JP2019008958A - Molten salt electrolyte and potassium molten salt battery - Google Patents

Abstract

To provide a molten salt electrolyte which enables the operation of a potassium molten salt battery at a temperature of a room temperature or under.SOLUTION: The molten salt electrolyte hereof is a molten salt electrolyte for a potassium molten salt battery, having a potassium ion conducting property. The molten salt electrolyte comprises salts including anions and cations. The cations include potassium ions and organic cations. The organic cation includes an organic cation having a nitrogen-containing heterocyclic skeleton. The mole fraction of the potassium ions to a total quantity of the potassium ions and the organic cations is over 0.2 up to 0.7.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a molten salt electrolyte for a potassium molten salt battery and a potassium molten salt battery using the same.

  In a potassium molten salt battery, potassium ions become electron carriers. Potassium has a high redox potential and a wide electrochemical window. Therefore, a high operating voltage and a high energy density are expected in a potassium molten salt battery. Further, since potassium ions have higher ion conductivity than lithium ions, it is considered that the output is high and high-speed charge / discharge is possible. Moreover, potassium is cheaper than lithium, and a reduction in battery cost is also expected.

  Patent Document 1 proposes a potassium molten salt battery using a metal complex as a positive electrode active material and hard carbon and / or graphite as a negative electrode active material. In Patent Document 1, potassium bis (fluorosulfonyl) amide is used as a molten salt electrolyte, and the battery is operated at 120 ° C. or 90 ° C.

JP-A-2015-64932

  Since potassium bis (fluorosulfonyl) amide has a high melting point, the operating temperature of the battery is relatively high such as 120 ° C. or 90 ° C. as in Patent Document 1. When a potassium molten salt battery is put into practical use, if the battery can be operated at a temperature below room temperature, the application can be easily expanded. For example, the melting point of the molten salt electrolyte can be lowered by using the molten salt electrolyte as a mixture of a plurality of ionic liquids. On the other hand, when the concentration of potassium ions is low, potassium is likely to locally precipitate during charging and discharging, and short-circuiting is likely to occur.

One aspect of the present invention is a molten salt electrolyte for a potassium molten salt battery having potassium ion conductivity,
The molten salt electrolyte includes a salt composed of an anion and a cation,
The cations include potassium ions and organic cations,
The organic cation includes an organic cation having a nitrogen-containing heterocyclic skeleton,
The molar fraction of the said potassium ion which occupies for the sum total of the said potassium ion and the said organic cation is related with molten salt electrolyte which is more than 0.2 and 0.7 or less.

Another aspect of the present invention includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator interposed between the positive electrode and the negative electrode, and the above molten salt electrolyte.
The positive electrode active material includes a material that reversibly absorbs and releases potassium ions,
The negative electrode active material relates to a potassium molten salt battery including a material that reversibly absorbs and releases potassium ions.

  According to the above aspect of the present invention, a molten salt electrolyte for a potassium molten salt battery capable of operating a potassium molten salt battery at a temperature below room temperature while suppressing a short circuit and a potassium molten salt battery using the same are provided. There is.

1 is a longitudinal sectional view schematically showing a potassium molten salt battery according to an embodiment of the present invention. 2 is a phase diagram of a molten salt electrolyte of Example 2. FIG.

[Description of Embodiment of the Invention]
First, the contents of the embodiment of the present invention will be listed and described.
One embodiment of the present invention relates to a molten salt electrolyte for a potassium molten salt battery having potassium ion conductivity. The molten salt electrolyte includes an anion and a cation, and the cation includes a potassium ion and an organic cation. The organic cation includes an organic cation having a nitrogen-containing heterocyclic skeleton. The molar fraction of potassium cations in the total of potassium ions and organic cations is greater than 0.2 and 0.7 or less.

  One embodiment of the present invention also includes a potassium molten salt battery including the above molten salt electrolyte. The potassium molten salt battery includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator interposed between the positive electrode and the negative electrode, and the molten salt electrolyte. The positive electrode active material includes a material that reversibly absorbs and releases potassium ions, and the negative electrode active material includes a material that reversibly absorbs and releases potassium ions.

  When the molar fraction of potassium ions in the total of potassium ions and organic cations is in the range of 0.2 to 0.7, at least a part of the molten salt electrolyte exists in a liquid state even at room temperature. Further, when the molar fraction of potassium ions is in the above range, a glass transition point (Tg) appears, and therefore the electrolyte is in a liquid state even at a temperature lower than room temperature due to supercooling. Therefore, when the above molten salt electrolyte is used for a potassium molten salt battery, the battery can be operated even at a temperature below room temperature. Moreover, when the molar fraction is in the above range, the local precipitation of potassium during charging / discharging or the like is suppressed, and a short circuit hardly occurs. In addition, when the molar fraction of potassium ions is a low concentration of 0.2 or less, potassium is likely to be locally deposited during charging / discharging, and short-circuiting is likely to occur. In the present embodiment, such a short circuit can be suppressed by setting the molar fraction of potassium ions to be greater than 0.2 and less than or equal to 0.7.

  Even when the molar fraction of potassium ions is within the above range, salt may precipitate at room temperature. However, even when the salt is deposited, some of the electrolyte can exist in a liquid state, so that the battery can be operated.

Here, the molten salt electrolyte means an electrolyte containing a molten salt. Moreover, a molten salt battery is a general term for batteries containing a molten salt as an electrolyte. The molten salt means a molten salt (or ionic liquid). The potassium molten salt battery includes a molten salt electrolyte exhibiting potassium ion conductivity, and potassium ions are involved in charge / discharge reactions and become charge carriers. The ionic liquid is a liquid composed of an anion and a cation.
Moreover, in this specification, room temperature shall mean the temperature of 25 to 35 degreeC.

  The molar fraction of potassium ions is preferably more than 0.2 and 0.4 or less. When the molar fraction of potassium ions is in such a range, even if a salt is precipitated, a large amount of electrolyte can be present in a liquid state, so that high ion conductivity is easily ensured.

  The organic cation preferably contains at least one selected from the group consisting of a pyrrolidinium cation and an imidazolium cation. The organic cation further preferably contains at least a pyrrolidinium cation. In these cases, when the molar fraction of potassium ions is within the above range, the molten salt electrolyte is easily kept in a liquid state at a temperature below room temperature, and high ion conductivity is easily secured.

  The anion preferably includes a bissulfonylamide anion. In this case, it is easy to keep the melting point of the molten salt electrolyte low. Moreover, since the viscosity of molten salt electrolyte can be made low, it becomes easy to ensure high ion conductivity.

  In a preferred embodiment, the molten salt electrolyte contains 80% by mass or more of an ionic liquid containing an anion and a cation. In this case, the flame retardancy of the molten salt electrolyte can be increased.

  In the potassium molten salt battery, the negative electrode active material preferably contains at least one selected from the group consisting of hard carbon and a carbonaceous material having a graphite-type crystal structure. Such a negative electrode active material can occlude and release potassium ions reversibly, and can be stably charged and discharged by combining with the above molten salt electrolyte.

[Details of the embodiment of the present invention]
Specific examples of a molten salt electrolyte for a potassium molten salt battery according to an embodiment of the present invention and a potassium molten salt battery using the molten salt electrolyte will be described below. In addition, this invention is not limited to these illustrations, is shown by the attached claim, and is intended that all the changes within the meaning and range equivalent to the claim are included. .

(Molten salt electrolyte)
Since the molten salt electrolyte needs to have ionic conductivity at least when it is melted, it contains ions (cations and anions) that serve as charge carriers for charge and discharge reactions in the molten salt battery. More specifically, the molten salt electrolyte includes a salt of a cation and an anion. In one embodiment of the present invention, the cation includes potassium ions because the molten salt electrolyte needs to have potassium ion conductivity. The molten salt electrolyte contains an organic cation in order to maintain a liquid state even at a relatively low temperature, and the organic cation contains an organic cation having a nitrogen-containing heterocyclic skeleton.

  Examples of the nitrogen-containing heterocyclic skeleton include 5- to 8-membered heterocycles having 1 or 2 nitrogen atoms as ring constituent atoms such as pyrrolidine, imidazoline, imidazole, pyridine and piperidine; Examples thereof include 5- to 8-membered heterocycles having two nitrogen atoms and other heteroatoms (oxygen atoms, sulfur atoms, etc.).

  Note that the nitrogen atom which is a constituent atom of the ring may have an organic group such as an alkyl group as a substituent. Examples of the alkyl group include alkyl groups having 1 to 10 carbon atoms such as a methyl group, an ethyl group, a propyl group, and an isopropyl group. 1-8 are preferable, as for carbon number of an alkyl group, 1-4 are more preferable, and it is especially preferable that it is 1, 2, or 3.

  Those having pyrrolidine or imidazole as the nitrogen-containing heterocyclic skeleton are preferred. The organic cation having a pyrrolidine skeleton preferably has two of the above alkyl groups on one nitrogen atom constituting the pyrrolidine ring. The organic cation having an imidazole skeleton preferably has one alkyl group on each of two nitrogen atoms constituting the imidazole ring.

Specific examples of the organic cation having a pyrrolidine skeleton include 1,1-dimethylpyrrolidinium cation, 1,1-diethylpyrrolidinium cation, 1-ethyl-1-methylpyrrolidinium cation, 1-methyl-1- Propyl pyrrolidinium cation (C ₃ C ₁ pyrr: 1-methyl-1-propylpyrrolidinium cation), 1-butyl-1-methylpyrrolidinium cation (C ₄ C ₁ pyrr: 1-butyl-1-ylpyrrolidinium cation), Examples include 1-ethyl-1-propylpyrrolidinium cation. Among these, pyrrolidinium cations having a methyl group and an alkyl group having 2 to 4 carbon atoms such as C ₃ C ₁ pyrr and C ₄ C ₁ pyrr are particularly preferable because of their high electrochemical stability. preferable.

Specific examples of the organic cation having an imidazole skeleton include 1,3-dimethylimidazolium cation, 1-ethyl-3-methylimidazolium cation (EMI ⁺ : 1-ethyl-3-methylimidazolium cation), and 1-methyl-3. -Propylimidazolium cation, 1-butyl-3-methylimidazolium cation (BMI ⁺ : 1-butyl-3-methylimidazolium cation), 1-ethyl-3-propylimidazolium cation, 1-butyl-3-ethylimidazolium And cations. Of these, an imidazolium cation having a methyl group such as EMI ⁺ or BMI ⁺ and an alkyl group having 2 to 4 carbon atoms is preferable.

  These organic cations can be used singly or in combination of two or more. Among these organic cations, an organic cation having a pyrrolidinium skeleton and / or an organic cation having an imidazole skeleton is preferable from the viewpoint of easily adjusting the melting point of the molten salt electrolyte to a low level. In particular, the organic cation has at least a pyrrolidinium skeleton. The case containing an organic cation is preferred.

  The organic cation may include an organic cation other than the organic cation having a nitrogen-containing heterocyclic skeleton. Examples of other organic cations include quaternary ammonium cations, sulfur-containing organic cations, and phosphorus-containing organic cations. Another organic cation may be used individually by 1 type, and may be used in combination of 2 or more type.

  The cation may contain other inorganic cations other than potassium ions. Examples of other inorganic cations include alkali metal cations (lithium ions, sodium ions, rubidium ions, cesium ions, etc.) other than potassium ions, ammonium cations, and the like. You may use another inorganic cation individually by 1 type or in combination of 2 or more types.

  The ratio of the organic cation having a nitrogen-containing heterocyclic skeleton in the entire organic cation is preferably 90% by mass or more, and the organic cation may be composed only of an organic cation having a nitrogen-containing heterocyclic skeleton.

  The total proportion of the potassium cation and the organic cation in the entire cation is preferably 80% by mass or more, may be 90% by mass or more, and only the potassium cation and the organic cation may be used as the cation.

  In the present embodiment, in the molten salt electrolyte, it is important that the molar fraction of potassium ions in the total of potassium ions and the organic cation is greater than 0.2 and 0.7 or less. By adjusting the molar fraction to such a range, the melting point of the molten salt electrolyte is lowered, and the molten salt electrolyte can be maintained in a liquid state even at a temperature below room temperature. In addition, since the molten salt electrolyte has Tg, the electrolytic solution can exist in a liquid state even at a low temperature by supercooling. Therefore, the battery can be operated even at a temperature lower than room temperature. Further, the concentration of potassium ions in the electrolyte is unlikely to be low, and the local precipitation of potassium during charging / discharging of the potassium molten salt battery is suppressed, so that a short circuit hardly occurs. In addition, Tg of molten salt electrolyte is -100 degreeC or more and -70 degreeC or less, for example.

  When the molar fraction of potassium ions is 0.2 or less, the concentration of potassium ions in the electrolyte tends to be low, and potassium is likely to locally precipitate during charge / discharge of a potassium molten salt battery. Is likely to occur. Further, when the molar fraction of potassium ions is 0.2 or less, the molten salt electrolyte does not have Tg, so that it becomes difficult to operate the battery at a low temperature.

  The molar fraction of potassium ions is preferably more than 0.2 and 0.4 or less. When the molar fraction is in such a range, even if salt is precipitated in the molten salt electrolyte at room temperature, most of the molten salt electrolyte can exist in a liquid state, ensuring high ionic conductivity. it can. The molar fraction of potassium ions is more preferably greater than 0.2 and less than or equal to 0.35, and more preferably greater than 0.2 and less than or equal to 0.3. When the molar fraction is within such a range, the melting point of the molten salt electrolyte can be kept low, and the precipitation of salt itself in the molten salt electrolyte can be suppressed. In the above range, the lower limit of the molar fraction of potassium ions is preferably 0.21 or more and may be 0.25 or more.

The anion of the molten salt electrolyte preferably contains a bissulfonylamide anion. When the molten salt electrolyte contains a bissulfonylamide anion, the viscosity of the molten salt electrolyte can be reduced, and high ion conductivity can be easily ensured. Examples of the bissulfonylamide anion include, for example, bis (fluorosulfonyl) amide anion [FSA ⁻ : bis (fluorosulfonyl) amide anion]], (fluorosulfonyl) (perfluoroalkylsulfonyl) amide anion [(fluorosulfonyl) (trifluoromethyl). Sulfonyl) amide anion ((FSO ₂ ) (CF ₃ SO ₂ ) N ⁻ ), (fluorosulfonyl) (pentafluoroethylsulfonyl) amide anion ((FSO ₂ ) (SO ₂ C ₂ F ₅ ) N ⁻ ) and the like], bis (perfluoroalkyl sulfonyl) amide anion [bis (trifluoromethylsulfonyl) amide anion ^{(TFSA -: bis (trifluoromethylsulfonyl)} amide anion), bis (pen Fluoro ethylsulfonyl) amide anion _{(N (SO 2 C 2 F} 5) 2 -) ], and others. From the viewpoint of easily reducing the viscosity and / or melting point of the electrolyte, in the bissulfonylamide anion having a perfluoroalkyl group, the carbon number of the perfluoroalkyl group is, for example, 1 to 10, preferably 1 to 8, and Preferably it is 1-4, especially 1, 2 or 3.
These anions can be used singly or in combination of two or more.

Among the bissulfonylamide anions mentioned above, bis (perfluoroalkylsulfonyl) amide anions such as FSA ⁻ ; TFSA ⁻ , bis (pentafluoroethylsulfonyl) amide anion, (fluorosulfonyl) (trifluoromethylsulfonyl) amide anion, etc. preferable. From the viewpoint of easily lowering the melting point, it is also preferable to use a bissulfonylamide anion having an asymmetric structure such as a (fluorosulfonyl) (trifluoromethylsulfonyl) amide anion and a (fluorosulfonyl) (pentafluoroethylsulfonyl) amide anion.

  The electrolyte used in the potassium molten salt battery can contain known additives as necessary, but most of the electrolyte is the molten salt (ionic liquid containing the cation and the anion). preferable. Content of the molten salt in electrolyte is 80 mass% or more (for example, 80-100 mass%), for example, Preferably it is 90 mass% or more (for example, 90-100 mass%). When content of molten salt is such a range, it is easy to improve the flame retardance of electrolyte.

(Potassium molten salt battery)
A potassium molten salt battery according to an embodiment of the present invention includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and the molten salt electrolyte having potassium ion conductivity. Hereinafter, each component other than the molten salt electrolyte will be described more specifically with reference to the drawings as appropriate.

(Negative electrode)
The negative electrode includes a negative electrode active material. As the negative electrode active material, a material that reversibly occludes (or inserts or adsorbs) and releases (or desorbs) potassium ions is used. As such a material, for example, a carbonaceous material having a graphite-type crystal structure and hard carbon are preferable. The graphite-type crystal structure means a layered crystal structure, and examples thereof include a cubic crystal structure and a rhombohedral crystal structure. Examples of the carbonaceous material having a graphite type crystal structure include natural graphite (such as flake graphite), artificial graphite, and graphitized mesocarbon microspheres. A negative electrode active material can be used individually by 1 type or in combination of 2 or more types.

When a negative electrode active material containing a carbonaceous material is used, in a potassium molten salt battery, potassium ions are inserted between the graphite-type crystal structure layers of the carbonaceous material during charging, and during discharge, potassium ions are inserted between the graphite-type crystal structure layers. Ions are released.
As one of the indicators of the degree of development of the graphite-type crystal structure in the carbonaceous material, an average interplanar spacing d ₀₀₂ of the (002) plane measured by an X-ray diffraction (XRD) spectrum of the carbonaceous material is It is used. From easily viewpoint potassium ion is inserted and released, the carbonaceous material used in the negative electrode active material, the average spacing d ₀₀₂ is preferably less than 0.337 nm. The lower limit of the average spacing d ₀₀₂ is not particularly limited, the average spacing d _002, for example, may be equal to or larger than 0.335 nm.

  Hard carbon, unlike graphite having a graphite-type crystal structure in which carbon network surfaces overlap in a layered manner, has a turbostratic structure in which the carbon network surfaces overlap in a three-dimensionally shifted state. Hard carbon does not change from a turbulent structure to a graphite structure even by heat treatment at a high temperature (for example, 3000 ° C.), and the development of graphite crystallites is not observed. Therefore, hard carbon is also referred to as non-graphitizable carbon.

Mean spacing d ₀₀₂ of the hard carbon having a turbostratic structure, for example, not less than 0.37 nm. The upper limit of the average spacing d ₀₀₂ of the hard carbon is not particularly limited, the average spacing d _002, for instance, can be less than or equal to 0.42 nm.

Note that hard carbon has a lower average specific gravity than graphite. The average specific gravity of graphite is about 2.1 to 2.25 g / cm ³ , while the average specific gravity of hard carbon is, for example, 1.4 to 1.7 g / cm ³ .

  In lithium ion batteries, graphite is used for the negative electrode, but lithium ions are inserted between layers of a graphite-type crystal structure (specifically, a layered structure of a carbon network surface) contained in graphite. When potassium ions are occluded in the hard carbon, the potassium ions are inserted between the layers of the graphite-type crystal structure that is slightly contained in the hard carbon, and in the turbulent layer structure (specifically, the layers of the graphite-type crystal structure It is considered that the hard carbon is occluded by entering into and / or adsorbing to the hard carbon.

  The negative electrode may include, for example, a negative electrode active material and a negative electrode current collector that fixes or supports the negative electrode active material. The negative electrode may contain a binder and / or a conductive aid as optional components.

As the current collector, a metal foil, a metal fiber nonwoven fabric, a metal porous sheet, or the like is used. As the metal constituting the negative electrode current collector, copper, copper alloy, nickel, nickel alloy, aluminum, or aluminum alloy is preferable because it is not alloyed with potassium and is stable at the negative electrode potential, but is not particularly limited. .
From the viewpoint of easily ensuring the capacity density and the utilization rate of the active material, the thickness of the metal foil serving as the current collector is, for example, 10 to 50 μm, and the thickness of the metal fiber nonwoven fabric and the metal porous sheet is respectively For example, it is 100-800 micrometers.

  The binder serves to bind the negative electrode active materials to each other and to fix the negative electrode active material to the negative electrode current collector. Examples of the binder include fluorine resins such as polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymer, and polyvinylidene fluoride; polyamide resins such as aromatic polyamide; polyimides (such as aromatic polyimide) and polyamideimides Examples thereof include polyimide resins such as styrene butadiene rubber, rubber-like polymers such as butadiene rubber, and cellulose derivatives (cellulose ether and the like) such as carboxymethyl cellulose and salts thereof (Na salt and the like). A binder can be used individually by 1 type or in combination of 2 or more types. From the viewpoint of easily ensuring high binding properties and capacity, the amount of the binder is preferably 1 to 10 parts by mass per 100 parts by mass of the negative electrode active material.

  As a conductive support agent, carbonaceous conductive support agents, such as carbon black and carbon fiber; Metal fiber etc. are mentioned, for example. A conductive support agent can be used individually by 1 type or in combination of 2 or more types. From the viewpoint of easily ensuring high conductivity and capacity, the amount of the conductive auxiliary agent can be appropriately selected from the range of, for example, 0 to 15 parts by mass per 100 parts by mass of the negative electrode active material.

  The negative electrode can be formed, for example, by applying a negative electrode mixture paste containing a negative electrode active material to the surface of the negative electrode current collector, drying, and rolling if necessary. The negative electrode mixture paste is obtained by dispersing a negative electrode active material, a binder as an optional component, and a conductive additive in a dispersion medium. Examples of the dispersion medium include ketones such as acetone; ethers such as tetrahydrofuran; nitriles such as acetonitrile; amides such as dimethylacetamide; N-methyl-2-pyrrolidone and the like. These dispersion media may be used individually by 1 type, and may be used in combination of 2 or more type.

(Positive electrode)
The positive electrode includes a positive electrode active material. The positive electrode active material includes a material that reversibly absorbs and releases potassium ions. Examples of such a material include a metal compound. Examples of the metal compound include K _α M _β A _γ (M: metal ion, A: monoatomic anion or polyatomic anion), metal complex, and the like. α, β, and γ are preferably 0 ≦ α ≦ 3, 0 <β ≦ 2, and 0 <γ ≦ 3, respectively.

The metal constituting the metal ion M is not particularly limited as long as it can have a trivalent valence, and examples thereof include transition metals (V, Cr, Mn, Fe, Co, Ni, Nb, Mo, and the like). Examples of the anion represented by A include a monoatomic anion (O ²⁻ , F ⁻ , S ^2−, etc.) and a polyatomic anion (PO ₄ ³⁻ , P ₂ O ₇ ⁴⁻ , PO ₄ F ⁴⁻ , SO _4). ^2- , SO ₄ F ^3-, etc.). Specific examples of K _α M _β A _γ include K _0.3 MnO ₂ , FePO ₄ , FeSO ₄ F, K ₃ V ₂ (PO ₄ ) ₃ , KVPO ₄ F, and the like.

  The metal complex can form a double complex containing potassium ions by occluding potassium ions. As such a metal complex, one that can occlude and release potassium ions at a higher potential than the negative electrode active material is used. The metal complex is insoluble or hardly soluble in the molten salt electrolyte. The metal complex includes a metal ion (cation) and a ligand coordinated to the metal ion (anionic ligand).

As a metal complex, following formula (1):
K _x M ¹ [M ² (CN) _6-y L _y ] _z (1)
(Wherein, M ¹ is a first metal ion, M ² is a second metal ion, L is CN ^- is a monovalent anion other than, x, y and z are each, 0 <x ≦ 2, 0 ≦ y <6 and 0.5 ≦ z ≦ 1.5 are satisfied).

  The ratio x of potassium ions is preferably 0 <x ≦ 1.5, and may be 0.1 ≦ x ≦ 1.1 or 0.1 ≦ x ≦ 0.7. In the case of a fully charged state, the potassium ion ratio x in the metal complex is the minimum (theoretically 0).

The first metal constituting the first metal ion is not particularly limited as long as it can have a trivalent valence. Transition metal (such as Cr, Mn, Fe, Co, Ni, and / or Cu), periodic table Typical metals of Group 12 to Group 15 (Zn, and / or Sn, etc.) are included. The metal complex may include one type of first metal ion or may include a plurality of types of first metal ions. Of these, ions of metals (such as Cr, Mn, Fe, Co, Ni, Cu, and / or Zn) in the fourth period of the periodic table are preferred, and in particular, Cr ³⁺ , Mn ³⁺ , Fe ³⁺ , and At least one selected from the group consisting of Co ³⁺ is preferred. The first metal ion preferably contains at least Fe ³⁺ . The ratio of Fe ³⁺ to the first metal ions is, for example, 80 to 100 mol%, preferably 90 to 100 mol%, and the first metal ions may contain only Fe ³⁺ .

The second metal constituting the second metal ion is not particularly limited as long as it can have a divalent valence, and is a transition metal (such as Cr, Mo, Mn, Fe, Ru, Co, Ni, and / or Cu). And typical metals of Group 12 to Group 15 of the periodic table (such as Zn and / or Sn). The metal complex may include one type of second metal ion or may include a plurality of types of second metal ions. Among these, ions such as metals (Cr, Mn, Fe, Co, Ni, Cu, and / or Zn, etc.) of the fourth period of the periodic table, metals (Mo, Ru, and / or Sn, etc.) of the fifth period, etc. In particular, at least one selected from the group consisting of Cr ²⁺ , Mn ²⁺ , Fe ²⁺ , Co ²⁺ , Ni ²⁺ , Cu ²⁺ , Zn ²⁺ and Sn ²⁺ is preferable. The second metal ion preferably contains at least Fe ²⁺ . The ratio of Fe ²⁺ to the second metal ions is, for example, 80 to 100 mol%, preferably 90 to 100 mol%, and the second metal ions may contain only Fe ²⁺ .

In the formula (1), the monovalent anion represented by L may be a monoatomic anion (such as a halogen anion such as Cl ⁻ , Br ⁻ and I ⁻ ), but a monovalent anion other than CN ^−. Polyatomic anions are preferred. Examples of the monovalent polyatomic anion include SCN ⁻ , OH ⁻ , SH ⁻ , oxo acid anion, carboxylate anion, and the like. Examples of the oxo acid anion include NO ₃ ⁻ , ClO ₄ ⁻ , HCO ₃ ⁻ , H ₂ PO ₄ ⁻ , H ₂ SO ₄ ^{− and the} like. Examples of the carboxylate anion include CH ₃ COO ⁻ and CH ₃ COCH ₂ COO ⁻ .

  Y representing the coordination number of the anion L is preferably 0 ≦ y ≦ 4, more preferably 0 ≦ y ≦ 2 or 0 ≦ y ≦ 1.

  In the formula (1), z is preferably 0.7 ≦ z ≦ 1.3, more preferably 0.85 ≦ z ≦ 1.15.

  Since such a metal complex forms a Prussian blue type crystal structure, it is also referred to as a Prussian blue type complex (or Prussian blue analog). Since this crystal structure has an open skeleton with a lattice structure like jungle gym, a metal complex can form a double complex with potassium ions by inserting potassium ions into the space in the open skeleton. In a metal complex, formation of a double complex can be performed reversibly. That is, it is considered that the metal complex takes a potassium ion into the open skeleton during discharge to form a double complex, and releases the potassium ion from within the open skeleton during charging. With such a mechanism, the potassium molten salt battery can be charged and discharged more stably.

The valences of the first metal ion and the second metal ion contained in the metal complex may change depending on the occlusion and release of potassium ions, respectively. For example, at least a part of the first metal ion may be a divalent metal ion. In addition, at least a part of the second metal ion may be a trivalent metal ion.
The metal complex may include coordinating water and / or bound water.

The positive electrode can include, for example, a positive electrode active material and a positive electrode current collector that fixes or supports the positive electrode active material.
As the current collector, a metal foil, a metal fiber nonwoven fabric, a metal porous sheet, or the like is used. The metal constituting the positive electrode current collector is preferably aluminum or an aluminum alloy because it is stable at the positive electrode potential, but is not particularly limited. The thickness of the current collector can be appropriately selected from the range described for the negative electrode current collector.

The positive electrode may contain a binder and / or a conductive aid as optional components. As a binder and a conductive support agent, it can select from the thing similar to what was illustrated about the negative electrode suitably.
A positive electrode can be formed according to the formation method of a negative electrode. Specifically, the positive electrode can be formed by applying a positive electrode mixture paste containing a positive electrode active material to the surface of the positive electrode current collector, drying, and rolling if necessary. The positive electrode mixture paste is obtained by dispersing a positive electrode active material, a binder as an optional component, and a conductive additive in a dispersion medium. As a dispersion medium, it can select from the thing similar to what was illustrated about the negative electrode suitably.

  From the viewpoint of increasing the capacity, it is preferable that at least one of the positive electrode active material and the negative electrode active material be pre-doped with potassium ions. When potassium ions are pre-doped into the negative electrode active material, pre-doping can be performed in the battery assembly process. For example, when potassium metal foil is attached to the surface of the negative electrode, and the battery is assembled together with the positive electrode and the electrolyte, potassium ions are eluted from the potassium metal foil and doped into the negative electrode active material. it can. In doping, electricity may be supplied as necessary.

  When the positive electrode active material is pre-doped with potassium ions, a potassium metal foil may be attached to the surface of the positive electrode and may be doped according to the case of pre-doping the negative electrode active material. It is preferable to prepare a complex in advance and use it as a positive electrode active material. A metal complex (double complex) in which potassium ions are occluded is, for example, a compound in which the coefficient x of potassium ions satisfies 0 <x ≦ 2 (preferably 0.5 ≦ x ≦ 2) in formula (1) It is.

(Separator)
As the separator, for example, a nonwoven fabric other than a resin microporous film can be used. The separator may be formed of only a microporous membrane layer or a non-woven fabric layer, or a laminate of a plurality of layers having different compositions and / or forms. Examples of the laminate include a laminate having a plurality of resin porous layers having different compositions and a laminate having a microporous membrane layer and a nonwoven fabric layer.

  Examples of the resin contained in the fibers forming the microporous membrane and the nonwoven fabric include polyolefin resin, polyphenylene sulfide resin, polyamide resin, and polyimide resin. One of these resins may be used alone, or two or more thereof may be used in combination. The fibers forming the nonwoven fabric may be inorganic fibers such as glass fibers. The separator is preferably formed of at least one selected from the group consisting of glass fiber, polyolefin resin, polyamide resin, and polyphenylene sulfide resin.

The separator may include an inorganic filler. Examples of the inorganic filler include ceramics (for example, silica, alumina, zeolite, titania, etc.), talc, mica, and / or wollastonite.
Although the thickness of a separator is not specifically limited, For example, it can select from the range of about 10-300 micrometers.

(Electrode group)
A molten salt battery is used in a state in which a positive electrode, a negative electrode, a separator interposed therebetween, and an electrolyte are accommodated in a battery case. An electrode group may be formed by interposing a positive electrode and a negative electrode with a separator between them, or by further laminating or winding them, and this electrode group may be housed in a battery case. At this time, while using a metal battery case, by making one of the positive electrode and the negative electrode conductive with the battery case, a part of the battery case can be used as the first external terminal. On the other hand, the other of the positive electrode and the negative electrode is connected to a second external terminal led out of the battery case in a state insulated from the battery case, using a lead piece or the like.

FIG. 1 is a longitudinal sectional view schematically showing a potassium molten salt battery.
The potassium molten salt battery includes a stacked electrode group, an electrolyte (not shown), and a rectangular aluminum battery case 10 that houses them. The battery case 10 includes a bottomed container body 12 having an open top and a lid 13 that closes the upper opening of the container body 12.

  When assembling a potassium molten salt battery, first, an electrode group is configured by laminating the positive electrode 2 and the negative electrode 3 with the separator 1 interposed therebetween, and the configured electrode group is a battery case 10. The container body 12 is inserted. Thereafter, a step of injecting molten salt into the container body 12 and impregnating the electrolyte in the gaps of the separator 1, the positive electrode 2 and the negative electrode 3 constituting the electrode group is performed. Alternatively, the electrode group may be impregnated with the molten salt, and then the electrode group containing the molten salt may be accommodated in the container body 12.

  In the center of the lid 13, a safety valve 16 is provided for releasing gas generated inside when the internal pressure of the battery case 10 rises. An external positive terminal 14 that penetrates the lid 13 while being insulated from the battery case 10 is provided near the one side of the lid 13 with the safety valve 16 in the center, at a position near the other side of the lid 13. Is provided with an external negative electrode terminal that penetrates the lid 13 in a state of being electrically connected to the battery case 10.

  The stacked electrode group is composed of a plurality of positive electrodes 2, a plurality of negative electrodes 3, and a plurality of separators 1 interposed between them, each having a rectangular sheet shape. In FIG. 1, the separator 1 is formed in a bag shape so as to surround the positive electrode 2, but the form of the separator is not particularly limited. The plurality of positive electrodes 2 and the plurality of negative electrodes 3 are alternately arranged in the stacking direction within the electrode group.

  A positive electrode lead piece 2 a may be formed at one end of each positive electrode 2. The plurality of positive electrodes 2 are connected in parallel by bundling the positive electrode lead pieces 2 a of the plurality of positive electrodes 2 and connecting them to the external positive terminal 14 provided on the lid 13 of the battery case 10. Similarly, a negative electrode lead piece 3 a may be formed at one end of each negative electrode 3. The plurality of negative electrodes 3 are connected in parallel by bundling the negative electrode lead pieces 3 a of the plurality of negative electrodes 3 and connecting them to the external negative terminal provided on the lid 13 of the battery case 10. The bundle of the positive electrode lead pieces 2a and the bundle of the negative electrode lead pieces 3a are desirably arranged on the left and right sides of one end face of the electrode group with an interval so as to avoid mutual contact.

  Both the external positive terminal 14 and the external negative terminal are columnar, and at least a portion exposed to the outside has a screw groove. A nut 7 is fitted in the screw groove of each terminal, and the nut 7 is fixed to the lid 13 by rotating the nut 7. A flange 8 is provided in a portion of each terminal housed in the battery case, and the flange 8 is fixed to the inner surface of the lid 13 via a washer 9 by the rotation of the nut 7.

  EXAMPLES Hereinafter, although this invention is demonstrated concretely based on an Example and a comparative example, this invention is not limited to a following example.

Example 1
(1) Preparation of electrolyte A liquid electrolyte was prepared at room temperature by mixing K · FSA and C ₃ C ₁ pyrr · FSA at a molar ratio of 3: 7.

(2) Production of negative electrode 100 parts by mass of hard carbon (manufactured by Kureha) and 5 parts by mass of polyvinylidene fluoride (binder) were dispersed in N-methyl-2-pyrrolidone to prepare a negative electrode mixture paste. The obtained negative electrode mixture paste was applied to both surfaces of an aluminum foil (10 cm long × 10 cm wide, 20 μm thick) as a negative electrode current collector, sufficiently dried, rolled, and negative electrode having a thickness of 15 μm on both surfaces. Nine negative electrodes having a mixture layer and a total thickness of 50 μm were prepared. Further, two negative electrodes were produced in the same manner as described above except that the negative electrode mixture layer was formed only on one surface of the negative electrode current collector. In addition, the lead piece for current collection was formed in the one side edge part of the one side of a negative electrode.

(3) Production of positive electrode 100 parts by mass of Prussian blue type metal complex (KFe [Fe (CN) ₆ ]) as positive electrode active material, 5 parts by mass of acetylene black (conducting aid) and polyvinylidene fluoride (binder) 5 A positive electrode mixture paste was prepared by dispersing parts by mass in N-methyl-2-pyrrolidone. The obtained positive electrode mixture paste was applied to both surfaces of an aluminum foil (vertical 10 cm × width 10 cm, thickness 20 μm) as a positive electrode current collector, sufficiently dried, rolled, and positive electrode having a thickness of 15 μm on both surfaces. Ten positive electrodes having a mixture layer and a total thickness of 50 μm were produced. A lead piece for current collection was formed at one end of one side of the positive electrode.

(4) Assembly of electrode group A separator is interposed between the positive electrode and the negative electrode, the positive electrode lead pieces and the negative electrode lead pieces overlap each other, and the positive electrode lead piece bundle and the negative electrode lead piece bundle are symmetrical. The electrode group was manufactured by stacking so as to be arranged at various positions. At both ends of the electrode group, a negative electrode having a negative electrode mixture layer only on one side was disposed so that the negative electrode mixture layer was opposed to the positive electrode. As the separator, a glass fiber nonwoven fabric (size 35 × 35 mm, thickness 200 μm) was used.

(5) Assembly of molten salt battery The electrode group obtained in (4) above and the electrolyte obtained in (1) above are accommodated in an aluminum container body, and the opening of the container body is made of aluminum. Was sealed with a lid body (sealing plate), and the potassium molten salt battery shown in FIG. 1 was completed.

  The potassium molten salt battery was charged at a constant current until it reached 3.8 V at a current value of a rate of 0.2C at 25 ° C., and then charged at a constant voltage of 3.8 V. And it discharged until it became 2.5V with the electric current value of the time rate 0.2C rate. Furthermore, said charging / discharging cycle was repeated 20 times. Thus, the obtained potassium molten salt battery was able to charge / discharge reversibly.

Example 2
Molten salt electrolytes having different molar ratios of K · FSA and C ₃ C ₁ pyrr · FSA were prepared. The melting point and Tg of each molten salt electrolyte were measured using a differential scanning calorimeter. Note that the melting point was measured melting start temperature T _{M_o} and melting completion temperature T _{m_e} the molten salt electrolyte.

  The phase diagram of Example 2 is shown in FIG. The horizontal axis in FIG. 2 is the molar fraction of potassium ions. From FIG. 2, when the ratio of K · FSA in the molten salt electrolyte (that is, the molar fraction of potassium ions in the cation contained in the molten salt electrolyte) is in the range of 0.2 to 0.7, the electrolyte is Tg (about 175-200K). Therefore, the electrolyte is maintained in a liquid state even at a temperature lower than room temperature due to supercooling. However, from the viewpoint of suppressing the occurrence of a short circuit due to the local precipitation of potassium during charging and discharging, the molar fraction of potassium ions needs to be greater than 0.2. As shown in FIG. 2, when the molar fraction of potassium ions is about 0.28 or less, the melting point of the electrolyte is 270 K or less, and the electrolyte is liquid at room temperature.

  As shown in FIG. 2, when the molar fraction of potassium ions is larger than 0.3, the melting point of the electrolyte is remarkably increased as compared with the case where the molar fraction is 0.3 or less, and exceeds 350K. Therefore, near the room temperature, the salt is deposited in the electrolyte. However, if the molar fraction of potassium ions is 0.7 or less, the battery can be operated because some liquid electrolyte exists. In view of the fact that there are many electrolytes in a liquid state at room temperature and it is easy to ensure ionic conductivity, the molar fraction of potassium ions is preferably 0.4 or less. In view of the fact that most of the electrolyte exists in liquid form at room temperature, the molar fraction of potassium ions is preferably 0.3 or less.

Comparative Example 1
K · FSA and C ₃ C ₁ pyrr · FSA were used at a molar ratio of 2: 8 to mix them together to prepare a molten salt electrolyte in a liquid state at room temperature. A potassium molten salt battery was produced and charged / discharged in the same manner as in Example 1 except that the obtained molten salt electrolyte was used, but the voltage did not increase during charging. This is probably because an internal short circuit occurred.

Example 3
K · FSA and EMI · FSA were used at a molar ratio of 3: 7, and both were mixed to prepare a molten salt electrolyte in a liquid state at room temperature. The melting point and Tg of the obtained molten salt electrolyte were measured using a differential scanning calorimeter. As a result, it was maintained in a liquid state even at low temperatures due to supercooling. Moreover, Tg was about -90 degreeC.

  According to the molten salt electrolyte which concerns on one Embodiment of this invention, a potassium molten salt battery can be operated at the temperature below room temperature. Therefore, a potassium molten salt battery can be utilized for various uses, such as a large-sized electric power storage apparatus for household use or industry, and the power supply of an electric vehicle or a hybrid vehicle.

1: Separator 2: Positive electrode 2a: Positive electrode lead piece 3: Negative electrode 3a: Negative electrode lead piece 7: Nut 8: Hook 9: Washer 10: Battery case 12: Container body 13: Cover body 14: External positive electrode terminal 16: Safety valve

Claims (8)

A molten salt electrolyte for a potassium molten salt battery having potassium ion conductivity,
The molten salt electrolyte includes a salt composed of an anion and a cation,
The cations include potassium ions and organic cations,
The organic cation includes an organic cation having a nitrogen-containing heterocyclic skeleton,
The molten salt electrolyte, wherein the molar fraction of the potassium ions in the total of the potassium ions and the organic cations is greater than 0.2 and less than or equal to 0.7.   The molten salt electrolyte according to claim 1, wherein a molar fraction of the potassium ions is greater than 0.2 and equal to or less than 0.4.   The molten salt electrolyte according to claim 1, wherein the organic cation includes at least one selected from the group consisting of an organic cation having a pyrrolidine skeleton and an organic cation having an imidazole skeleton.   The molten salt electrolyte according to any one of claims 1 to 3, wherein the organic cation includes an organic cation having at least a pyrrolidine skeleton.   The molten salt electrolyte according to claim 1, wherein the anion includes a bissulfonylamide anion.   The molten salt electrolyte according to any one of claims 1 to 5, comprising 80% by mass or more of the salt. A positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator interposed between the positive electrode and the negative electrode, and the molten salt electrolyte according to claim 1,
The positive electrode active material includes a material that reversibly absorbs and releases potassium ions,
The said negative electrode active material is a potassium molten salt battery containing the material which occludes and discharge | releases potassium ion reversibly.   The potassium molten salt battery according to claim 7, wherein the negative electrode active material includes at least one selected from the group consisting of hard carbon and a carbonaceous material having a graphite-type crystal structure.

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