JP2020035633A - Positive active material for secondary batteries and secondary battery using the same - Google Patents

Abstract

To provide a novel electrode active material for secondary batteries, and a secondary battery using the same.SOLUTION: The electrode active material for secondary batteries contains a graphite intercalation compound represented by the general formula M1M2C. Here, M1 is at least one selected from the group consisting of alkaline earth metals and alkali metals, and M2 is at least one selected from the group consisting of alkaline earth metals, x and y satisfies 0<x≤3.2, and 0.7≤y≤2.3, respectively.SELECTED DRAWING: None

Description

  The present disclosure relates to a positive electrode active material for a secondary battery and a secondary battery using the same.

  In recent years, development of a secondary battery using a two-electron reaction of an alkaline earth metal has been expected as a new battery having a capacity exceeding that of an existing lithium ion secondary battery. However, there are still few reports of positive electrode active materials capable of inserting and removing alkaline earth metal ions.

For example, Non-Patent Document 1 discloses V ₂ O ₅ as a positive electrode active material for a magnesium secondary battery.

Long Yu et al. , "Electrochemical Insertion of Magnesium Ions into V2O5 from aproto-electrochemicals with valued water content, 1, 200, 1800, International, Republic of Ireland, 2nd ed.

  An object of the present disclosure is to provide an electrode active material that can improve the discharge capacity of a secondary battery.

Rechargeable battery electrode active material according to an aspect of the present disclosure includes a graphite intercalation compound represented by the general formula M1 _x M2 _y C _6. Here, M1 is at least one selected from the group consisting of alkaline earth metals and alkali metals, M2 is at least one selected from the group consisting of alkaline earth metals, and x and y are each , 0 <x ≦ 3.2 and 0.7 ≦ y ≦ 2.3.

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

FIG. 1 is a cross-sectional view schematically illustrating a configuration example of a secondary battery. FIG. 2 is a diagram showing XRD charts of Sample 1 and Sample 2 before and after charging.

  Hereinafter, an electrode active material according to an embodiment and a secondary battery using the same will be described in detail with reference to the drawings.

  The following description is a comprehensive or specific example. The numerical values, composition, shape, film thickness, electrical characteristics, structure of the secondary battery, and the like shown below are merely examples, and do not limit the present disclosure. In addition, components not described in the independent claim indicating the top concept are optional components.

[1. Electrode active material]
Electrode active material according to the present embodiment includes the general formula M1 _x M2 graphite intercalation compound represented by _y C ₆ a (graphite intercalation compound). Here, M1 is at least one selected from the group consisting of alkaline earth metals and alkali metals, M2 is at least one selected from the group consisting of alkaline earth metals, and x and y are each , 0 <x ≦ 3.2 and 0.7 ≦ y ≦ 2.3. This graphite intercalation compound can insert and remove the M1 cation, and can exhibit a large charge / discharge capacity.

  Examples of alkaline earth metals include magnesium, calcium, strontium, and barium. Examples of alkali metals include lithium, sodium, and potassium.

  In the above general formula, when M1 contains an alkaline earth metal, the alkaline earth metal contained in M1 and the alkaline earth metal of M2 may be different elements.

In the above general formula, M1 may be magnesium. That is, the graphite intercalation compound may be represented by the general formula _Mg x _M2 y _{C 6.} This realizes a graphite intercalation compound capable of inserting and removing magnesium ions. The electrode active material containing the graphite intercalation compound is used, for example, as a positive electrode active material or a negative electrode active material of a magnesium secondary battery, and can exhibit a larger charge / discharge capacity than an existing active material.

In the above general formula, M1 may be magnesium and lithium. That is, the graphite intercalation compound may be represented by the general formula Mg _x1 Li _x2 M2 _y C ₆ (where x = x1 + x2). This realizes a graphite intercalation compound capable of inserting and removing magnesium ions and lithium ions. The electrode active material containing the graphite intercalation compound is used, for example, as a positive electrode active material or a negative electrode active material of a magnesium secondary battery, and can exhibit a larger charge / discharge capacity than an existing active material.

In the above general formula, M1 may be lithium. That is, the graphite intercalation compound may be represented by the general formula _Li x _M2 y _{C 6.} This realizes a graphite intercalation compound capable of inserting and removing lithium ions. The electrode active material containing the graphite intercalation compound is used, for example, as a negative electrode active material of a lithium ion secondary battery, and can exhibit a charge / discharge capacity exceeding the theoretical capacity of graphite.

  In the above general formula, M2 may be calcium. Calcium can increase the interlayer spacing of the graphite layer of the graphite intercalation compound and promote insertion and removal of lithium and / or magnesium. Calcium may be inserted and removed from the graphite intercalation compound according to the charge and discharge reaction of the electrode active material.

  The graphite intercalation compound may include a plurality of graphite layers containing carbon. The atoms or ions of the metals represented by M1 and M2 in the above general formula may be arranged between these graphite layers. For example, M2 ions may increase the spacing of the graphite layers, thereby making it easier for M1 ions to move within an intercalation layer.

  At least a portion of the graphite intercalation compound may undergo a reversible phase change between the first phase and the second phase according to the charge / discharge reaction of the electrode active material. For example, desorption of M1 ions from the first phase causes a change from the first phase to the second phase, and insertion of M1 ions into the second phase causes the first phase to change from the second phase to the first phase. Phase may be changed. The amount of M2 contained in the first phase and the amount of M2 contained in the second phase may be different. Both the first phase and the second phase may satisfy the above general formula. The graphite intercalation compound may contain both an α phase and a β phase in a charged state and / or a discharged state. In this case, the ratio of the α phase and the β phase may change according to the charge / discharge reaction.

When the composition of the graphite intercalation compound is represented by Li _x Ca _y C _6, the first phase may be a β-phase, the second phase may be a α-phase. In Li _x Ca _y C _6, the α-phase is a phase in which the interlayer spacing of the graphite layer has a structure which is 7.6～8.0A, the β phase is interlayer spacing of the graphite layer from 9.5 to 9. This is a phase having a structure of 9 °.

of β-phase _Li x _Ca y _{C 6,} for example, 1.8 ≦ x ≦ 3.2, and may satisfy 1.7 ≦ y ≦ 2.3. _Li x _Ca y _{C 6} of α-phase, for example, 0.2 ≦ x ≦ 0.8, and may satisfy 0.7 ≦ y ≦ 2.1. However, the amount of lithium and calcium inserted between the graphite layers can vary depending on the synthesis conditions, the battery system, and / or the electrical conditions during charging and discharging. For example, when the graphite intercalation compound is charged and discharged in an electrolyte in which lithium is more abundant than calcium, lithium can be easily inserted between the graphite intercalations. For example, when almost all lithium is desorbed from the graphite intercalation compound, x approaches zero.

  The structure of the graphite intercalation compound can be specified or estimated from, for example, an XRD chart obtained by X-ray diffraction (XRD) measurement. The composition of the graphite intercalation compound according to the present embodiment can be specified, for example, from a spectrum obtained by ICP (inductively coupled plasma) emission spectroscopy.

[2. Method for producing electrode active material]
[2-1. Method for producing Li _x Ca _y C _6]
First, a carbon source, a calcium source, and a lithium source are prepared as raw materials.

  Examples of carbon sources include graphite, amorphous carbon materials, and organic materials. Examples of graphite include massive graphite, flaky graphite, and artificial graphite. The artificial graphite may be, for example, pyrolytic graphite such as HOPG or PGCCL. Examples of the amorphous carbon material include activated carbon, carbon fiber, carbon black, and coke. Examples of the organic material include a synthetic resin such as polyvinyl alcohol.

  Examples of calcium sources include calcium metal, calcium hydride, calcium hydroxide, calcium carbide, and calcium carbonate.

  Examples of lithium sources include lithium metal, lithium hydride, lithium hydroxide, lithium carbide, and lithium carbonate.

  The carbon source, the calcium source, and the lithium source may not be provided as separate materials. For example, calcium carbide may serve both as a carbon source and a calcium source.

  Examples of the shape of each raw material include a particle shape, a fiber shape, and a sheet shape. From the viewpoint of ease of processing after forming the electrode active material, their shapes may be, for example, particles having an average particle diameter of 1 μm to 100 μm, or an average fiber diameter of 1 μm to 100 μm. It may be fibrous. In the present disclosure, the “average particle diameter” means a median diameter equivalent to a sphere volume.

  Each raw material is weighed according to the target composition of the graphite intercalation compound or its precursor.

  Next, the weighed mixture of the carbon source, the calcium source, and the lithium source is melted. These raw materials may be mixed and then melted, or a raw material having a relatively low melting point may be melted, and then another raw material may be added to the melted raw material while gradually increasing the heating temperature. You may. For example, the calcium source and the lithium source may be melted first, and then the carbon source may be added thereto to increase the heating temperature.

  Although the heating method varies depending on the type of the raw material and / or the target composition, for example, the heating may be performed as follows. First, lithium metal is heated to 180 to 250 ° C. and melted. Add calcium metal to the molten lithium metal and heat to 300-350 ° C. As a result, a molten lithium-calcium alloy is obtained. The decomposable graphite is immersed in the molten alloy, heated to 300 ° C. to 400 ° C., and held for 400 to 500 hours.

  The heat treatment may be performed in an inert atmosphere. Examples of the inert gas include a nitrogen gas, an argon gas, a helium gas, and a neon gas.

Thus, the graphite intercalation compound of lithium and calcium are disposed between the graphite layers Li _x Ca _y C ₆ is obtained.

[2-2. Method for producing a Mg _x Ca _y C _6]
The alkaline earth metal may be inserted into the synthesized graphite intercalation compound by a chemical or electrochemical method.

For example, an electrode comprising a Li _x Ca _y C _6, and the other electrode, the electrochemical cell having an electrolyte solution containing magnesium ions is prepared. The electrochemical cell is, for example, [3. Secondary Battery].

By applying a predetermined voltage between the electrodes of the electrochemical cell, magnesium ions in the electrolyte are inserted into the graphite intercalation compound. For example, if the electrochemical cell is a secondary battery, the Li _x Ca _y C _6, is charged and discharged in an electrolyte containing magnesium ions. As a result, first, part or all of lithium ions are irreversibly desorbed from the graphite layer, and then magnesium ions are reversibly inserted. That _is, lithium Li x _Ca y _{C 6} is replaced with magnesium.

Thus, the graphite intercalation compound magnesium and calcium are disposed between the graphite layers _Mg x _Ca y _{C 6, or, Mg x1 Li x2 Ca y C} 6 is obtained.

[2-3. Supplement]
The method for producing an electrode active material according to the present embodiment is not limited to the above example.

For example, [2-1. In li _x Ca _y production method of the production method] of C _6, instead of the "lithium source", by selecting any of the alkali metal source or alkaline earth metal source, also, instead of the "calcium source", By selecting an arbitrary alkaline earth metal source, a desired graphite intercalation compound can be produced.

For example, [2-2. In mg _x Ca _y production method of the production method] of C _6, instead of the "magnesium ions", by selecting any alkali metal ions or alkaline earth metal ions, can be inserted a desired metal to graphite intercalation compound .

[3. Secondary battery]
[3-1. overall structure]
The electrode active material according to the present embodiment can be used for a secondary battery. That is, the secondary battery includes a positive electrode, a negative electrode, and an electrolyte, and the positive electrode or the negative electrode is formed as described in [1. Electrode Active Material].

  The secondary battery may be an alkaline earth metal secondary battery or an alkaline metal secondary battery. Examples of the alkaline earth metal secondary battery include a magnesium secondary battery and a calcium secondary battery. Examples of the alkali metal secondary battery include a lithium secondary battery and a sodium secondary battery.

  For example, a magnesium secondary battery includes a positive electrode, a negative electrode including the electrode active material according to the present embodiment, and an electrolyte having magnesium ion conductivity. Alternatively, a magnesium secondary battery includes a positive electrode including the electrode active material according to the present embodiment, a negative electrode, and an electrolyte having magnesium ion conductivity.

  For example, a lithium ion secondary battery includes a positive electrode, a negative electrode including the electrode active material according to the present embodiment, and an electrolyte having lithium ion conductivity.

  FIG. 1 is a cross-sectional view schematically illustrating a configuration example of the secondary battery 10.

  The 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. The separator 14 is disposed between the positive electrode 21 and the negative electrode 22. The positive electrode 21, the negative electrode 22, and the separator 14 are impregnated with a non-aqueous electrolyte, and are contained in the case 11. The case 11 is closed by a gasket 18 and a sealing plate 15.

  The structure of the secondary battery 10 may be, for example, a cylindrical type, a square type, a button type, a coin type, or a flat type.

[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 contains a positive electrode active material capable of inserting and removing alkali metal ions and / or alkaline earth metal ions.

  The positive electrode active material is, for example, as described in [1. Electrode Active Material].

  Alternatively, the positive electrode active material may be another material. Examples of other materials include metal composite oxides, metal halides, metal oxyfluorides, and polyanion compounds.

When the secondary battery 10 is a magnesium secondary battery, an example of the positive electrode active material is MgM ₂ O ₄ (where M is at least one selected from Mn, Co, Cr, Ni, and Fe). , MgMO ₂ (where M is at least one selected from Mn, Co, Cr, Ni and Al), and MgMSiO ₄ (where M is selected from Mn, Co, Ni and Fe) At least one).

When the secondary battery 10 is a lithium ion secondary battery, examples of the positive electrode active material include a lithium-cobalt composite oxide (for example, LiCoO ₂ ) and a lithium-nickel-cobalt-manganese composite oxide (for example, LiNi _{0. 5} Co _0.2 Mn _0.3 O ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ or LiNi _0.4 Co _0.2 Mn _0.4 O ₂ ), lithium-nickel-cobalt -Aluminum composite oxide (for example, LiNi _0.8 Co _0.15 Al _0.05 O ₂ and LiNi _0.8 Co _0.18 Al _0.02 O ₂ ).

  The positive electrode active material layer 13 may further include a conductive agent, a binder, and / or an ionic conductor as necessary.

  Examples of the conductive material include a carbon material, a metal, an inorganic compound, and a conductive polymer. Examples of the carbon material include graphite such as natural graphite (eg, massive graphite and flaky graphite) and artificial graphite, acetylene black, carbon black, Ketjen black, carbon whisker, needle coke, carbon nanotube, carbon fiber, graphene, fullerene. , And graphite oxide. Examples of metals include copper, nickel, aluminum, silver, and gold. Examples of the inorganic compound include tungsten carbide, titanium carbide, tantalum carbide, molybdenum carbide, titanium boride, and titanium nitride. Examples of the conductive polymer include polyaniline, polypyrrole, and polythiophene. These materials may be used alone or in combination of two or more.

  Examples of the binder include polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, polytetrafluoroethylene, carboxymethyl cellulose, polyacrylic acid, and styrene-butadiene copolymer. Examples include polymerized rubber, polypropylene, polyethylene, and polyimide. These materials may be used alone or in combination of two or more.

  Examples of the ion conductor include a polymer electrolyte and an inorganic solid electrolyte. The polymer electrolyte may be a gel polymer electrolyte or a solid polymer electrolyte. Examples of the polyelectrolyte host polymer include polymethyl methacrylate, polyvinylidene fluoride, and polyethylene oxide. The electrolyte held by the host polymer may be a solution in which an alkali metal salt or an alkaline earth metal salt is dissolved in a non-aqueous solvent, or may be an ionic liquid. The material of the electrolyte is [3-5. Electrolyte] may be selected from the materials described below.

  Examples of the solvent in which the positive electrode active material, the conductive material, and the binder are dispersed include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methylethylketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N, N-dimethyl Aminopropylamine, ethylene oxide, and tetrahydrofuran. For example, a thickener may be added to the dispersant. Examples of the thickener include carboxymethylcellulose and methylcellulose.

  The positive electrode active material layer 13 is formed, for example, as follows. First, a positive electrode active material, a conductive material, and a binder are mixed. Next, an appropriate solvent is added to this mixture, whereby a paste-like positive electrode mixture is obtained. Next, this positive electrode mixture is applied to the surface of the positive electrode current collector 12 and dried. Thereby, the 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, but 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 metal or an alloy. More specifically, the material of the positive electrode current collector 12 is copper, chromium, nickel, titanium, platinum, gold, aluminum, tungsten, iron, and a metal containing at least one selected from the group consisting of molybdenum or It may be an alloy. The material of the positive electrode current collector 12 may be, for example, stainless steel.

  The positive electrode current collector 12 may be plate-shaped or foil-shaped, and may be porous, mesh, or non-porous. The positive electrode current collector 12 may be a laminated film. The positive electrode current collector 12 may have a layer made of a carbon material such as carbon on a surface that contacts the positive electrode active material layer 13.

  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 contains a negative electrode active material capable of inserting and removing alkali metal ions and / or alkaline earth metal ions.

  The negative electrode active material is, for example, as described in [1. Electrode Active Material].

  Alternatively, the negative electrode active material may be another material. Examples of other materials include a carbon material. Examples of the carbon material include graphite, non-graphite carbon such as hard carbon and coke, and graphite intercalation compounds.

  The negative electrode active material layer 17 may further include a conductive agent and / or a binder as necessary. Examples of the conductive material, the binder, the solvent, and the thickener include [2-3. Positive electrode] can be appropriately used.

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

  When the secondary battery 10 is an alkaline earth metal secondary battery, the negative electrode active material may be, for example, an alkaline earth metal or an alkaline earth metal alloy. The alkaline earth metal alloy is, for example, an alloy of an alkaline earth metal 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, the above [3-2. Positive electrode] 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 have a plate shape or a foil shape.

  When the case 11 also serves as the 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 capable of dissolving and depositing an alkaline earth metal on its surface, the negative electrode active material layer 17 may be omitted. That is, the negative electrode 22 may include only the negative electrode current collector 16 capable of dissolving and depositing the alkaline earth metal. In this case, the negative electrode current collector 16 may be stainless steel, nickel, copper, or iron.

[3-4. Separator]
Examples of the material of the separator 14 include a porous film, a woven fabric, and a nonwoven fabric. Examples of the nonwoven fabric include a resin nonwoven fabric, a glass fiber nonwoven fabric, and a paper nonwoven fabric. The material of the separator 14 may be a polyolefin such as polypropylene or polyethylene. The thickness of the separator 14 is, for example, 10 to 300 μm. The separator 14 may be a single-layer film made of one type of material, or a composite film (or a multi-layered film) made of two or more types of materials. The porosity of the separator 14 is in the range of, for example, 30 to 70%.

[3-5. Electrolytes]
The electrolyte may be any material having alkali metal ion conductivity or alkaline earth metal ion conductivity.

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

  Examples of the material of the non-aqueous solvent include cyclic ether, chain ether, cyclic carbonate, chain carbonate, cyclic carboxylate, chain carboxylate, pyrocarbonate, phosphate, borate, and sulfuric acid. Esters, sulfites, cyclic sulfones, linear sulfones, nitriles, amides, and sultones.

  Examples of the cyclic ether include 1,3-dioxolan, 4-methyl-1,3-dioxolan, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,4-dioxane, 1,3,3 5-trioxane, furan, 2-methylfuran, 1,8-cineole, and crown ethers, and derivatives thereof. Examples of chain ethers include 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl Phenyl ether, methoxytoluene, benzyl ethyl 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-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tet Ethylene glycol dimethyl, and derivatives thereof.

  Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, 4,5-difluoroethylene carbonate, 4,4,4-trifluoroethylene carbonate, fluoromethylethylene carbonate, and trifluorocarbonate. Examples include methyl ethylene carbonate, 4-fluoropropylene carbonate, 5-fluoropropylene carbonate, and derivatives thereof. Examples of the chain carbonate include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl isopropyl carbonate, and derivatives thereof.

  Examples of the cyclic carboxylate include γ-butyrolactone, γ-valerolactone, γ-caprolactone, ε-caprolactone, α-acetolactone, and derivatives thereof. Examples of linear carboxylic esters include methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, and derivatives thereof.

  Examples of pyrocarbonates include diethyl pyrocarbonate, dimethyl pyrocarbonate, di-tert-butyl dicarbonate, and derivatives thereof. Examples of phosphate esters include trimethyl phosphate, triethyl phosphate, hexamethylphosphoramide, and derivatives thereof. Examples of borate esters include trimethyl borate, triethyl borate, and derivatives thereof. Examples of sulfate esters include trimethyl sulfate, triethyl sulfate, and derivatives thereof. Examples of sulfites include ethylene sulfite and its derivatives.

  Examples of cyclic sulfones include sulfolane and its derivatives. Examples of linear sulfones include alkyl sulfones and derivatives thereof. Examples of nitriles include acetonitrile, valeronitrile, propionitrile, trimethylacetonitrile, cyclopentanecarbonitrile, adiponitrile, pimeronitrile and derivatives thereof. Examples of amides include dimethylformamide and its derivatives. Examples of sultone include 1,3-propane sultone and its derivatives.

  As the solvent, only one of the above substances may be used, or two or more of them may be used in combination.

When the secondary battery 10 is a magnesium secondary battery, examples of magnesium salts include MgBr ₂ , MgI ₂ , MgCl ₂ , Mg (AsF ₆ ) ₂ , Mg (ClO ₄ ) ₂ , Mg (PF ₆ ) ₂ , Mg (BF ₄ ) ₂ , Mg (CF ₃ SO ₃ ) ₂ , Mg [N (CF ₃ SO ₂ ) ₂ ] ₂ , Mg (SbF ₆ ) ₂ , Mg (SiF ₆ ) ₂ , Mg [C (CF ₃ SO ₂ ) ₃ ] ₂ , Mg [N (FSO ₂ ) 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 , and Mg [PF ₃ ( CF ₂ CF _{3) 3] 2} and the like. As the magnesium salt, only one of the above substances may be used, or two or more of them may be used in combination.

When the secondary battery 10 is a lithium ion secondary battery, examples of the lithium salt include LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiSO ₃ CF ₃ , LiN (SO ₂ F) ₂ , and LiN (SO _2). CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiN (SO ₂ CF ₃ ) (SO ₂ C ₄ F ₉ ), LiC (SO ₂ CF ₃ ) ₃ , and LiBF ₂ (C ₂ O ₄ ) Is mentioned. As the lithium salt, only one of the above substances may be used, or two or more of them may be used in combination.

  Alternatively, the electrolyte may be a solid electrolyte.

When the secondary battery 10 is a magnesium secondary battery, examples of the solid electrolyte include Mg _2-1.5x Al _x SiO ₄ (where 0.1 ≦ x ≦ 1) and Mg _{2-1.5x-0. .5y Al x-y Zn y SiO} 4 ( however, 0.5 ≦ x ≦ 1, 0.5 ≦ y ≦ 0.9, x-y ≧ 0, x + y ≦ 1), MgZr 4 (PO 4) 6, MgMPO ₄ (although, M is, Zr, at least one selected from Nb and _{Hf), Mg 1-x a} x M (M'O 4) 3 ( however, a is, Ca, Sr, Ba, and At least one selected from Ra, M is at least one selected from Ze and Hf, M ′ is at least one selected from W and Mo, and 0 ≦ x <1) , and _{Mg (BH 4) (NH 2} ) can be mentioned.

When the secondary battery 10 is a lithium ion secondary battery, examples of the solid electrolyte include a NASICON type solid electrolyte (for example, LiTi ₂ (PO ₄ ) ₃ or a substitute thereof) and a perovskite type electrolyte (for example, (LaLi)). TiO ₃ ), LISICON type solid electrolyte (for example, Li ₁₄ ZnGe ₄ O ₁₆ , Li ₄ SiO ₄ , LiGeO ₄ , or a substitute thereof), garnet-type solid electrolyte (for example, Li ₇ La ₃ Zr ₂ O ₁₂ or Substitutes thereof), lithium phosphate oxide nitride (LiPON), and a sulfide solid electrolyte (for example, Li ₂ S—P ₂ S ₅ ).

[4. Experimental result]
[4-1. Sample 1]
First, highly oriented pyrolytic graphite (HOPG), calcium metal (Ca), and lithium metal (Li) were prepared as raw materials. 42.7 mg of HOPG, 190 mg of calcium metal, and 66 mg of lithium metal were weighed.

  The weighed lithium metal was put into a stainless steel container, and the container was heated to 200 ° C. using a ribbon heater. After the lithium metal was melted, the weighed calcium metal was added and heated to 350 ° C. As a result, a Li-Ca molten alloy was obtained. HOPG was put into a container, and HOPG was immersed in the molten Li-Ca alloy, and the container was placed in a stainless steel reactor and sealed. The reactor was placed in an electric furnace installed in a glove box, the temperature in the furnace was set to 350 ° C., and held for 500 hours. Thereafter, the heating was stopped and the inside of the furnace was naturally cooled, and the product was taken out. The product taken out was crushed in an agate mortar to obtain a sample 1 of an electrode active material.

Sample 1, acetylene black, and polyvinylidene fluoride (PVDF) were weighed to a mass ratio of 8: 1: 1, and mixed using an agate mortar. As a result, a mixture was obtained. The obtained mixture was dispersed in an N-methyl-2-pyrrolidone solvent to prepare a slurry. The slurry was applied on a copper foil and dried to obtain an electrode sheet. The electrode sheet was rolled by a rolling mill and punched into a 20 mm × 20 mm square shape. Thus, an electrode (that is, a working electrode) including Sample 1 was obtained. A lithium metal foil having a thickness of 300 μm was pressed on a square nickel mesh of 20 mm × 20 mm. Thereby, a counter electrode was obtained. A lithium metal reference electrode was obtained in the same manner as the counter electrode. Ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate were mixed at a volume ratio of 5:70:25 to obtain a mixed solvent. In this mixed solvent, lithium hexafluorophosphate (LiPF ₆ ) was dissolved at a concentration of 1.4 mol / liter. Thus, a non-aqueous electrolyte was obtained. A separator of a polyethylene microporous membrane was prepared and impregnated with a non-aqueous electrolyte. The working electrode, counter electrode, reference electrode, and separator were arranged in a three-electrode beaker cell (manufactured by EC Frontier Inc.). Thereby, the battery cell 1 for evaluation was obtained. Note that all the battery cells 1 were prepared in a glove box in an Ar atmosphere having a dew point of −60 degrees or less and an oxygen value of 1 ppm or less.

The current density with respect to the working electrode was set to 1 mA / cm ² , and the battery cell 1 was charged until the potential difference between the working electrode and the reference electrode reached 0V. After suspending for 20 minutes, the current density for the working electrode was set to 1 mA / cm ² , and the battery cell 1 was discharged until the potential difference between the working electrode and the reference electrode reached 3 V. The charge / discharge operation was performed in a constant temperature bath at 25 ° C.

  The charge capacity of the battery cell 1 was 711 mAh / g, and the discharge capacity was 629 mAh / g.

[4-2. Sample 2]
As an electrode active material sample 2, a graphite powder having an average particle diameter of 20 μm was prepared.

  Except that Sample 2 was used instead of Sample 1, the above [4-1. Battery cell 2 was produced in the same manner as the method for producing battery cell 1 described in [Sample 1].

  [4-1. Battery cell 2 was charged and discharged under the same conditions as the charging and discharging operation described in [Sample 1].

  The charge capacity of the battery cell 2 was 351 mAh / g, and the discharge capacity was 345 mAh / g.

[4-3. Sample 3]
[4-1. Sample 1 of an electrode active material was produced in the same manner as in the method of synthesizing Sample 1 described in [Sample 1].

  The point that sample 3 was used instead of sample 1, the point that a magnesium ribbon was used as a counter electrode and a reference electrode instead of lithium foil, and that 0.3 mol of bis (trifluoromethylsulfonyl) imide magnesium was added to triethylene glycol dimethyl ether / 4-1. Except that a non-aqueous electrolyte dissolved at a concentration of 1 / liter was used. Sample 1] was used to prepare a battery cell 3 using Sample 3 as an electrode active material.

  Two cycles of charge / discharge operation were performed on the battery cell 3 in a 60 ° C. constant temperature bath.

First, the current density with respect to the working electrode was set to 0.1 mA / cm ² , and the battery cell 3 was charged until the charging capacity reached 450 mAh / g. After resting for 20 minutes, the current density for the working electrode was set to 0.1 mA / cm ² , and the battery cell 3 was discharged until the discharge capacity reached 450 mAh / g.

Next, the current density with respect to the working electrode was set to 0.1 mA / cm ² , and the battery cell 3 was charged until the potential difference between the working electrode and the reference electrode reached 0V. After resting for 20 minutes, the current density with respect to the working electrode was set to 0.1 mA / cm ² , and the battery cell 3 was discharged until the potential difference between the working electrode and the reference electrode reached 3V.

  In the second charge / discharge operation, the charge capacity of the battery cell 3 was 316 mAh / g, and the discharge capacity was 179 mAh / g.

  Sample 3 was a graphite intercalation compound composed of carbon, lithium, and calcium before the charge / discharge operation, and it is considered that magnesium was inserted into this graphite intercalation compound by the first charge / discharge operation. Thereafter, it is considered that magnesium was desorbed from the graphite layer in the second charge / discharge operation, and magnesium was re-inserted between the graphite layers.

[4-4. Sample 4]
As the electrode active material sample 4, graphite powder having an average particle diameter of 20 μm was prepared.

  Except that the sample 4 was used instead of the sample 3, the above [4-3. Battery cell 4 was produced in the same manner as the method for producing battery cell 3 described in [Sample 3].

  [4-3. Battery cell 4 was charged and discharged under the same conditions as the second charging and discharging operation described in [Sample 3].

  The charge capacity of the battery cell 4 was 3 mAh / g, and the discharge capacity was 1 mAh / g.

[4-5. Sample 5]
As the electrode active material sample 5, a vanadium oxide (V ₂ O ₅ ) powder having an average particle size of 50 μm was prepared.

  Except that the sample 5 was used instead of the sample 1, [4-3. The battery cell 5 was produced in the same manner as the method for producing the battery cell 3 described in [Sample 3].

  [4-3. The battery cell 5 was charged and discharged under the same conditions as the second charging and discharging operation described in [Sample 3].

  The charge capacity of the battery cell 5 was 54 mAh / g, and the discharge capacity was 46 mAh / g.

[4-6. Comparison of charge and discharge capacity]
Samples 1 and 2 were charged and discharged in the electrolyte containing lithium ions as described above. Sample 1 of the graphite intercalation compound exhibited a larger charge / discharge capacity than Sample 2 of graphite.

  Samples 3 to 5 were charged and discharged in the electrolyte containing magnesium ions as described above. Sample 3 of the graphite intercalation compound exhibited a larger charge / discharge capacity than Sample 4 of graphite and Sample 5 of vanadium oxide.

[4-7. Composition analysis]
The composition before charging and the composition after charging of Sample 1 were analyzed using an ICP (inductively coupled plasma) emission spectroscopy analyzer (CIROS-120 manufactured by Spectro).

The composition of Sample 1 before charging was Li _2.1 Ca _2.0 C ₆ , and the composition of Sample 1 after charging was Li _0.5 Ca _1.4 C ₆ . The results show that (i) lithium between the graphite layers was desorbed by the charging operation, and (ii) the graphite intercalation compound was not oxidized to graphite but oxidized to a composition in which part of lithium and calcium remained. It suggests. By leaving lithium and calcium, a wide interlayer spacing can be maintained even when charge and discharge are repeated.

[4-8. X-ray diffraction measurement]
An X-ray diffraction (XRD) analysis was performed on Sample 1 before charging, Sample 1 after charging, and Sample 2. FIG. 2 shows an XRD chart for each sample.

  The XRD chart of Sample 1 before charging showed peaks at 2θ = 27.3 °, 36.6 °, and 46.4 °. These peaks are considered to belong to the (003) plane, (004) plane, and (005) plane of the β phase of the Li—Ca graphite intercalation compound, respectively. That is, this result indicates that Sample 1 before charging has a β-phase structure. The interlayer distance between the graphite layers of Sample 1 before charging was 9.8 °.

  The XRD chart of Sample 1 after charging showed peaks at 2θ = 11.2 °, 35.1 °, and 47.6 ° in addition to the positions where Sample 1 before charging showed peaks. It is considered that these peaks belong to the (001), (003), and (004) planes of the α phase of the Li—Ca graphite intercalation compound, respectively. That is, this result shows that at least a part of the charged sample 1 has an α-phase structure. The interlayer distance between the graphite layers of Sample 1 after charging was 7.9 °.

  The XRD chart of Sample 2 showed a peak at the position of 2θ = 26 °. The interlayer distance between the graphite layers of Sample 2 was 3.4 °.

  From the above results, the interlayer spacing of the graphite layer of Sample 1 was wider than the interlayer spacing of graphite (ie, Sample 2) regardless of before / after charging. A part of Sample 1 was changed from β phase to α phase by the charging operation.

[4-9. Supplement]
Sample 1 and Sample 3 after the charge / discharge operation correspond to Examples of the electrode active material according to the present embodiment, and Samples 2, 4, and 5 correspond to Comparative Examples.

  The electrode active material of the present disclosure can be used for a secondary battery.

REFERENCE SIGNS LIST 10 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

Claims (9)

Including general formula _M1 x _M2 graphite intercalation compound represented by y _{C 6,} the electrode active material for a secondary battery.
Here, M1 is at least one selected from the group consisting of alkaline earth metals and alkali metals, M2 is at least one selected from the group consisting of alkaline earth metals, and x and y are each , 0 <x ≦ 3.2 and 0.7 ≦ y ≦ 2.3. M1 is at least one selected from the group consisting of magnesium and lithium;
The electrode active material for a secondary battery according to claim 1. M2 is calcium,
The electrode active material for a secondary battery according to claim 2. The graphite intercalation compound includes a plurality of graphite layers each containing carbon,
M1 and M2 are located between the plurality of graphite layers,
The electrode active material for a secondary battery according to claim 1. A negative electrode comprising the electrode active material for a secondary battery according to any one of claims 1 to 4,
A positive electrode,
And an electrolyte having magnesium ion conductivity,
M1 contains magnesium;
Rechargeable battery. A positive electrode comprising the electrode active material for a secondary battery according to any one of claims 1 to 4,
A negative electrode,
And an electrolyte having magnesium ion conductivity,
M1 contains magnesium;
Rechargeable battery. Depending on the charging and discharging of the secondary battery, the amount of magnesium contained in the graphite intercalation compound changes,
The secondary battery according to claim 5. A negative electrode comprising the electrode active material for a secondary battery according to any one of claims 1 to 4,
A positive electrode,
An electrolyte having lithium ion conductivity,
M1 is lithium;
Rechargeable battery. At least a portion of the graphite intercalation compound reversibly changes between a first phase and a second phase in accordance with charging and discharging of the secondary battery,
x and y are 0 <x ≦ 3.2 and 0.7 ≦ y ≦ 2, regardless of whether the at least a part of the graphite intercalation compound is the first phase or the second phase. .3 is satisfied,
The secondary battery according to claim 5.