrode active material. Non-Patent Document 2 describes a magnesium secondary battery using V ₂ O ₅ which is an oxide as a positive electrode active material.

Prototype systems for rechargeable magnesium batteries, Nature, vol.407, pp.724-727 (2000) An artificial interphase enables reversible magnesium chemistry in carbonate electrolytes, Nature Chemistry, vol.10, pp.532-539 (2018)

The present disclosure provides a novel positive electrode active material for a magnesium ion secondary battery, and a magnesium ion secondary battery using the same.

This disclosure:
A positive electrode active material for a magnesium ion secondary battery is provided, which is represented by the following compositional formula (1).
_{MgM2O6 ...} (1)
In formula (1), M is V, Nb, or Ta.

According to the present disclosure, a novel positive electrode active material for a magnesium ion secondary battery and a magnesium ion secondary battery using the same can be provided.

FIG. 1 is a cross-sectional view schematically showing a configuration example of a magnesium ion secondary battery. FIG. 2(a) is a diagram showing the results of powder X-ray diffraction measurement of the positive electrode active material according to the example, and FIG. 2(b) is a diagram showing the results of powder X-ray diffraction measurement of MgNb ₂ O ₆ obtained by simulation. FIG. FIG. 3 is a schematic diagram showing a schematic configuration of a beaker cell according to an example. FIG. 4 is a graph showing the results of a charge/discharge test of the magnesium ion secondary battery according to the example.

(Knowledge that led to this disclosure)
In recent years, multivalent ion secondary batteries using multivalent ions as carriers have been actively researched. Multivalent ion secondary batteries include calcium secondary batteries using Ca ²⁺ as a carrier, beryllium secondary batteries using Be ²⁺ as a carrier, manganese secondary batteries using Mn ²⁺ as a carrier, and Ni ²⁺ as a carrier. A nickel secondary battery with Zn ²⁺ as a carrier, a yttrium secondary battery with Y ³⁺ as a carrier, an aluminum secondary battery with Al ³⁺ as a carrier, and a Mg ²⁺ carrier Examples include magnesium secondary batteries. Recently, research on magnesium ion secondary batteries has been attracting attention.

The theoretical capacity per unit weight and theoretical capacity per unit volume of magnesium are large. Furthermore, magnesium exhibits a relatively base redox potential. Therefore, a secondary battery using magnesium as a negative electrode is expected to have high energy density. In particular, the theoretical capacity per unit volume of magnesium is higher than the theoretical capacity per unit volume of lithium. Therefore, magnesium ion secondary batteries allow large-capacity batteries to be installed in limited spaces such as in electric vehicles. Moreover, the reserves of magnesium in the earth's crust are greater than those of lithium. Therefore, the magnesium secondary battery can overcome the problems of resource depletion and cost, which are disadvantages of lithium ion secondary batteries.

The melting point of magnesium is about 650°C. The melting point of lithium is about 180°C. The melting point of sodium is approximately 98°C. The melting point of magnesium is much higher than that of lithium and sodium. Since the melting point is an indicator of the stability of a metal, the safety of the secondary battery can be improved by using magnesium in the secondary battery. Furthermore, lithium and sodium react violently with moisture in the air. On the other hand, magnesium is stable in air and can therefore be easily handled. For these reasons, magnesium ion secondary batteries are being actively researched as a secondary battery to replace lithium ion secondary batteries.

However, there are many issues that must be overcome in order to realize multivalent ion secondary batteries such as magnesium ion secondary batteries. In particular, the development of positive electrode materials is important. The Coulomb interaction between multivalent ions and anions contained in the positive electrode material is stronger than the Coulomb interaction between lithium ions and anions contained in the positive electrode material. Therefore, the diffusion rate of multivalent ions in the positive electrode material is slower than that of lithium ions. This can be a factor that greatly limits the reaction.

Non-Patent Document 1 discloses a compound containing a soft base such as a sulfur ion and having a Chevrel phase. According to Non-Patent Document 1, the reaction potential and reversible capacity of this compound are about 1.1 V and about 116 mAh/g, respectively. That is, the reaction potential and reversible capacity of this compound are low.

Non-Patent Document 2 discloses a positive electrode containing V ₂ O ₅ which is an oxide. According to Non-Patent Document 2, the reaction potential and reversible capacity of this compound are about 1.5 V and about 75 mAh/g, respectively. Although the reaction potential of V ₂ O ₅ is higher than that of the compound described in Non-Patent Document 1, the reversible capacity of V ₂ O ₅ is low.

From the above viewpoints, there is a need to develop a positive electrode material that has an excellent diffusion rate of magnesium ions, a high reversible capacity, and a high reaction potential.

As a result of intensive research, the present inventors have found that the above-described problems can be solved by a magnesium compound having a specific composition, and have completed the present disclosure.

(Summary of one aspect of the present disclosure)
The positive electrode active material for a magnesium ion secondary battery according to the first aspect of the present disclosure includes:
It is represented by the following compositional formula (1).
_{MgM2O6 ...} (1)
In formula (1), M is V, Nb, or Ta.

According to the first aspect, a magnesium ion secondary battery having high reversible capacity, high reaction potential, and high energy density can be provided.

In the second aspect of the present disclosure, for example, in the positive electrode active material for a magnesium ion secondary battery according to the first aspect, the M may be Nb. The positive electrode active material represented by MgNb ₂ O ₆ can have high reversible capacity, high reaction potential, and high energy density.

The positive electrode for a magnesium ion secondary battery according to the third aspect of the present disclosure includes:
A current collector;
a positive electrode active material for a magnesium ion secondary battery according to the first or second aspect disposed on the current collector;
including.

According to the third aspect, a positive electrode for a magnesium ion secondary battery having high reversible capacity, high reaction potential, and high energy density can be provided.

The magnesium ion secondary battery according to the fourth aspect of the present disclosure includes:
A positive electrode for a magnesium ion secondary battery according to a third aspect,
a negative electrode;
electrolyte and
It is equipped with

According to the fourth aspect, a magnesium ion secondary battery having high reversible capacity, high reaction potential, and high energy density can be provided.

Hereinafter, a positive electrode active material according to an embodiment and a magnesium ion secondary battery using the same (hereinafter referred to as a magnesium secondary battery) will be described in detail using the drawings.

All of the following descriptions are intended to be inclusive or specific. The numerical values, composition, shape, film thickness, electrical properties, structure of the secondary battery, etc. shown below are examples, and do not limit the present disclosure. In addition, elements not recited in the top-level independent claim are optional elements.

[1. Cathode active material]
Magnesium secondary batteries are expected to be put into practical use as high-capacity secondary batteries because they can utilize the two-electron reaction of magnesium. However, since the interaction between divalent magnesium ions and anions such as oxygen ions in the active material is large, it is difficult for the magnesium ions to move within the active material, making it difficult for electrode reactions in the active material to proceed.

In contrast, the present inventors have discovered the following novel positive electrode active material.

The positive electrode active material for a magnesium ion secondary battery according to the present embodiment is a composite oxide represented by the following compositional formula. In formula (1), the transition metal M is vanadium (V), niobium (Nb), or tantalum (Ta). These elements are Group 5 elements of the periodic table.

_{MgM2O6 ...} (1)

According to the above configuration, a magnesium ion secondary battery having high reversible capacity, high reaction potential, and high energy density can be provided.

In the positive electrode active material represented by formula (1), the transition metal M may be niobium (Nb). MgNb ₂ O ₆ can be produced easily and in high yield. MgNb ₂ O ₆ can have high reversible capacity, high reaction potential, and high energy density.

[2. Manufacturing method of positive electrode active material]
The positive electrode active material according to this embodiment is manufactured, for example, by mixing a raw material containing Mg and a raw material containing transition metal M.

The raw material containing Mg is, for example, a magnesium oxide. Examples of magnesium oxides include MgO and _MgO2 .

The raw material containing M is, for example, an oxide of transition metal M. Examples of the oxide of transition metal M include MO, M ₃ O ₄ , M ₂ O ₃ , MO ₂ and M ₂ O ₅ .

Weigh these raw materials. The amount of each raw material is adjusted as appropriate depending on the composition of the target composite oxide.

The weighed raw materials are mixed. For example, a ball mill, rod mill, bead mill, or jet mill is used for mixing. The mixing method may be a dry method or a wet method. The mixing time is, for example, 4 hours or more and 12 hours or less.

Thereby, a composite oxide containing Mg and transition metal M is obtained. Note that the composite oxide may be heat-treated thereafter.

A composite oxide may be synthesized by mixing a raw material containing Mg and a raw material containing transition metal M using a dry method and then performing a heat treatment such as a calcination method.

Firing furnaces used in the firing method include electric furnaces, gas furnaces, and high-frequency induction heating furnaces. The mixed powder is placed in a crucible or boat and fired. The firing temperature is, for example, 800°C or higher and 1300°C or lower. The firing time is, for example, 2 hours or more and 24 hours or less.

The composition of the obtained composite oxide can be determined, for example, by ICP emission spectrometry. The crystal structure of the obtained composite oxide can be determined, for example, by powder X-ray diffraction.

[3. Magnesium secondary battery]
[3-1. overall structure]
The positive electrode active material according to this embodiment can be used in a magnesium secondary battery. A magnesium secondary battery includes a positive electrode containing a positive electrode active material, a negative electrode, and an electrolyte having magnesium ion conductivity.

FIG. 1 is a cross-sectional view schematically showing a configuration example of a magnesium secondary battery 10. As shown in FIG.

The magnesium secondary battery 10 includes a positive electrode 21, a negative electrode 22, a separator 14, a case 11, a sealing plate 15, and a gasket 18. Separator 14 is arranged between positive electrode 21 and negative electrode 22. The positive electrode 21, the negative electrode 22, and the separator 14 are impregnated with a non-aqueous electrolyte, and are housed in the case 11. The case 11 is closed by a gasket 18 and a sealing plate 15.

The structure of the magnesium secondary battery 10 may be cylindrical, square, button, coin, or flat.

[3-2. Positive electrode]
The positive electrode 21 includes a positive electrode current collector 12 and a positive electrode active material layer 13 disposed on the positive electrode current collector 12. The positive electrode active material layer 13 is arranged between the positive electrode current collector 12 and the separator 14.

The positive electrode active material layer 13 is formed by the above [1. Positive electrode active material] Contains the positive electrode active material described in [Cathode active material]. According to such a configuration, a positive electrode for a magnesium ion secondary battery having high reversible capacity, high reaction potential, and high energy density can be provided.

The positive electrode active material layer 13 may further contain a conductive material and/or a binder, if necessary.

Examples of the conductive material include carbon materials, metals, inorganic compounds, and conductive polymers. Examples of carbon materials include graphite, acetylene black, carbon black, Ketjenblack, carbon whisker, needle coke, and carbon fiber. Examples of graphite include natural graphite and artificial graphite. Examples of natural graphite include lump graphite and flaky graphite. Metals include copper, nickel, aluminum, silver, and gold. Inorganic compounds include tungsten carbide, titanium carbide, tantalum carbide, molybdenum carbide, titanium boride, and titanium nitride. These materials may be used alone or in combination.

Examples of the binder include fluorine-containing resins, thermoplastic resins, ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, and natural butyl rubber (NBR). Examples of the fluororesin include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and fluororubber. Thermoplastic resins include polypropylene and polyethylene. These materials may be used alone or in combination.

As a solvent for dispersing the positive electrode active material, conductive material, and binder, N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methylethylketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N,N-dimethylaminopropylamine , ethylene oxide, and tetrahydrofuran. A thickener may be added to the dispersant. Thickeners include carboxymethylcellulose and methylcellulose.

The positive electrode active material layer 13 is formed, for example, by the following method. First, these materials are mixed to obtain a mixture of the positive electrode active material, the conductive material, and the binder. Next, a suitable solvent is added to this mixture to obtain a paste-like positive electrode mixture. Next, this positive electrode mixture is applied to the surface of the positive electrode current collector 12 and dried. As a result, a positive electrode active material layer 13 is formed on the positive electrode current collector 12 . Note that the positive electrode active material layer 13 may be compressed in order to increase the electrode density.

The thickness of the positive electrode active material layer 13 is not particularly limited, and is, for example, 1 μm or more and 100 μm or less.

The material of the positive electrode current collector 12 is, for example, a single metal or an alloy. More specifically, the material of the positive electrode current collector 12 is a single metal containing at least one selected from the group consisting of copper, chromium, nickel, titanium, platinum, gold, aluminum, tungsten, iron, and molybdenum. It may be an alloy. The material of the positive electrode current collector 12 may be stainless steel.

The positive electrode current collector 12 may be plate-shaped or foil-shaped. The positive electrode current collector 12 may be a laminated film.

When the case 11 also serves as a positive electrode current collector, the positive electrode current collector 12 may be omitted.

[3-3. Negative electrode]
The negative electrode 22 includes, for example, a negative electrode active material layer 17 containing a negative electrode active material and a negative electrode current collector 16. The negative electrode active material layer 17 is arranged between the negative electrode current collector 16 and the separator 14.

The negative electrode active material layer 17 contains a negative electrode active material that can insert and desorb magnesium ions. A carbon material is mentioned as a negative electrode active material. Examples of carbon materials include graphite, non-graphitic carbon, and graphite intercalation compounds. Examples of non-graphitic carbon include hard carbon and coke.

The negative electrode active material layer 17 may further contain a conductive material and/or a binder, if necessary. The conductive material, binder, solvent, and thickener are, for example, [3-2. The conductive material, binder, solvent, and thickener described in [Positive Electrode] can be used as appropriate.

The thickness of the negative electrode active material layer 17 is not particularly limited, and is, for example, 1 μm or more and 50 μm or less.

Alternatively, the negative electrode active material layer 17 contains a negative electrode active material that can precipitate and dissolve magnesium. In this case, examples of the negative electrode active material include Mg metal and Mg alloy. The Mg alloy is, for example, an alloy of magnesium and at least one selected from the group consisting of aluminum, silicon, gallium, zinc, tin, manganese, bismuth, and antimony.

The material of the negative electrode current collector 16 is, for example, [3-2. The same material as the positive electrode current collector 12 described in [Positive Electrode] can be appropriately used. The negative electrode current collector 16 may be plate-shaped or foil-shaped.

When the sealing plate 15 also serves as a negative electrode current collector, the negative electrode current collector 16 may be omitted.

When the negative electrode current collector 16 is made of a material that can deposit and dissolve magnesium on its surface, the negative electrode active material layer 17 may be omitted. That is, the negative electrode 22 may be composed only of the negative electrode current collector 16 that can precipitate and dissolve magnesium. In this case, the negative electrode current collector 16 may be made of stainless steel, nickel, copper, or iron.

[3-4. Separator]
Materials for the separator 14 include microporous thin films, woven fabrics, and nonwoven fabrics. The material of the separator 14 may be polyolefin such as polypropylene or polyethylene. The thickness of the separator 14 is, for example, 10 μm or more and 300 μm or less. The separator 14 may be a single layer film made of one type of material, a composite film or a multilayer film made of two or more types of materials. The porosity of the separator 14 is, for example, 30% or more and 70% or less.

[3-5. Electrolytes]
The electrolyte can be a material that has magnesium ion conductivity.

The electrolyte is, for example, a non-aqueous electrolyte. The non-aqueous electrolyte includes a non-aqueous solvent and a magnesium salt dissolved in the non-aqueous solvent.

As a non-aqueous solvent, cyclic ether, chain ether, cyclic carbonate, chain carbonate, cyclic carboxylate, chain carboxylate, pyrocarbonate, phosphate, borate, sulfate, sulfite, Included are cyclic sulfones, linear sulfones, nitriles, and sultones.

As a cyclic ether, 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,4-dioxane, 1,3,5-trioxane , furan, 2-methylfuran, 1,8-cineole, crown ether, and derivatives thereof.

As chain ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethylphenyl ether, butylphenyl ether, pentylphenyl ether, Methoxytoluene, benzylethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxy Examples include methane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl, and derivatives thereof.

As the cyclic carbonate ester, ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, 4,5-difluoroethylene carbonate, 4,4,4-trifluoroethylene carbonate, fluoromethylethylene carbonate, trifluoromethylethylene carbonate , 4-fluoropropylene carbonate, 5-fluoropropylene carbonate, and derivatives thereof.

Examples of chain carbonate esters include dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, methylisopropyl carbonate, and derivatives thereof.

Examples of the cyclic carboxylic acid ester include γ-butyrolactone, γ-valerolactone, γ-caprolactone, ε-caprolactone, α-acetolactone, and derivatives thereof.

Examples of chain carboxylic acid esters include methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, and derivatives thereof.

Examples of the pyrocarbonate include diethylpyrocarbonate, dimethylpyrocarbonate, di-tert-butyl dicarbonate, and derivatives thereof. Phosphate esters include trimethyl phosphate, triethyl phosphate, hexamethylphosphoramide, and derivatives thereof. Examples of boric acid esters include trimethylborate, triethylborate, and derivatives thereof. Examples of sulfuric esters include trimethyl sulfate, triethyl sulfate, and derivatives thereof. Examples of sulfites include ethylene sulfite and its derivatives.

Examples of the cyclic sulfone include sulfolane and its derivatives. Examples of chain sulfones include alkyl sulfones and derivatives thereof. Nitriles include acetonitrile, valeronitrile, propionitrile, trimethylacetonitrile, cyclopentanecarbonitrile, adiponitrile, pimelonitrile and derivatives thereof. Examples of the sultone include 1,3-propane sultone and its derivatives.

As the solvent, only one type of the above-mentioned substances may be used, or two or more types may be used in combination.

Examples of magnesium salts include _MgBr2 , _MgI2 , _MgCl2 , Mg( _AsF6 ) ₂ , Mg( _ClO4 ) ₂ , Mg( _PF6 ) ₂ , Mg( _BF4 ) ₂ , Mg( _CF3SO3 ) _{2 ,} Mg[N( _{CF3SO2 ) 2} ] ₂ , Mg( _SbF6 ) ₂ , Mg( _SiF6 ) ₂ , Mg[C( _{CF3SO2 ) 3} ] ₂ , Mg[N( _FSO2 ) ₂ ] ₂ , Mg[N(C ₂ F ₅ SO ₂ ) ₂ ] ₂ , MgB ₁₀ Cl ₁₀ , MgB ₁₂ Cl ₁₂ , Mg[B(C ₆ F ₅ ) ₄ ] ₂ , Mg[B(C ₆ H ₅ ) ₄ ] ₂ , Mg[N(SO ₂ CF ₂ CF ₃ ) ₂ ] ₂ , Mg[BF ₃ C ₂ F ₅ ] ₂ , Mg[PF ₃ (CF ₂ CF ₃ ) ₃ ] ₂ , and Mg[B(OCH(CF ₃ ) ₂ ) ₄ ] ₂ are listed. As the magnesium salt, only one type of the above-mentioned substances may be used, or two or more types may be used in combination.

The electrolyte may be a solid electrolyte. In this case, the solid electrolyte is Mg _2-1.5x Al _x SiO ₄ , Mg _2-1.5y-0.5z Al _yz Zn _z SiO ₄ , MgZr ₄ (PO ₄ ) ₆ , MgM1PO ₄ , Mg _1-a M2 _a M3 (M4O ₄ ) ₃ and Mg(BH ₄ )(NH ₂ ). x satisfies 0.1≦x≦1. y satisfies 0.5≦y≦1. z satisfies 0.5≦z≦0.9. yz satisfies yz≧0. y+z satisfies y+z≦1. M1 is at least one selected from the group consisting of Zr, Nb, and Hf. M2 is at least one selected from the group consisting of Ca, Sr, Ba, and Ra. M3 is at least one selected from the group consisting of Zr and Hf. M4 is at least one selected from the group consisting of W and Mo. a satisfies 0≦a<1.

(Manufacture of positive electrode active material)
The positive electrode active material according to this embodiment is manufactured, for example, by mixing an oxide containing Mg and an oxide containing Nb.

MgO was used as the Mg-containing oxide.

Niobium pentoxide Nb ₂ O ₅ was used as the Nb-containing oxide.

MgO and Nb ₂ O ₅ were weighed at a molar ratio of MgO:Nb ₂ O ₅ =1:1.

Each weighed raw material was sealed in a planetary ball mill container (manufactured by Fritsch, PL-7). Zr balls with a diameter of 5 mm were placed in a container as media, and mixed at 400 rpm for 4 hours. The mixed powder was sieved to remove agglomerated powder.

The mixed powder was placed in a crucible and fired at 1000° C. for 6 hours using an electric furnace (SUPER BURN, manufactured by Motoyama) to obtain MgNb ₂ O ₆ powder.

Powder X-ray diffraction measurement was performed on the obtained composite oxide.

The structure of the crystal was identified by analyzing the X-ray diffraction pattern using an X-ray diffraction device (MiNi Flex, manufactured by Rigaku Corporation). FIG. 2 shows the results of powder X-ray diffraction measurement of the positive electrode active material according to the example and the results of powder X-ray diffraction of MgNb ₂ O ₆ obtained by simulation. FIG. 2(a) is a diagram showing the results of powder X-ray diffraction measurement of the positive electrode active material according to the example. FIG. 2(b) is a diagram showing the results of a simulation of the powder X-ray diffraction spectrum of MgNb ₂ O ₆ . The results of powder X-ray diffraction measurement of the positive electrode active material according to the example were in good agreement with the simulation results. From analysis of the results of powder X-ray diffraction measurements, it was confirmed that MgNb ₂ O ₆ powder with a purity of 99.9% or higher was synthesized.

(Preparation of magnesium secondary battery)
FIG. 3 is a schematic diagram showing a schematic configuration of a beaker cell according to an example.

The beaker cell 30 includes a positive electrode 31, a negative electrode 34, and a non-aqueous electrolyte 35. The non-aqueous electrolyte 35 is stored in a beaker. The positive electrode 31 includes an aluminum mesh 32 and a positive electrode mixture 33. The positive electrode mixture 33 is placed at the tip of the aluminum mesh 32. The positive electrode 31 and the negative electrode 34 are immersed in a non-aqueous electrolyte 35.

MgNb ₂ O ₆ , acetylene black, and polytetrafluoroethylene were weighed in a weight ratio of 8:1:1. The weighed raw materials were mixed in a mortar to obtain a positive electrode mixture. The positive electrode mixture was rolled to a thickness of 30 μm using a roll press machine to obtain a positive electrode mixture sheet. The rolled positive electrode mixture sheet was punched into a 5 mm x 5 mm square. The punched positive electrode mixture sheet was placed on the tip of a 5 mm x 30 mm aluminum mesh, and was crimped with a pressure of 5 MPa. Thereby, a positive electrode 31 containing MgNb ₂ O ₆ was obtained. The positive electrode 31 was dried at 105° C. for 6 hours or more under vacuum.

A magnesium ribbon having a thickness of 300 μm was cut into a size of 5 mm×40 mm to obtain a magnesium foil. The surface of the magnesium foil was scraped to remove the oxide film, and the surface was washed with acetone. As a result, a negative electrode 34 was obtained.

A non-aqueous electrolyte 35 was stored in a glass beaker. The non-aqueous electrolyte 35 used 1,2-dimethoxyethane (DME) as a non-aqueous solvent. 0.3 mol/L of Mg[B(OCH(CF ₃ ) ₂ ) ₄ ] 2.3DME, which is an organic boron ate complex salt in which 1,2-dimethoxyethane is coordinated, was added to _1,2 -dimethoxyethane. A non-aqueous electrolyte 35 was obtained by dissolving at a certain concentration.

The positive electrode 31 and the negative electrode 34 were immersed in a non-aqueous electrolyte 35 to produce a magnesium secondary battery 30 having the configuration shown in FIG. 3.

The magnesium secondary battery was manufactured under an argon atmosphere.

(Charge/discharge test)
A charge/discharge test of the produced magnesium secondary battery was conducted at a temperature of 60° C. in an argon atmosphere.

FIG. 4 is a graph showing the results of a charge/discharge test of the magnesium secondary battery according to the example.

A charge/discharge test was conducted using a charge/discharge device (manufactured by Bio-Logic, VSP-300). First, since the theoretical capacity was 175 mAh/g, the C rate was set to 0.01 and the magnesium secondary battery according to the example was discharged. Specifically, the end-of-discharge voltage was set to 0.1V, and the magnesium secondary battery according to the example was discharged. The discharge capacity was 164mAh/g. After discharging, the open circuit condition was maintained for 5 hours. Next, the C rate was set to 0.01 and the magnesium secondary battery according to the example was charged. The charging capacity was 164mAh/g. A reversible charge-discharge reaction was confirmed in the charge-discharge test. The reaction potential, which is the average value of the charging potential and the discharging potential, was about 2.1V.

The density of Mo ₆ S ₈ , which is a sulfide having a Chevrel phase described in Non-Patent Document 1, is 5.20 g/cm ³ . According to Non-Patent Document 1, the reversible capacity and reaction potential of Mo ₆ S ₈ are about 116 mAh/g and about 1.1 V, respectively. The density of vanadium pentoxide V ₂ O ₅ , which is an oxide described in Non-Patent Document 2, is 3.37 g/cm ³ . According to Non-Patent Document 2, the reversible capacity and reaction potential of V ₂ O ₅ are about 75 mAh/g and about 1.5 V, respectively. The density of MgNb ₂ O ₆ , which is a compound according to an example of the present disclosure, is 5.00 g/cm ³ . The reversible capacity and reaction potential of MgNb ₂ O ₆ were 164 mAh/g and about 2.1 V, respectively. As these results show, MgNb ₂ O ₆ according to the example had a high reversible capacity and a high reaction potential.

The volume energy density can be determined by multiplying the density of the compound by the reversible capacity and reaction potential. The volume energy density of V ₂ O ₅ described in Non-Patent Document 2 was approximately 379 mWh/cm ³ . The volume energy density of MgNb ₂ O ₆ according to the example was about 1722 mWh/cm ³ . The volumetric energy density of MgNb ₂ O ₆ according to the example was about 4.5 times that of V ₂ O ₅ .

It is thought that MgNb ₂ O ₆ allows magnesium ions to easily diffuse inside the positive electrode active material. According to MgNb ₂ O ₆ , the bonding force between niobium (Nb) and oxygen is stronger than the bonding force between magnesium and oxygen, so it is thought that magnesium can be easily ionized. As a result, the positive electrode active material represented by MgNb ₂ O ₆ is believed to have high reversible capacity, high reaction potential, and high energy density. Niobium (Nb) is a Group 5 element of the periodic table. Vanadium (V) and tantalum (Ta), which are Group 5 elements of the periodic table, are thought to have high reversible capacity and high reaction potential, similar to Nb. Therefore, MgV ₂ O ₆ and MgTa ₂ O ₆ are considered to have high volumetric energy density.

The positive electrode active material of the present disclosure can be used in a magnesium secondary battery.

10 Magnesium secondary battery 11 Case 12 Positive electrode current collector 13 Positive electrode active material layer 14 Separator 15 Sealing plate 16 Negative electrode current collector 17 Negative electrode active material layer 18 Gasket 21 Positive electrode 22 Negative electrode 30 Beaker cell 31 Positive electrode 32 Aluminum mesh 33 Positive electrode mixture 34 Negative electrode 35 Nonaqueous electrolyte

Claims (4)

A positive electrode active material for a magnesium ion secondary battery represented by the following compositional formula (1).
Mg Ta ₂ O ₆ ...(1 ) A current collector;
The positive electrode active material for a magnesium ion secondary battery according to claim 1, disposed on the current collector;
Positive electrode for magnesium ion secondary batteries, including: A current collector;
a positive electrode active material disposed on the current collector;
including ;
A positive electrode for a magnesium ion secondary battery, wherein the positive electrode active material is composed only of MgNb ₂ O ₆ . A positive electrode for a magnesium ion secondary battery according to claim 2 or 3,
a negative electrode;
electrolyte and
Magnesium ion secondary battery with

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