JP7723953B2 - Positive electrode active material for secondary battery and fluoride ion secondary battery using said positive electrode active material - Google Patents

Description

The present invention relates to a positive electrode active material for a secondary battery and a fluoride ion secondary battery using the positive electrode active material.

Lithium-ion secondary batteries have a high voltage per cell and a high energy density, and are therefore used in a wide range of applications, including portable electronic devices, electric vehicles, and power storage batteries.
Lithium-ion secondary batteries are secondary batteries that utilize the reaction between lithium ions and a positive electrode active material and the reaction between lithium ions and a negative electrode active material. Similarly, development of sodium-ion secondary batteries and potassium-ion secondary batteries is underway as cation-based batteries that use alkali metal ions as carriers, using sodium and potassium, which are alkali metals like lithium.
In lithium-ion secondary batteries, a principle known as the intercalation reaction is used in the electrodes to accommodate the lithium ions that move back and forth between the two electrodes, the positive and negative electrodes. In intercalation reaction batteries, lithium ions are inserted and stored in the gaps in the crystalline structure of the active material. Because the electrodes themselves do not change significantly even with repeated charging and discharging, phenomena such as electrode deformation caused by repeated charging and discharging, which occurred in conventional electrode-dissolving secondary batteries, are suppressed. The distance between the electrodes, which was previously set larger than necessary to account for electrode deformation, can now be reduced, allowing for a more compact battery design.

Conventional technology

[ Lithium-ion secondary batteries and cyanometallates ]
Metal complexes in which metal ions are bridged with cyano groups (hereinafter referred to as cyanometallates) are promising candidates for intercalation in the positive electrodes of lithium-ion, sodium-ion, and potassium-ion secondary batteries. This is because the bridged metal ions form a three-dimensional structure resembling a jungle gym (see Figure 1(A)), allowing charge carriers to be inserted into the gaps in the jungle gym structure.

For example, Patent Document 1 discloses a battery cathode material composed of an iron-based cyanometallate salt, represented by the general formula A _x Mn[Fe(CN) ₆ ]y·zH ₂ O. Here, A represents an alkali cation and an alkaline earth cation, x = 1 to 2, y = 0.5 to 1, and z = 0 to 3.5. Patent Document 1 also describes the charge and discharge characteristics of a secondary battery fabricated using a cathode containing A _x Mn[Fe(CN) ₆ ]y·zH ₂ O, an organic electrolyte containing A, and a negative electrode. According to the document, the charge and discharge characteristics exhibit a single plateau discharge curve, where the output voltage is nearly constant, and this single plateau discharge curve covers a range of 85% to 15% of the total discharge capacity. In terms of discharge characteristics, at the 85% point of the single plateau curve, the secondary battery had a discharge capacity exceeding 90 milliampere-hours per gram of cathode active material (mAh/g). When a secondary battery was fabricated using sodium as A, Na _x Mn[Fe(CN) ₆ ] _{y.zH 2} O as the positive electrode active material, an organic electrolyte solution, and a negative electrode, a large discharge capacity of 170 milliampere-hours per gram (mAh/g) was obtained, and the output voltage was 2 to 4.2 V.

[ Increasing the output voltage of lithium-ion secondary batteries: The first problem with lithium-ion secondary batteries ]
Increasing the output voltage is an effective way to increase the energy density of a battery. In theory, a higher output voltage can be obtained by increasing the difference in redox potential between the materials used in the positive and negative electrodes. This can be achieved by using a material with a low redox potential for the negative electrode and a material with a high redox potential for the positive electrode.

In the invention disclosed in Patent Document 1, the following oxidation-reduction reaction of iron complex ions or manganese complex ions coordinated with a cyano group is used as a positive electrode active material, and is responsible for the charge-discharge reaction.
[M ^III (CN) ₆ ] ^3- + e ^- ⇔ [M ^II (CN) ₆ ] ^4- (where M is Fe or Mn)
In this reaction, the higher the redox potential of M(CN) ₆ , the better. However, as disclosed in Non-Patent Document 1, when M is iron (Fe), the redox potential is +0.355 V (vs. standard hydrogen electrode potential), and when M is manganese (Mn), the redox potential is −0.240 V (vs. standard hydrogen electrode potential), which is not particularly high compared to that of other cyanometallates.

In order to improve the redox potential, it is considered to use molybdenum Mo for M. The reason for this is that the redox reaction of the molybdenum complex ion coordinated with a cyano group, represented by the following formula, [ ^MoV (CN) ₈ ] ^3- + e- ⇔ [ ^MoIV (CN) ₈ ] ^4-
This is because, as disclosed in Non-Patent Document 2, the oxidation-reduction potential of the iron complex ion (Mo) is +0.725 V (vs. standard hydrogen electrode potential), which is higher than the oxidation-reduction potentials of the iron complex ion and manganese complex ion. Therefore, if molybdenum (Mo) is used for M, the output voltage of the secondary battery can be increased compared to when iron (Fe) or manganese (Mn) is used, and as a result, a secondary battery with a high energy density can be obtained.
Therefore, if it were possible to synthesize a material such as A _x Mn[Mo(CN) ₈ ] y.zH ₂ O (where A is an alkali cation such as lithium Li, sodium Na, or potassium K, or an alkaline earth cation), it would be possible to expect an improvement in output voltage by applying this directly to a secondary battery having the configuration shown in Patent Document 1. The applicant has diligently searched for such a material, but has not been able to find a substance that has the structure of [Mo(CN) ₈ ] and contains both A and Mn.

However, Non-Patent Document 3 discloses [Mn(H ₂ O)] [Mn(HCOO) _2/3 (H ₂ O) _2/3 ] _3/4 [Mo(CN) ₈ ]·H ₂ O as an example of a cyano acid salt containing pentavalent molybdenum, and shows that lithium ions and sodium ions can be reversibly inserted and extracted into and from this substance. Although Non-Patent Document 3 does not show any prototype secondary battery examples, this substance contains a molybdenum complex ion framework [Mo(CN) ₈ ] structure, which is key to improving output voltage, and is capable of intercalation reactions of lithium ions and sodium ions. Therefore, if this molybdenum-based cyano acid salt is applied to a secondary battery with the configuration shown in Patent Document 1, it may be possible to realize a lithium ion secondary battery with a high output voltage.

However, the [Mn(H ₂ O)] [Mn(HCOO) _2/3 (H ₂ O) _2/3 ] _3/4 [Mo(CN) ₈ ]·H ₂ O disclosed in Non-Patent Document 3 has drawbacks. First, because the raw material for molybdenum-based cyanate salts is generally obtained as a compound containing tetravalent molybdenum, cyanate salts containing pentavalent molybdenum require a complicated synthesis procedure, such as using raw materials that have been pre-oxidized. Second, since the above material does not contain lithium ions, sodium ions, or potassium ions during production, when the above molybdenum-based cyanate salt is used as a positive electrode active material, the initial reaction inevitably involves the insertion of lithium ions, sodium ions, and potassium ions into the material, while at the same time, molybdenum accepts electrons from the electrode side and is reduced from pentavalent to tetravalent ([Mo ^V (CN) ₈ ] ³⁻ → [Mo ^IV (CN) ₈ ] ⁴⁻ ). This means that the initial reaction is discharge. In this case, the negative electrode must also be in a dischargeable state (reduced state). Examples of dischargeable negative electrode materials include graphite absorbing lithium ions, sodium ions, and potassium ions. However, such compounds are not commonly available on the market and are unstable in the atmosphere. Negative electrode active materials commonly available on the market are limited to materials in an oxidized state (rechargeable state). Therefore, it is difficult to fabricate a lithium-ion secondary battery using [Mn(H ₂ O)] [Mn(HCOO) _2/3 (H ₂ O) _2/3 ] _3/4 [Mo(CN) ₈ ]·H ₂ O, as disclosed in Non-Patent Document 3.

As described above, it is believed that it is theoretically possible to fabricate a lithium ion secondary battery using a cyano acid salt containing a molybdenum complex ion skeleton as a positive electrode active material.
However, there are many challenges to actually producing such a battery, such as the need for anode materials that are difficult to obtain and unstable in the atmosphere.

Although it is known that the use of a molybdenum complex ion framework [Mo(CN) ₈ ] may enable the realization of high voltage lithium ion secondary batteries, the fact that this has not yet been realized is the "first problem" with lithium ion secondary batteries.

[ Safety of Lithium-ion Secondary Batteries: The Second Problem with Lithium-ion Secondary Batteries ]
Furthermore, lithium-ion secondary batteries have the unavoidable problem of low safety due to their structure and physical properties. This problem stems from the fact that they must use organic electrolytes, which can be flammable.

Essentially, only organic electrolytes can be used in lithium-ion secondary batteries. This is because when an aqueous electrolyte is used, the reaction in which water is reduced and hydrogen is generated is more likely to occur than the reaction in which lithium ions are reduced and absorbed into the negative electrode. As a result, lithium is not absorbed into the negative electrode, and charge cannot be transferred between the electrode and the electrolyte using lithium ions.

Next, we will explain the causes of organic electrolyte ignition. When a lithium-ion secondary battery is overcharged, metallic lithium that can no longer be absorbed in the negative electrode can precipitate on the negative electrode. If metallic lithium grows across the separator and reaches the positive electrode, an internal short circuit occurs within the battery, which generates heat. This heat can cause the organic electrolyte to ignite.

Furthermore, when oxide-based materials are used as the positive electrode active material, it is known that oxygen atoms in the positive electrode active material are released at high temperatures. The released oxygen atoms are highly reactive and can react with the organic electrolyte, causing combustion.

Thus, despite their extremely high performance, lithium-ion secondary batteries have unavoidable safety issues.

International Publication No. 2014/118854 Japanese Patent Application Laid-Open No. 2019-204775 Japanese Patent Application Laid-Open No. 2017-220301

Handbook of Chemical Equilibria in Analytical Chemistry, Wiley, New York, 1985. Electrochemistry Handbook, 5th Edition, Maruzen, Tokyo, 2020 Inorganic Chemistry, vol. 52, pp. 3772-3779, 2013.

[ Anion-based secondary batteries: a method to simultaneously solve the first and second problems ]
The present applicant has conducted extensive research into the above two problems that have been associated with conventional lithium ion secondary batteries, and as a result has discovered a method for simultaneously solving the first and second problems.

In other words, the carrier is an anion instead of a cation, and an aqueous electrolyte is used.

The first problem is that the initial reaction results in discharging. In order for the reaction in which charge carriers are inserted into the positive electrode to result in charging for a secondary battery, the charge carriers must be anions rather than cations.

The second problem is the low safety of lithium-ion secondary batteries. This is due to the fact that the charge carriers are cations. In other words, when the charge carriers are cations, the charge carriers are lithium ions, sodium ions, potassium ions, etc., so organic solvents must be used in the electrolyte, which poses a risk of burning organic solvents.
If the carrier is selected from anions rather than cations and an aqueous electrolyte can be used, it may be possible to avoid the electrolyte from igniting.

[ Fluoride ion secondary battery ]
Fluoride ion secondary batteries, which utilize the reaction of fluoride ions, have been studied as anion-based secondary batteries that use anions as charge carriers. Generally, anions have a large ionic radius and are considered unsuitable for intercalation reactions. However, fluoride ions, the smallest stable anions, have an atomic radius comparable to that of sodium and potassium ions, making them the only anions that can freely enter and exit the gaps in the molecular structure of intercalation electrodes. The use of fluoride ions may enable the realization of anion-based intercalation electrodes.

Patent Document 2 discloses metal fluorides and metal oxyfluorides containing at least one metal selected from the group consisting of alkali metals, alkaline earth metals, scandium, yttrium, and lanthanides, a first transition metal, a second transition metal different from the first transition metal, and fluorine as positive electrode materials for fluoride ion batteries. Patent Document 2 also describes that when such a positive electrode active material is used, a large discharge capacity of 165 milliampere-hours per gram (mAh/g) is obtained, and speculates that an intercalation reaction may be occurring in the electrode as the carrier storage mechanism.

Patent Document 3 discloses a battery cathode material made of a metal oxyfluoride A _n+1 B _n O _3n+1-α F _x having a layered perovskite structure, where A is at least one of an alkaline earth metal element and a rare earth element, B is at least one of Mn, Co, Ti, Cr, Fe, Cu, Zn, V, Ni, Zr, Nb, Mo, Ru, Pd, W, Re, Bi, and Sb, n is 1 or 2, 0≦α≦3.5, and 0≦x≦5.5.
Patent Document 3 discloses that when the active material is charged, fluoride ions are inserted by an intercalation reaction, and when charging is continued, some of the O elements are replaced by F elements, resulting in a new high-potential electrode reaction, and further that as a result of this two-stage electrode reaction, the charge capacity becomes 225 milliampere-hours per gram (mAh/g).

The secondary batteries disclosed in Patent Documents 2 and 3 disclose that fluoride ions are inserted into the positive electrode by an intercalation reaction, but the positive electrode does not contain a cyanometallate, and naturally, the output voltage is not improved by using a molybdenum complex ion framework [Mo(CN) ₈ ].

Furthermore, the secondary batteries disclosed in Patent Documents 2 and 3 use metal fluorides as the positive electrode active material, which has the disadvantage of being easily decomposed by absorbing moisture in the air. Furthermore, the synthesis of the material requires numerous steps, such as combustion in a fluorine gas stream or in an inert atmosphere.

In particular, the metal oxyfluoride disclosed in Patent Document 3 contains oxygen, and furthermore, an organic solvent is used in the electrolyte, which can cause combustion and explosion. In other words, no solution to the second problem mentioned above is recognized.

As such, while intercalation-type secondary batteries that use fluoride ions as charge carriers have been disclosed in prior art secondary batteries, none of them simultaneously solve both the first and second problems mentioned above.

As a result of extensive research, the present inventors have succeeded in solving the first and second problems by using a positive electrode containing a molybdenum-based cyanometallate having a molybdenum complex ion framework [Mo(CN) ₈ ] and using fluoride ions as a carrier. That is, they have succeeded in providing a positive electrode active material that can obtain a satisfactorily high output voltage, can be charged in a state immediately after manufacture, can be constructed using commercially available negative electrode materials, and has high safety by preventing the electrolyte from igniting, as well as a fluoride ion secondary battery that uses the positive electrode active material.

The invention according to claim 1 relates to a positive _electrode active material for a secondary battery, characterized in that it is a metal complex represented by the general formula _MxMo (CN) _8.yH2O , in which M is one element selected from the group consisting of Fe, Co, Ni, and Zn, x and y satisfy the relationships 1.95<x<2.05 and 0≦y≦9, respectively, and is in an oxidizable state, i.e., a chargeable state, in an electrochemical reaction using a fluoride ion as an intercalating species.

The invention according to claim 2 relates to a fluoride ion secondary battery using the positive electrode active material for secondary batteries described in claim 1, characterized in that the fluoride ion secondary battery comprises a positive electrode, a negative electrode, an electrolyte, and a separator, the positive electrode containing the positive electrode active material for secondary batteries, the electrolyte being an aqueous solution in which a fluoride salt is dissolved, and the separator is provided between the positive electrode and the negative electrode.

The invention of claim 3 relates to the fluoride ion secondary battery described in claim 2, characterized in that it is a sealed battery prepared by impregnating the separator with the electrolyte.

The invention according to claim 4 relates to the fluoride ion secondary battery according to claim 2 or 3, characterized in that the fluoride salt is one or more selected from the group consisting of lithium fluoride, sodium fluoride, potassium fluoride, rubidium fluoride, and cesium fluoride.

According to the invention of claim 1, a fluoride _{ion secondary} battery is produced using a _positive electrode containing a positive electrode active material for a secondary battery, characterized in that the positive electrode active material is a metal complex represented by the general formula _MxMo (CN)8.yH2O, wherein M is one element selected from the group consisting of Fe, Co, Ni, and Zn, and x and y satisfy the relationships 1.95<x<2.05 and 0≦y≦9, respectively, and is in an oxidizable state, i.e., a chargeable state, in an electrochemical reaction in which a fluoride ion serves as an insertion species. This solves the first and second problems of conventional lithium ion secondary batteries. That is, a satisfactorily high output voltage can be obtained, the positive electrode of the fluoride ion secondary battery is chargeable immediately after production, and therefore the negative electrode can be constructed using commercially available negative electrode materials. Furthermore, a positive electrode active material necessary for producing a secondary battery with high safety, capable of avoiding explosive combustion and ignition of the electrolyte, can be provided.
In the present invention, the positive electrode active material being "in a chargeable state" means that there are surplus portions in which negative ions (anions) such as fluoride ions can be inserted into voids in the crystal structure of the positive electrode active material due to a charge compensation reaction accompanying the oxidation reaction of the positive electrode active material.

The fluoride ion secondary battery according to claim 2 comprises a positive electrode, a negative electrode, an electrolyte, and a separator. Because the positive electrode contains the secondary battery's positive electrode active material, it can output a satisfactorily high voltage. Furthermore, because the electrolyte dissolves fluoride salt, the initial reaction is charging. Therefore, it can be constructed using commercially available negative electrode materials. Furthermore, because the fluoride ion secondary battery according to claim 2 uses water as a solvent, there is no need to use a non-aqueous electrolyte, such as an organic electrolyte. Therefore, even if heat is generated, the electrolyte will not ignite, making it highly safe.

According to claim 3, a highly practical, compact fluoride ion secondary battery is provided, characterized in that it is a sealed battery prepared by impregnating the separator with the electrolyte.

According to the invention of claim 4, the fluoride salt of the electrolyte is one or more selected from lithium fluoride, sodium fluoride, potassium fluoride, rubidium fluoride, and cesium fluoride. These fluoride salts are readily available on the market and are stable in air.

FIG. 1(A) is a schematic diagram showing the molecular structure of a metal complex according to the present invention, which is represented by the general formula M _x Mo(CN) _{8.yH 2} O. FIG. 1B is a schematic diagram showing a charged state in which fluoride ions are inserted into the positive electrode containing the positive electrode active material of the fluoride ion secondary battery according to the present invention. FIG. 2(A) is a schematic diagram showing the flow of charge carriers during charging in the fluoride ion secondary battery according to the present invention. FIG. 2B is a schematic diagram showing the state of charge carriers during discharge in the fluoride ion secondary battery according to the present invention. FIG. 3A is a graph showing a cyclic voltammogram (solid line) of Zn ₂ Mo(CN) ₈ according to Example 2 of the present invention. FIG. 3B is a graph showing a cyclic voltammogram of Zn ₂ Mo(CN) ₈ according to Comparative Example 2. FIG. 3C is a graph showing a cyclic voltammogram of ZnFe(CN) ₆ according to Comparative Example 3. FIG. 10 is a schematic diagram of a sealed fluoride ion secondary battery according to a third embodiment of the present invention. 10 is a graph showing the charge/discharge characteristics of a sealed fluoride ion secondary battery according to Example 3 of the present invention. 10 is a graph showing a cyclic voltammogram of Fe ₂ Mo(CN) ₈ according to Example 4 of the present invention. 10 is a graph showing a cyclic voltammogram of Co ₂ Mo(CN) ₈ according to Example 5 of the present invention. 10 is a graph showing a cyclic voltammogram of Ni ₂ Mo(CN) ₈ according to Example 6 of the present invention.

Embodiments of the positive electrode active material according to the present invention and a fluoride ion secondary battery using the positive electrode active material will be described in detail below with reference to the accompanying drawings.

[Embodiment]
<1. Cathode active material＞
1A is a schematic diagram showing the molecular structure of a metal complex (5) according to this embodiment, represented by the general formula _MxMo (CN) _8.yH2O . The metal complex contains an M ion (3), a molybdenum ion ( ₂ ), and a cyano group (4). Fluoride ions (1), which are charge carriers, are inserted into gaps in the three-dimensional, jungle-gym-like structure of the metal complex (5).
Generally, anions have a larger ionic radius than cations, making it difficult for them to be inserted into or removed from compounds. However, the fluoride ion (1) has an ionic radius similar to that of a potassium ion, making it possible for it to be inserted into compounds.

1B is a diagram showing the operation of a positive electrode containing the positive electrode active material for a secondary battery according to this embodiment. Molybdenum (2) in a metal complex represented by the general formula _MxMo (CN) _8.yH2O releases electrons ( ₆ ) to the electrode and is oxidized from a tetravalent to a pentavalent (Mo4 ⁺ → Mo5 ⁺ ), and fluoride ions are inserted into the metal complex.

2. Carrier flow in fluoride ion secondary batteries
FIG. 2(A) shows the flow of charge carriers, including the external circuit. Electrons (6) flow out of the positive electrode (11) into the external circuit, and simultaneously, fluoride ions (1) are inserted into the positive electrode active material. Electrons (6) released from the positive electrode (11) flow out of the positive electrode (11) into the external circuit and then flow into the negative electrode (12) via the external circuit. The movement of electrons (6) from the positive electrode (11) to the negative electrode (12) means that current (15) flows from the negative electrode (12) to the positive electrode (11). This corresponds to the state in which the secondary battery is being charged using an external power source (16).

In the fluoride ion secondary battery shown in Figure 2(A), a common metal zinc electrode is used as the negative electrode (12). Electrons (6) flowing into the negative electrode (12) from an external circuit combine with zinc ions (17) in the electrolyte or with zinc ions released from zinc fluoride ZnF2 ( ₁₉ ) deposited on the surface of the negative electrode (12) to form zinc Zn. At the same time, fluoride ions (1) are released into the solution.

Figure 2 (B) shows the flow of charge carriers, including the external circuit. Electrons (6) emitted from the negative electrode (12) flow out of the negative electrode (12) into the external circuit and then flow into the positive electrode via the external circuit. The movement of electrons from the negative electrode (12) to the positive electrode (11) means that current is flowing from the positive electrode (11) to the negative electrode (12). This corresponds to a state in which current is flowing from the secondary battery to the load (18).

In the fluoride ion secondary battery shown in Figure 2(B), electrons (6) are released from the metallic zinc electrode of the negative electrode (12) into the external circuit, and at the same time, zinc dissolves into the solution as zinc ions (17). The dissolved zinc ions (17) immediately combine with fluoride ions (1) in the solution to form zinc fluoride _ZnF2 (19), which precipitates on the surface of the negative electrode. Because the solubility of zinc fluoride (19) is extremely low, this reaction occurs immediately.

3. Sealed Fluoride Ion Secondary Battery
FIG. 4 shows a schematic diagram of a sealed fluoride ion secondary battery (20). The sealed fluoride ion secondary battery consists of a positive electrode (11), a negative electrode (12), and a separator (13). The positive electrode (11) is arranged at the top and the negative electrode (12) at the bottom, with a separator (13) between the positive electrode (11) and the negative electrode (12). The separator (13) is impregnated with an electrolyte (not shown). The positive electrode (11) and the negative electrode (12) are in contact with each other via the separator (13), and the positive electrode (11), the negative electrode (12), the electrolyte, and the separator (13) are sealed in the same area by an exterior body (14).

<3-1. Positive electrode>
The positive electrode (11) is a conductive substrate to which a positive electrode mixture is fixed. The positive electrode mixture is made from a positive electrode active material containing a _{metal complex represented by the general formula MxMo(CN)8.yH2O according to} this embodiment, a conductive additive, and a binder.

<3-2. Conductive additive>
Suitable examples of the conductive additive used in this embodiment include carbon materials such as carbon blacks, graphites, carbon nanotubes (CNTs), and vapor-grown carbon fibers (VGCFs). Examples of carbon blacks include acetylene black, oil furnace black, and ketjen black. The conductive additive may be used alone or in combination with two or more. The mixing ratio of the positive electrode active material and the conductive additive is not particularly limited. However, the content of the conductive additive in the positive electrode is preferably 1% by mass to 500% by mass, more preferably 1% by mass to 300% by mass, even more preferably 1% by mass to 200% by mass, and particularly preferably 1% by mass to 150% by mass, relative to the total mass of the positive electrode active material contained in the positive electrode.

<3-3. Binder>
The binder is not particularly limited, and examples of known binders include polymer compounds, and preferred examples include fluororesins, polyolefin resins, rubber polymers, polyamide resins, polyimide resins (such as polyamideimides), and cellulose ethers.

Specific examples of binders include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene fluororubber (VDF-HFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFP-TFE fluororubber), polyethylene, aromatic polyamide, cellulose, styrene-butadiene rubber, isoprene rubber, butadiene rubber, ethylene-propylene rubber, styrene-butadiene-styrene block copolymers, their hydrogenated products, and styrene-ethylene-butadiene. -styrene copolymers, styrene-isoprene-styrene block copolymers, their hydrogenated products, syndiotactic 1,2-polybutadiene, ethylene-vinyl acetate copolymers, propylene-α-olefin (carbon number 2 to 12) copolymers, starch, methyl cellulose, carboxymethyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, carboxymethylhydroxyethyl cellulose, nitrocellulose, polyacrylic acid, sodium polyacrylate, polyacrylonitrile, etc.

The binder may be used alone or in combination with two or more types.

There are no particular restrictions on the mixing ratio between the positive electrode active material and the binder, but the content of the binder in the positive electrode is preferably 1% to 50% by mass, and more preferably 1% to 25% by mass, relative to the total mass of the positive electrode active material contained in the positive electrode.

<3-4. Positive electrode current collector>
Examples of the positive electrode current collector include foils, meshes, expanded grids (expanded metals), punched metals, nonwoven fabrics, porous bodies, etc., made of conductive materials such as carbon paper, nickel, and stainless steel (SUS). The mesh size, wire diameter, number of meshes, porosity, etc. are not particularly limited, and conventionally known current collectors can be used.

There are no particular restrictions on the shape of the positive electrode current collector, and it can be selected to match the desired shape of the positive electrode. Examples include foil and plate shapes.

<3-5. Negative electrode>
The negative electrode (12) may be any material that undergoes oxidation and reduction at a potential lower than that of the positive electrode active material, such as a metal fluoride containing one or more metals selected from the group consisting of zinc (Zn), tin (Sn), and lead (Pb).

<3-6. Separator>
The separator (13) serves to physically separate the positive electrode and the negative electrode to prevent internal short circuits. The separator (13) is impregnated with an electrolyte and has fluoride ion permeability to ensure the battery reaction.

As the separator (13), for example, a resin porous membrane or a nonwoven fabric can be used. The separator may be formed of only a porous membrane layer or a nonwoven fabric layer, or may be formed as a laminate of multiple layers with different compositions and forms. Suitable laminates include, but are not limited to, laminates having multiple resin porous layers with different compositions, and laminates having a porous membrane layer and a nonwoven fabric layer.

The material of the separator (13) can be selected taking into consideration the battery's operating temperature, electrolyte composition, etc.

The resin contained in the fibers forming the porous membrane and nonwoven fabric may be one or more selected from the group consisting of derivatives such as cellulose and nitrocellulose; polyolefin resins such as polyethylene, polypropylene, and ethylene-propylene copolymer; polyphenylene sulfide resins such as polyphenylene sulfide and polyphenylene sulfide ketone; polyamide resins such as aromatic polyamide resins (such as aramid resin); and polyimide resins. The resin may be hydrophilized.

The shape and size of the separator (13) are not particularly limited and can be selected appropriately to suit the desired battery shape, etc.

<3-7. Electrolyte>
The electrolyte solution is prepared by dissolving a fluoride salt in water. The water is preferably water from which impurities have been removed, such as ion-exchanged water or ultrapure water. The electrolyte solution must contain fluoride ions and must be neutral to weakly alkaline so that the positive electrode active material does not dissolve. The fluoride salt may contain at least one of lithium fluoride, sodium fluoride, potassium fluoride, rubidium fluoride, and cesium fluoride. The concentration is preferably 1 mol/L to 13.5 mol/L, more preferably 5 mol/L to 13 mol/L, and even more preferably 10 mol/L to 13 mol/L. Furthermore, if the negative electrode active material dissolves in the electrolyte solution, the negative electrode active material compound may be saturated in advance.

The above-mentioned fluoride salts are more readily available on the market and more stable in the atmosphere than hexafluorophosphates (LiPF ₆ , NaPF ₆ , KPF ₆ ) and tetrafluoroborates (LiBF ₄ , NaBF ₄ , KBF ₄ ) used in lithium ion secondary batteries and the like.

The fluoride ion secondary battery according to this embodiment uses water as a solvent. There is no need to use a non-aqueous electrolyte solution, such as an organic electrolyte solution. Therefore, even if heat is generated, the electrolyte solution will not ignite, making it highly safe.

Example 1: Preparation of positive electrode active material
A positive electrode active material (Zn ₂ Mo(CN) ₈ ) was synthesized as follows.

A molybdenum source solution prepared by dissolving 10 g of homemade potassium octacyanomolybdate ( _K4Mo (CN) _{8.2H2O )} in 200 mL of ultrapure water was mixed with a zinc source solution prepared by dissolving 60 g of zinc _sulfate ( _ZnSO4.7H2O , Kishida Chemical) in 200 mL of ultrapure water. The resulting precipitate was washed with water, filtered, and dried to synthesize a positive electrode active material. This process is referred to as the Zn process.

In the above Zn step, zinc sulfate ZnSO ₄ ·7H ₂ O was converted to iron sulfate FeSO ₄ ·7H ₂ O to synthesize Fe ₂ Mo(CN) ₈ .

In the above Zn step, zinc sulfate ZnSO ₄ ·7H ₂ O was changed to cobalt sulfate CoSO ₄ ·7H ₂ O to synthesize Co ₂ Mo(CN) ₈ .

In the above Zn step, zinc sulfate ZnSO ₄ ·7H ₂ O was changed to nickel sulfate NiSO ₄ ·6H ₂ O to synthesize Ni ₂ Mo(CN) ₈ .

Example 2: Evaluation of positive electrode containing positive electrode active material Zn ₂ Mo(CN) ₈
Cyclic voltammograms were performed on the _Zn2Mo (CN) ₈ prepared in Example 1. A three-electrode electrochemical cell consisting of a positive electrode as the working electrode, a platinum counter electrode, and a lead | lead fluoride reference electrode (hereinafter referred to as Pb|Pb2 ⁺ ) was filled with a 10 mol/L potassium fluoride aqueous solution as the electrolyte, and cyclic voltammograms were performed using a potentiostat manufactured by BioLogic (France). The charge-discharge characteristics were evaluated by examining the redox response in the voltage range from 1.0 V (Pb|Pb2 ⁺ ) to 1.6 or 1.65 V.
The results are shown by the solid line in FIG.

Paired oxidation and reduction peaks were confirmed at approximately 1.46 V and 1.31 V. This indicates that this compound is reversibly oxidized and reduced, meaning it can be charged and discharged.

Comparative Example 1: Evaluation of Positive Electrode Not Containing Positive Electrode Active Material
A positive electrode was prepared by applying a positive electrode mixture slurry that did not contain the positive electrode active material Zn ₂ Mo(CN) ₈ to carbon paper. A cyclic voltammogram of the positive electrode was performed using an evaluation apparatus with the same configuration as in Example 2.
The results are shown by the dashed line in FIG.

In this result, neither an oxidation peak nor a reduction peak was confirmed, and it was found that the oxidation peak and reduction peak confirmed in Example 2 were due to the positive electrode active material Zn ₂ Mo(CN) ₈ .

Comparative Example 2: Evaluation of a positive electrode containing Zn ₂ Mo(CN) ₈ as a positive electrode active material (when the electrolyte was an aqueous potassium sulfate solution)
Cyclic voltammograms were performed using an evaluation device with the same configuration as in Example 2, except that a saturated potassium sulfate aqueous solution was used as the electrolyte and a mercury/mercury oxide electrode was used as the reference electrode.
The results are shown in Figure 3(B).

In these results, a weak oxidation wave associated with the oxidation of water was observed, but no corresponding reduction wave was observed. This indicates that the presence of fluoride ions is essential for the oxidation peak and reduction peak observed in Example 2.

Comparative Example 3: Evaluation of positive electrode containing positive electrode active material ZnFe(CN) ₆
One of the objectives of the present invention is to obtain a high output voltage of a secondary battery, and as mentioned above, the molybdenum complex ion framework [Mo(CN) ₈ ] coordinated with a cyano group was selected for this purpose. Therefore, it is necessary to verify whether the oxidation peak and reduction peak potentials obtained in Example 2 are attributable to the molybdenum framework.
For this verification, as a comparative example, ZnFe(CN)6 was synthesized, which consisted of an iron complex ion framework [Fe(CN) ₆ ] coordinated with a cyano group instead of the molybdenum complex ion framework [Mo(CN) _{8 ]} coordinated with a cyano group. Next, a cyclic voltammogram was performed using the same measurement conditions and evaluation equipment as in Example 2, except that the positive electrode active material _Zn2Mo (CN) ₈ was changed to ZnFe(CN) ₆ .
The results are shown in Figure 3(C).

Paired oxidation and reduction peaks were observed at 1.09 V and 0.95 V. Compared to the case where the positive electrode active material _Zn2Mo (CN) ₈ was used (Example 2), both the oxidation and reduction peak potentials were found to be 0.36 V lower. The oxidation and reduction peak potentials obtained in Example 2 indicate that the molybdenum complex ion framework [Mo(CN) ₈ ] coordinated with cyano groups contributes to the oxidation and reduction peak potentials.
That is, by using a compound having Zn ₂ Mo(CN) ₈ as the main structure, the redox potential of the positive electrode can be increased by 0.36 V compared to when a compound having ZnFe(CN) ₆ as the main structure is used.
Furthermore, the results of this experiment show that when this molybdenum complex ion framework is used, a higher output voltage of the secondary battery can actually be obtained compared to when other cyano-coordinated complex ion frameworks are used.

Example 3: Fabrication of sealed fluoride ion secondary battery
The method for producing a sealed fluoride ion secondary battery is described below.

A positive electrode mixture slurry was prepared by mixing 40 parts by weight of _Zn2Mo (CN) ₈ as the positive electrode active material, 50 parts by weight of Denka Black (Denka Co., Ltd.) as a conductive additive, and 10 parts by weight of polyvinylidene fluoride (Kureha Corporation, #1100) as a binder in n-methylpyrrolidone solvent (three times the weight of the solid content). The positive electrode mixture slurry was applied to a carbon paper (Toray Industries, Inc., TGP-H090) positive electrode current collector and dried to prepare a positive electrode.

The negative electrode was made by fixing zinc metal foil to carbon paper as the negative electrode current collector.

The positive electrode and negative electrode prepared by the positive electrode preparation procedure were laminated together with a hydrophilized polyolefin nonwoven fabric separator, and a 10 mol/L aqueous potassium fluoride solution (saturated with zinc fluoride) as an electrolyte was sealed in the laminate film to prepare a sealed fluoride ion secondary battery.
A schematic diagram of a sealed fluoride ion secondary battery is shown in FIG.

A charge/discharge test was carried out on the obtained sealed fluoride ion secondary battery using a potentiostat (described above).
The results are shown in Figure 5.

The midpoint voltage of charge determined from the charge curve was 1.6 V, the midpoint voltage of discharge determined from the discharge curve was 1.15 V, and the discharge capacity per gram of positive electrode active material was 40 mAh/g.

Example 4: Evaluation of positive electrode containing positive electrode active material _Fe2Mo (CN) ₈
A cyclic voltammogram of _Fe2Mo (CN) ₈ was performed using the same evaluation device and conditions as in Example 2, except that the positive electrode active material _Zn2Mo (CN) ₈ was changed to _Fe2Mo (CN) ₈ .
The results are shown in Figure 6.

A pair of oxidation peaks with a maximum at 1.37 V and reduction peaks with a plateau between 1.2 V and 1.3 V was confirmed. It was found that Fe ₂ Mo(CN) ₈ was reversibly oxidized and reduced, that is, capable of being charged and discharged.

Example 5: Evaluation of positive electrode containing positive electrode active material Co ₂ Mo(CN) ₈
A cyclic voltammogram of Co2Mo(CN) ₈ was performed using the same evaluation device and conditions as in Example 2, except that the positive electrode active material _Zn2Mo (CN) ₈ was changed to _Co2Mo (CN) _{8 .}
The results are shown in Figure 7.

A pair of oxidation peaks with two maxima at 1.35 V and 1.48 V and a reduction peak with a maximum at 1.31 V were confirmed. It was found that _{Co 2} Mo(CN) ₈ can be reversibly oxidized and reduced, that is, can be charged and discharged.

Example 6: Evaluation of positive electrode containing positive electrode active material Ni ₂ Mo(CN) ₈
A cyclic voltammogram of Ni ₂ Mo(CN) ₈ was performed using the same evaluation device and conditions as in Example 2, except that the positive electrode active material Zn ₂ Mo(CN) ₈ was changed to Ni ₂ Mo(CN) ₈ .
The results are shown in Figure 8.

A pair of an oxidation peak having a maximum at 1.47 V and a reduction peak having a maximum at 1.39 V was confirmed. It was found that Ni ₂ Mo(CN) ₈ was reversibly oxidized and reduced, that is, capable of being charged and discharged.

A positive electrode active material and a fluoride ion secondary battery using this active material can be produced that can provide a satisfactorily high output voltage, can be charged immediately after manufacture, can be constructed using commercially available negative electrode materials, and are highly safe, preventing the electrolyte from igniting.

REFERENCE SIGNS LIST 1 Fluoride ion 2 Molybdenum ion 3 M ion 4 Cyano group 5 Metal complex represented by _MxMo (CN) _{8· yH2O} 11 Positive electrode 12 Negative electrode 13 Separator 20 Sealed fluoride ion secondary battery

Claims (4)

A positive electrode active material for a secondary battery containing an aqueous electrolyte solution,
A positive _{electrode active material for a secondary battery, characterized in that it is a metal complex represented by the general formula MxMo(CN)8.yH2O , in} which M is one element selected from the group consisting of Fe, Co, Ni, and Zn, and x and y satisfy the relationships 1.95<x<2.05 and 0≦y≦9, respectively, and is in an oxidizable state, i.e., a chargeable state, in an electrochemical reaction using a fluoride ion as an intercalating species. A fluoride ion secondary battery using the positive electrode active material for secondary batteries according to claim 1, wherein the fluoride ion secondary battery comprises:
a positive electrode, a negative electrode, the aqueous electrolyte solution, and a separator;
the positive electrode contains the positive electrode active material for a secondary battery,
the aqueous electrolyte is an aqueous solution in which a fluoride salt is dissolved,
The separator is provided between the positive electrode and the negative electrode.
A fluoride ion secondary battery characterized by: 3. The fluoride ion secondary battery according to claim 2, wherein the battery is a sealed battery prepared by impregnating the separator with the aqueous electrolyte solution. The fluoride ion secondary battery according to claim 2 or 3, characterized in that the fluoride salt is one or more selected from the group consisting of lithium fluoride, sodium fluoride, potassium fluoride, rubidium fluoride, and cesium fluoride.