ELECTROLYTE SOLUTION

Abstract

There are provided an aqueous electrolyte solution having an extended potential window, in particular, an aqueous electrolyte solution whose potential window is further wider than those exhibited by conventional concentrated aqueous electrolyte solutions, and an aqueous electrolyte solution in which the cycle characteristics can be improved. A non-aqueous electrolyte solution capable of achieving a higher energy density is provided, the non-aqueous electrolyte solution containing easily available and inexpensive materials and having further improved characteristics. One aqueous electrolyte solution of the present embodiment contains a salt of at least one selected from the group consisting of sodium, magnesium, potassium and lithium, and a chaotropic additive. One other non-aqueous electrolyte solution of the present embodiment contains a salt of at least one selected from the group consisting of sodium, magnesium, potassium and lithium, and a chaotropic additive.

Claims

1. An electrolyte solution, comprising a salt of at least one selected from the group consisting of sodium, magnesium, potassium and lithium, and a chaotropic additive.

2. The electrolyte solution according to claim 1, wherein the salt is at least one selected from the group consisting of sulfates, nitrates, acetates, chlorates, perchlorates, hypochlorites, hydroxide salts, chloride salts, fluoride salts and imide salts containing at least one selected from the group consisting of sodium, magnesium, potassium and lithium.

3. The electrolyte solution according to claim 1, wherein a mass concentration of the salt in the electrolyte solution is 1 mol/kg or more and 30 mol/kg or less.

4. The electrolyte solution according to claim 1, wherein the chaotropic additive is at least one selected from the group consisting of amines, alcohols and acids that exhibit chaotropic properties.

5. The electrolyte solution according to claim 1, wherein the chaotropic additive is at least one selected from the group consisting of amines and alcohols that exhibit chaotropic properties.

6. The electrolyte solution according to claim 1, wherein the chaotropic additive is at least one selected from the group consisting of urea, thiourea, acetamide, trifluoroacetamide, 1,1-dimethylurea, guanidium and guanidium salts.

7. The electrolyte solution according to claim 1, wherein the electrolyte solution is an aqueous electrolyte solution.

8. The electrolyte solution according to claim 1, wherein the electrolyte solution is a non-aqueous electrolyte solution.

9. A method for producing the electrolyte solution according to claim 1, the method comprising a step of mixing the salt of at least one selected from the group consisting of sodium, magnesium, potassium and lithium, and the chaotropic additive.

10. An electrochemical device, comprising the electrolyte solution according to claim 1.

11. A method for producing an electrochemical device, comprising using the electrolyte solution according to claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0032] FIG. 1 is a graph illustrating the charge-discharge cycle characteristics of sodium secondary batteries in Example 1-3 and Comparative example 1-5.

[0033] FIG. 2 is a graph illustrating the cyclic voltammograms of non-aqueous magnesium electrolyte solutions of Example 2-1 and Comparative example 2-1.

DESCRIPTION OF EMBODIMENTS

[0034] An electrolyte solution, a method for producing an electrolyte solution, an electrochemical device including an electrolyte solution and a method for producing an electrochemical device will be described in detail below. However, the description of components described below is an example (representative example) as one embodiment of the present disclosure, and is not limited to the contents thereof.

[0035] In this specification, for example, the expression of a numerical range of “1 to 100” includes both the lower limit “1” and the upper limit “100”.

Electrolyte Solution

[0036] An electrolyte solution of the present embodiment is characterized by containing a salt of at least one selected from the group consisting of sodium, magnesium, potassium and lithium, preferably a salt of at least one selected from the group consisting of sodium, magnesium and lithium (hereinafter, also referred to as an “electrolyte salt”), and a chaotropic additive.

[0037] An aqueous electrolyte solution of the present embodiment contains the chaotropic additive and thus can have a wider potential window than chaotropic additive-free aqueous electrolyte solutions. In addition, a non-aqueous electrolyte solution of the present embodiment contains the chaotropic additive and thus can have improved characteristics as compared to chaotropic additive-free non-aqueous electrolyte solutions, and can achieve a higher energy density.

[0038] In this specification, the term “aqueous electrolyte solution” refers to an aqueous solution having electrical conductivity, preferably an aqueous solution used as an electrolyte solution for an electrochemical device, more preferably an aqueous solution used as an electrolyte solution for a charge-discharge device.

[0039] In the aqueous electrolyte solution of the present embodiment, the main solvent is water, and the solvent is preferably water only. In the aqueous electrolyte solution of the present embodiment, an example thereof is water having an electrical resistance of 1 mS or less at room temperature. Specific examples of water include distilled water, ion-exchanged water, pure water and ultrapure water.

[0040] In this specification, the term “non-aqueous electrolyte solution” refers to a non-aqueous solution having electrical conductivity, preferably a non-aqueous solution used as an electrolyte solution for an electrochemical device, more preferably a non-aqueous solution used as an electrolyte solution for a charge-discharge device.

[0041] In the non-aqueous electrolyte solution of the present embodiment, the main solvent is a non-aqueous solvent, and the solvent is preferably a non-aqueous solvent only.

Non-Aqueous Solvent

[0042] Examples of the non-aqueous solvent include organic solvents, such as esters, ethers, carbonates, nitriles, sulfolanes, furans and dioxolanes. Specific examples thereof include carbonates, such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, chloroethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl n-butyl carbonate, methyl tert-butyl carbonate, diisopropyl carbonate and tert-butyl isopropyl carbonate; esters, such as γ-butyrolactone, γ-valerolactone, methyl formate, methyl acetate, ethyl acetate and methyl butyrate; ethers, such as dimethoxyethane, ethoxymethoxyethane and diethoxyethane; nitriles, such as acetonitrile and benzonitrile; furans, such as tetrahydrofuran and methyltetrahydrofuran; sulfolanes, such as sulfolane and tetramethylsulfolane; and dioxolanes, such as 1,3-dioxolane and methyldioxolane.

[0043] These non-aqueous solvents may be used alone or in combination as a mixture of two or more.

Electrolyte Salt

[0044] The electrolyte salt used in the electrolyte solution of the present embodiment is not particularly limited as long as a salt that functions as an electrolyte is contained. Specific examples of the electrolyte salt include sulfates, nitrates, acetates, chlorates, perchlorates, hypochlorites, hydroxide salts, chloride salts, fluoride salts and imide salts containing cations, such as cations of sodium, magnesium, potassium and lithium, preferably cations, such as cations of sodium, magnesium and lithium (hereinafter, also referred to as “charge-discharge cations”). Among these, chlorates, perchlorates, hypochlorites, hydroxide salts and chloride salts, containing charge-discharge cations are preferred. Chlorates, perchlorates and hypochlorites, containing charge-discharge cations are more preferred.

[0045] As described above, examples of the charge-discharge cations include sodium, magnesium, potassium and lithium, and further include sodium, magnesium and lithium. Among these, one or more selected from the group consisting of sodium, magnesium and potassium are preferred. At least one of sodium and magnesium is preferred. Sodium is more preferred. In another embodiment, at least one of sodium and potassium is preferred, and potassium is more preferred.

[0046] Specific examples of the electrolyte salt containing sodium serving as the charge-discharge cation include sodium hexafluorophosphate, sodium tetrafluoroborate, sodium trifluoromethanesulfonate, sodium sulfate, sodium nitrate, sodium acetate, sodium chlorate, sodium perchlorate, sodium hypochlorite, sodium hydroxide, sodium chloride, sodium fluoride, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide and sodium bis(pentafluoroethanesulfonyl)imide. Among these, sodium chlorate, sodium perchlorate, sodium hypochlorite, sodium hydroxide and sodium chloride are preferred. Sodium chlorate, sodium perchlorate and sodium hypochlorite are more preferred. These electrolyte salts may be used alone or in combination as a mixture of two or more.

[0047] Specific examples of the electrolyte salt containing magnesium serving as the charge-discharge cation include magnesium trifluoromethanesulfonate, magnesium sulfate, magnesium nitrate, magnesium acetate, magnesium chlorate, magnesium perchlorate, magnesium hypochlorite, magnesium hydroxide, magnesium chloride, magnesium fluoride, magnesium bis(fluorosulfonyl)imide, magnesium bis(trifluoromethanesulfonyl)imide and magnesium bis(pentafluoroethanesulfonyl)imide. Among these, magnesium chlorate, magnesium perchlorate, magnesium hypochlorite, magnesium hydroxide and magnesium chloride are preferred. Magnesium chlorate, magnesium perchlorate and magnesium hypochlorite are more preferred. These electrolyte salts may be used alone or in combination as a mixture of two or more.

[0048] Specific examples of the electrolyte salt containing potassium serving as a charge-discharge cation include potassium hexafluorophosphate, potassium tetrafluoroborate, potassium trifluoromethanesulfonate, potassium sulfate, potassium nitrate, potassium acetate, potassium chlorate, potassium perchlorate, potassium hypochlorite, potassium hydroxide, potassium chloride, potassium fluoride, potassium bis(fluorosulfonyl)imide, potassium bis(trifluoromethanesulfonyl)imide and potassium bis(pentafluoroethanesulfonyl)imide. Among these, potassium chlorate, potassium perchlorate, potassium hypochlorite, potassium hydroxide and potassium chloride are preferred. Potassium chlorate, potassium perchlorate and potassium hypochlorite are more preferred. These electrolyte salts may be used alone or in combination as a mixture of two or more.

[0049] Specific examples of the electrolyte salt containing lithium serving as a charge-discharge cation include lithium hexafluorophosphate, lithium tetrafluoroborate, lithium trifluoromethanesulfonate, lithium sulfate, lithium nitrate, lithium acetate, lithium chlorate, lithium perchlorate, lithium hypochlorite, lithium hydroxide, lithium chloride, lithium fluoride, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide and lithium bis(pentafluoroethanesulfonyl)imide. Among these, lithium chlorate, lithium perchlorate, lithium hypochlorite, lithium hydroxide and lithium chloride are preferred. Lithium chlorate, lithium perchlorate and lithium hypochlorite are more preferred. These electrolyte salts may be used alone or in combination as a mixture of two or more.

[0050] Any concentration of the electrolyte salt in the electrolyte solution of the present embodiment may be used. To provide an electrolyte solution having a wider potential window, the concentration of the electrolyte salt is preferably a saturation concentration, more preferably a concentration more than the saturation concentration. In this specification, the expression “concentration more than the saturation concentration” refers to a concentration more than the maximum dissolution concentration of the electrolyte salt when the electrolyte salt is dissolved alone (that is, in the absence of a chaotropic additive) in a solvent (water for an aqueous electrolyte solution, or a non-aqueous solvent for a non-aqueous electrolyte solution, the same applies hereinafter).

[0051] The concentration of the electrolyte salt in the electrolyte solution of the present embodiment is preferably 1.2 or more times the saturation concentration, more preferably 1.4 or more times the saturation concentration, although the solubility varies in accordance with the type of the electrolyte salt. The concentration of the electrolyte salt in the electrolyte solution of the present embodiment is preferably, for example, 4 or less times the saturation concentration, more preferably 3.5 or less times the saturation concentration, although the solubility varies in accordance with the type of the electrolyte salt.

[0052] When the electrolyte salt is a perchlorate, the mass concentration of the electrolyte salt in the electrolyte solution is preferably 1 mol/kg or more, more preferably 5 mol/kg or more, even more preferably 10 mol/kg or more. When the electrolyte salt is a perchlorate, the mass concentration of the electrolyte salt in the electrolyte solution is preferably 30 mol/kg or less, more preferably 28 mol/kg or less, even more preferably 26 mol/kg or less.

Auxiliary Electrolyte Salt

[0053] In the present embodiment, the non-aqueous electrolyte solution may contain a quaternary onium salt as an auxiliary electrolyte salt. This makes it easier to achieve a higher energy density when the non-aqueous electrolyte solution of the present embodiment is used as a non-aqueous electrolyte solution for an electric double-layer capacitor.

[0054] Specific examples of the quaternary onium salt include (C.sub.2H.sub.5).sub.4NBF.sub.4, (C.sub.2H.sub.5).sub.4NPF.sub.6, (C.sub.2H.sub.5).sub.4NClO.sub.4, (C.sub.2H.sub.5).sub.4PBF.sub.4, (C.sub.2H.sub.5).sub.4PPF.sub.6, (C.sub.2H.sub.5).sub.4PClO.sub.4, (C.sub.3H.sub.7).sub.4PBF.sub.4, (C.sub.3H.sub.7).sub.4PPF.sub.6, (C.sub.3H.sub.7).sub.4PClO.sub.4, (C.sub.3H.sub.7).sub.4NBF.sub.4, (C.sub.3H.sub.7).sub.4NPF.sub.6, (C.sub.3H.sub.7).sub.4NClO.sub.4, (CH.sub.3).sub.4NBF.sub.4, (CH.sub.3).sub.4NPF.sub.6, (CH.sub.3).sub.4NClO.sub.4, (CH.sub.3).sub.4PBF.sub.4, (CH.sub.3).sub.4PPF.sub.6 and (CH.sub.3).sub.4PClO.sub.4. Among these, (C.sub.2H.sub.5).sub.4NBF.sub.4, (C.sub.2H.sub.5).sub.4NPF.sub.6, (C.sub.2H.sub.5).sub.4NClO.sub.4, (CH.sub.3).sub.4NBF.sub.4, (CH.sub.3).sub.4NPF.sub.6 and (CH.sub.3).sub.4NClO.sub.4 are preferred. These auxiliary electrolyte salts may be used alone or in combination as a mixture of two or more.

Chaotropic Additive

[0055] Chaotropic additives are substances that denature biopolymers, such as DNA and proteins, and are compounds that disrupt their steric structure and biological functions by intervening in the hydrogen bonding network. The chaotropic additive used in the electrolyte solution of the present embodiment is a compound containing at least one of cations and anions on the right side of the Hofmeister series, and furthermore, a compound containing at least one of cations and anions exhibiting chaotropic properties. In this specification, the term “chaotropic properties” refers to properties possessed by chaotropic additives, which can intervene in the hydrogen bonding network of, for example, biopolymers and water to change their structures.

[0056] Examples of the chaotropic additive include amines, alcohols, and acids that exhibit chaotropic properties. Among these, amines and alcohols that have hydrogen-bonding functional groups and that exhibit chaotropic properties are preferred.

[0057] Specific examples of the chaotropic additive include acetamide, N-methylacetamide, oxalic acid, malonic acid, malic acid, xylitol, urea, 1,1-dimethylurea, guanidium, guanidium salts, isosorbide, tartaric acid, tricarbaryl acid, thiourea, thiocyanic acid, trifluoroacetamide, benzoic acid, itaconic acid, citric acid, imidazole, 2-imidazolidinone, 4-hydroxybenzoic acid, cinnamic acid, ethylene glycol, propyleneurea, resorcinol, phenylacetic acid, D-sorbitol, lactic acid, 1,3-dimethylurea, levulinic acid, gallic acid, caffeic acid, 1-methylurea, glycerol, succinic acid, caproic acid, coumaric acid, stearic acid, adipic acid, oleic acid, suberic acid, linoleic acid and decanoic acid. Among these, acetamide, N-methylacetamide, xylitol, urea, 1,1-dimethylurea, guanidium, guanidium salts, isosorbide, thiourea, trifluoroacetamide, imidazole, 2-imidazolidinone, ethylene glycol, propyleneurea, resorcinol, D-sorbitol, 1,3-dimethylurea and glycerol are preferred. Urea, thiourea, acetamide, trifluoroacetamide, 1,1-dimethylurea, guanidium and guanidium salts are more preferred in view of handleability. From the viewpoint, of availability, urea and thiourea are particularly preferred, and urea is particularly preferred. These chaotropic additives may be used alone or in combination as a mixture of two or more.

[0058] The chaotropic additive is preferably one or more selected from the group consisting of ethylene glycol, diethylene glycol, triethylene glycol, poly(ethylene glycol), glycerol, erythritol and threitol, more preferably one or more selected from the group consisting of ethylene glycol and poly(ethylene glycol), and even more preferably ethylene glycol, because the cycle characteristics of an electrochemical device, such as a sodium battery, containing the chaotropic additive in the electrolyte solution tend to be improved.

Additive other than Chaotropic Additive

[0059] The electrolyte solution of the present embodiment can contain additives other than chaotropic additives as needed to adjust physical properties and characteristics, such as storage stability and battery characteristics. The amount of an optional additive other than the chaotropic additive contained in the electrolyte solution is, but not particularly limited to, for example, 0.01% by mass or more and 10% by mass or less based on the mass of the electrolyte solution.

Characteristics of Aqueous Electrolyte Solution

Potential Window

[0060] The potential window of the aqueous electrolyte solution of the present embodiment is not particularly limited as long as it is wider than the potential window (1.23 V) of water as a solvent, and is, for example, 2.0 V or more, preferably 2.5 V or more. The potential window can vary in accordance with the charge-discharge cations in the electrolyte salt, and is not particularly limited. For example, when the charge-discharge cations are sodium, the potential window is 2.4 V or more and 3.5 V or less. When the charge-discharge cations are magnesium, the potential window is 2.1 V or more and 3.0 V or less. When the charge-discharge cations are potassium, the potential window is 1.9 V or more and 2.5 V or less. When the charge-discharge cations are lithium, the potential window is 1.3 V or more and 3.5 V or less.

Oxygen Evolution Potential and Hydrogen Evolution Potential

[0061] The oxygen evolution potential and the hydrogen evolution potential of the aqueous electrolyte solution of the present embodiment vary in accordance with the concentration of the electrolyte salt. For example, at a temperature of 25° C., the oxygen evolution potential is preferably 1.2 V or more, more preferably 1.4 V or more, with respect to a silver/silver chloride electrode. For example, at a temperature of 25° C., the hydrogen evolution potential is preferably −1.0 V or less, more preferably −1.2 V or less, with respect to the silver/silver chloride electrode.

[0062] In the aqueous electrolyte solution of the present embodiment, it is considered that oxygen evolution and hydrogen evolution are suppressed by a chaotropic effect provided by the presence of both of the chaotropic additive and the electrolyte salt in the aqueous electrolyte solution. At the same time, it is considered that the potential window can also be extended by the formation of an SEI, which is a passive film, on a negative electrode current collector due to the precipitation of cations associated with charging and discharging.

Characteristics of Non-Aqueous Electrolyte Solution

[0063] The non-aqueous electrolyte solution of the present embodiment can have improved characteristics, such as a wider potential window, and the improved characteristics can lead to achieving a higher energy density, as compared with, for example, a chaotropic additive-free non-aqueous electrolyte solution having the same electrolyte salt concentration.

[0064] In the non-aqueous electrolyte solution of the present embodiment, it is considered that the decomposition of the non-aqueous solvent is suppressed by a chaotropic effect provided by the presence of both of the chaotropic additive and the electrolyte salt in the non-aqueous electrolyte solution. Moreover, it is considered that an SEI, which is a passive film, can be formed on a negative electrode current collector due to the precipitation of cations associated with charging and discharging.

Method for Producing Electrolyte Solution

[0065] A method for producing the aqueous electrolyte solution of the present embodiment can be freely selected. It is sufficient to mix an electrolyte salt, a chaotropic additive, and a solvent. Any mixing method can be employed. Examples of the mixing method include a method in which an electrolyte salt, a chaotropic additive and a solvent are mixed together to achieve a desired electrolyte concentration; a method in which an electrolyte salt and a chaotropic additive are dissolved in a solvent and then the resulting solution is diluted to achieve the desired electrolyte concentration; and a method in which a chaotropic additive and a solvent are mixed together and then an electrolyte salt is mixed therewith.

[0066] A specific example of a method for producing an electrolyte solution of the present embodiment is as follows: For example, an electrolyte salt and a chaotropic additive are added to a solvent (pure water for an aqueous electrolyte solution or a non-aqueous solvent for a non-aqueous electrolyte solution). The mixture is stirred at room temperature (for example, 20° C. to 30° C.) until the electrolyte salt and the chaotropic additive are dissolved. In particular, when a concentrated solution is produced, the precipitated salt is preferably removed by, for example, filtration.

Electrochemical Device

[0067] The electrochemical device in the present embodiment only needs to include the electrolyte solution of the present embodiment. In this specification, the term “electrochemical device” refers to a device that converts chemical energy into electrical energy. Examples thereof include charge-discharge devices, such as capacitors, air batteries, primary batteries and secondary batteries, preferably capacitors and secondary batteries, more preferably secondary batteries. In the case where the electrolyte solution of the present embodiment is an aqueous electrolyte solution, when it is used for these applications, the aqueous electrolyte solution can also be regarded as, for example, an aqueous electrolyte solution for electrochemical devices, an aqueous electrolyte solution for charge-discharge devices, an aqueous electrolyte solution for capacitors, an aqueous electrolyte solution for air batteries, an aqueous electrolyte solution for primary batteries, and an aqueous electrolyte solution for secondary batteries. In the case where the electrolyte solution of the present embodiment is a non-aqueous electrolyte solution, the non-aqueous electrolyte solution can also be regarded as, for example, a non-aqueous electrolyte solution for electrochemical devices, a non-aqueous electrolyte solution for charge-discharge devices, a non-aqueous electrolyte solution for capacitors, a non-aqueous electrolyte solution for air batteries, a non-aqueous electrolyte solution for primary batteries, and a non-aqueous electrolyte solution for secondary batteries.

[0068] An electrochemical device including the electrolyte solution of the present embodiment will be described below by taking a secondary battery as an example.

Secondary Battery

[0069] The secondary battery of the present embodiment only needs to include the electrolyte solution of the present embodiment as described above.

[0070] The secondary battery of the present embodiment includes an electrolyte solution containing a salt of at least one selected from the group consisting of sodium, magnesium, potassium and lithium, preferably a salt of at least one selected from the group consisting of sodium, magnesium and lithium, and a chaotropic additive, a positive electrode and a negative electrode.

[0071] The secondary battery of the present embodiment is What is called an aqueous secondary battery when the electrolyte solution is an aqueous electrolyte solution, or is what is called a non-aqueous secondary battery when the electrolyte solution is a non-aqueous electrolyte solution. The secondary battery of the present embodiment can also be regarded as a sodium secondary battery or sodium ion battery when the electrolyte salt is a salt of sodium, a magnesium secondary battery or magnesium ion battery when the electrolyte salt is a salt of magnesium, a potassium secondary battery or magnesium ion battery when the electrolyte salt is a salt of potassium, or a lithium secondary battery or lithium ion battery when the electrolyte salt is a salt of lithium.

Positive Electrode

[0072] The positive electrode of the secondary battery of the present embodiment includes a positive electrode active material layer containing at least a positive electrode active material.

[0073] The positive electrode active material can be any substance capable of intercalating and deintercalating charge-discharge cations or capable of forming a compound reversibly with charge-discharge cations, and any known positive electrode active material used in secondary batteries can be used.

[0074] The positive electrode active material is preferably capable of reversibly intercalating and deintercalating as many cations as possible in a potential range in which oxygen is not generated by electrolysis of the solvent.

[0075] Examples of the positive electrode active material include oxides, fluorides, halides, polyanionic compounds, Prussian blue, Prussian blue analogues and organic compounds. Among these, fluorides, Prussian blue, Prussian blue analogues and polyanionic compounds are preferred. In this specification, Prussian blue analogues are each a compound in which one or more Fe atoms in Prussian blue (Fe.sub.4[Fe(CN).sub.6].sub.3) are replaced with a transition metal element other than Fe.

[0076] When the charge-discharge cations are sodium, specific examples of the positive electrode active material include Na.sub.0.44MnO.sub.2, MnO.sub.2, Na.sub.2.7Ru.sub.4O.sub.9, Na.sub.2FeP.sub.2O.sub.7, NaCo.sub.1/3Ni.sub.1/3Mn.sub.1/3PO.sub.4, Na.sub.xFePO.sub.4, Na.sub.XMnPO.sub.4, Na.sub.3FePO.sub.4CO.sub.3, Na.sub.3MnPO.sub.4CO.sub.3, Na.sub.2FePO.sub.4F, Na.sub.2MnPO.sub.4F, Na.sub.3V.sub.2(PO.sub.4).sub.3, NaVPO.sub.4F, Na.sub.2Mn[Mn(CN).sub.6] and Na.sub.2Mn[Fe(CN).sub.6]. Among these, Na.sub.3FePO.sub.4CO.sub.3, Na.sub.2FePO.sub.4F, Na.sub.3V.sub.2(PO.sub.4).sub.3, NaVPO.sub.4F, Na.sub.2Mn[Mn(CN).sub.6] and Na.sub.2Mn[Fe(CN).sub.6] are preferred. These positive electrode active materials may be used alone or in combination as a mixture of two or more.

[0077] When the charge-discharge cations are magnesium, specific examples of the positive electrode active material include MgFeMn.sub.2O.sub.4, MgMn.sub.2O.sub.4, MgFeSiO.sub.4, MgMnSiO.sub.4, MgFePO.sub.4F, MgMnPO.sub.4F, Mg.sub.xLiV.sub.2(PO.sub.4).sub.3 and nickel hexacyanoferrate (NiHCF). Among these, MgFeSiO.sub.4, MgFePO.sub.4F and Mg.sub.xLiV.sub.2(PO.sub.4).sub.3 are preferred. These positive electrode active materials may be used alone or in combination as a mixture of two or more.

[0078] When the charge-discharge cations are potassium, specific examples of the positive electrode active material include K.sub.0.44MnO.sub.2, MnO.sub.2, K.sub.2FeP.sub.2O.sub.7, KCo.sub.1/3Ni.sub.1/3Mn.sub.1/3PO.sub.4, K.sub.xFePO.sub.4, K.sub.xMnPO.sub.4, K.sub.2FePO.sub.4F, K.sub.2MnPO.sub.4F, K.sub.2CoPO.sub.4F, K.sub.3V.sub.2(PO.sub.4).sub.3, KVPO.sub.4F, K.sub.2Mn[Mn(CN).sub.6] and K.sub.2Mn[Fe(CN).sub.6]. Among these, K.sub.2FePO.sub.4F, K.sub.3V.sub.2(PO.sub.4).sub.3, KVPO.sub.4F, K.sub.2Mn[Mn(CN).sub.6] and K.sub.2Mn[Fe(CN).sub.6] are preferred. These positive electrode active materials may be used alone or in combination as a mixture of two or more.

[0079] When the charge-discharge cations are lithium, specific examples of the positive electrode active material include LiMn.sub.2O.sub.4, LiMnO.sub.2, MnO.sub.2, LiCoO.sub.2, LiNi.sub.1/3Co.sub.1/3Mn.sub.1/3O.sub.2, Na.sub.1.16V.sub.3O.sub.8, LiFePO.sub.4, FePO.sub.4, LiMnPO.sub.4, Li(Fe,Mn)PO.sub.4, LiCoPO.sub.4, LiNiPO.sub.4, LiCo.sub.1/2Ni.sub.1/2PO.sub.4, Li.sub.2FePO.sub.4F, Li.sub.2MnPO.sub.4F, Li.sub.3FePO.sub.4CO.sub.3 and Li.sub.3MnPO.sub.4CO.sub.3. Among these, LiMn.sub.2O.sub.4, LiFePO.sub.4, Li.sub.2FePO.sub.4F and Li.sub.3FePO.sub.4CO.sub.3 are preferred. These positive electrode active materials may be used alone or in combination as a mixture of two or more.

[0080] The physical properties, such as the particle size and specific surface area, of the positive electrode active material may be adjusted according to the purpose.

[0081] The positive electrode active material layer may contain, for example, a binder, a conductive material and an additive.

[0082] The binder may be any binder used for positive electrodes of secondary batteries. Examples thereof include fluorine-containing resins, such as poly(vinylidene fluoride) (PVDF), polytetrafluoroethylene (PTFE) and ethylene tetrafluoroethylene (ETFE) fluorine, polyethylene, polypropylene, SBR-based materials and imide-based materials.

[0083] The conductive material may be any conductive material used for positive electrodes of secondary batteries. Examples thereof include conductive fibers composed of at least one of carbon materials and metal fibers; powders of metals, such as copper, silver, nickel and aluminum; and organic conductive materials, such as polyphenylene derivative. Examples of carbon materials include graphite, soft carbon, hard carbon, carbon black, Ketjen black, acetylene black, graphite, activated carbon, carbon nanotubes, carbon fiber, aromatic ring-containing synthetic resins, and mesoporous carbon obtained by heating petroleum-based pitch.

Negative Electrode

[0084] The negative electrode of the secondary battery of the present embodiment includes a negative electrode active material layer that contains at least a negative electrode active material.

[0085] The negative electrode active material can be any negative electrode active material used for secondary batteries. Examples thereof include substances capable of intercalating and deintercalating charge-discharge cations, substances that form compounds reversibly with charge-discharge cations, substances that are reversibly alloyed with charge-discharge cations, platinum, zinc, carbon materials, materials that form alloys with charge-discharge cations, transition metal oxides containing charge-discharge cations, polyanionic materials containing, charge-discharge cations, Prussian blue, Prussian blue analogues and organic compounds. Among these, carbon materials, polyimides, transition metal-containing cyano compounds, transition metal-containing polyanionic compounds, Prussian blue and Prussian blue analogues are preferred.

[0086] When the charge-discharge cations are sodium, specific examples of the negative electrode active material are as follows: Examples of carbon materials include activated carbon. Examples of transition metal-containing cyano compounds include Na.sub.2Mn[Mn(CN).sub.6] and K.sub.0.11Mn[Mn(CN).sub.6].sub.0.38. Examples of transition metal-containing polyanionic compounds include NaTi.sub.2(PO.sub.4).sub.3, Na.sub.2V.sub.6O.sub.16.nH.sub.2O and C.sub.14H.sub.10N.sub.2O.sub.6. Among these, NaTi.sub.2(PO.sub.4).sub.3 is preferred. These negative electrode active materials may be used alone or in combination as a mixture of two or more.

[0087] When the charge-discharge cations are magnesium, specific examples of the negative electrode active material include Mg.sub.1.5MnO.sub.2, MgMn.sub.2O.sub.4, Mo.sub.6S.sub.8, Mg.sub.1.03Mn.sub.0.97SiO.sub.4, poly(pyromellitic anhydride) (PPMDA), CuFeHCF and NiHCF. Among these, Mg.sub.1.03Mn.sub.0.97SiO.sub.4, poly(pyromellitic dianhydride) (PPMDA) and nickel hexacyanoferrate (NiHCF) are preferred. These negative electrode active materials may be used alone or in combination as a mixture of two or more.

[0088] When the charge-discharge cations are potassium, specific examples of the negative electrode active material are as follows: Examples of carbon materials include activated carbon. Examples of organic compounds include K.sub.2C.sub.8H.sub.4O.sub.4 and C.sub.14H.sub.10N.sub.2O.sub.6. Examples of transition metal-containing cyano compounds include K.sub.2Mn[Mn(CN).sub.6] and K.sub.0.11Mn[Mn(CN).sub.6].sub.0.38. An example of transition metal-containing polyanionic compounds is KTi.sub.2(PO.sub.4).sub.3. Among these, KTi.sub.2(PO.sub.4).sub.3 is preferred. These negative electrode active materials may be used alone or in combination as a mixture of two or more.

[0089] When the charge-discharge cations are lithium, specific examples of the negative electrode active material include graphite, hard carbon, MCMB, Si, SiO, Li.sub.4Ti.sub.5O.sub.12, VO.sub.2(B), Li.sub.2Mn.sub.2O.sub.4, γ-LiV.sub.3C.sub.8, H.sub.2V.sub.2O.sub.8, Na.sub.1+xV.sub.3O.sub.8, VO.sub.2, V.sub.2O.sub.5, TiO.sub.2, TiP.sub.2O.sub.7, LiTi.sub.2(PO.sub.4).sub.3, polypyrroles and polyimides. Among these, graphite, Si, TiO.sub.2 and Li.sub.4Ti.sub.5O.sub.12 are preferred. These negative electrode active materials may be used alone or in combination as a mixture of two or more.

[0090] The physical properties, such as the particle size and specific surface area, of the positive electrode active material may be adjusted according to the purpose.

[0091] The negative electrode active material layer may contain, for example, a binder, a conductive material and an additive. As the binder, the conductive material and the additive, those similar to those contained in the positive electrode active material layer can be used.

[0092] As components other than those described above in the secondary battery of the present embodiment, for example, a separator, a positive electrode current collector, a negative electrode current collector, a casing and a lead, those used for known secondary batteries can be used. For example, the separator may be any separator that transmits cations and electrically separates the positive electrode and the negative electrode from each other, and may be any separator used in batteries including electrolyte solutions. Specific examples of the separator include porous membranes, such as porous polyethylene membranes, porous polypropylene membranes, porous polytetrafluoroethylene membranes, porous aramid resin membranes and porous ceramic membranes; and nonwoven fabric membranes, such as nonwoven polyethylene fabrics, nonwoven polypropylene fabrics, nonwoven glass fiber fabrics and nonwoven cellulose fabrics.

[0093] A method of producing the secondary battery of the present embodiment is freely selected, and any known method can be employed.

EXAMPLES

[0094] While the present embodiment will be described below by means of examples, the present embodiment is not limited to the following examples.

Evaluation of Aqueous Electrolyte Solution

[0095] The potential windows of aqueous electrolyte solutions in Examples 1-1 and 1-2 and Comparative examples 1-1 to 1-4 were measured by linear sweep voltammetry. The measurement conditions are described below.

[0096] Working electrode: Ti mesh (trade name: Expanded Metal, available from Thank Metal Co., Ltd.)

[0097] Counter electrode: zinc plate (trade name: ZN483384, available from The Nilaco Corp.)

[0098] Reference electrode: silver/silver chloride electrode (trade name: RE-1CP, available from BAS Inc.)

[0099] Potential scan rate: 0.5 mV/sec

[0100] Measurement temperature: 25° C.

[0101] Measurement device: potentio-galvanostat (device name: Versastat 3, available from AMETEC Inc.)

Example 1-1

[0102] Urea was dissolved in water to prepare an aqueous urea solution having a urea concentration of 18 mol/kg. At room temperature, NaClO4 was dissolved in the aqueous urea solution until the solution was saturated therewith, thereby preparing an aqueous electrolyte solution having a urea concentration of 18 mol/kg and a NaClO4 concentration of 25 mol/kg.

[0103] The resulting aqueous electrolyte solution had a hydrogen evolution potential of −1.45 V, an oxygen evolution potential of 1.54 V and a potential window of 2.99 V.

Comparative Example 1-1

[0104] At room temperature, NaClO.sub.4 was dissolved in water until the solution was saturated therewith, preparing an aqueous electrolyte solution having a NaClO.sub.4 concentration of 17 mol/kg.

[0105] The resulting aqueous electrolyte solution had a hydrogen evolution potential of −0.62 V, an oxygen evolution potential of 1.86 V and a potential window of 2.48 V.

[0106] Comparison of Example 1-1 and Comparative example 1-1 revealed that the presence of urea allowed the electrolyte salt to dissolve beyond the saturation concentration, up to 1.17 times the saturation concentration, and moreover, allowed both of the oxygen evolution potential and the hydrogen evolution potential to shift to extend the potential window.

Comparative Example 1-1a

[0107] At room temperature, NaClO.sub.4 was dissolved in water until the solution was saturated, thereby preparing an aqueous electrolyte solution having a NaClO.sub.4 concentration of 17 mol/kg. When ammonium sulfate was added to the electrolyte solution, a salt was precipitated. The supernatant solution had a hydrogen evolution potential of −0.30 V, an oxygen evolution potential of 1.30 V and a potential window of 1.60 V.

Example 1-2

[0108] Urea was dissolved in water to prepare an aqueous urea solution having a urea concentration of 25 mol/kg. At room temperature, Mg(ClO.sub.4).sub.2 was dissolved in the aqueous urea solution until the solution was saturated therewith, thereby preparing an aqueous electrolyte solution having a urea concentration of 25 mol/kg and a Mg(ClO.sub.4).sub.2 concentration of 14 mol/kg.

[0109] The resulting aqueous electrolyte solution had a hydrogen evolution potential of −1.00 V, an oxygen evolution. potential of 1.50 V and a potential window of 2.50 V.

Comparative Example 1-2

[0110] At room temperature, Mg(ClO.sub.4).sub.2 was dissolved in water until the solution was saturated therewith, thereby preparing an aqueous electrolyte solution having a Mg(ClO.sub.4).sub.2 concentration of 4 mol/kg.

[0111] The resulting aqueous electrolyte solution had a hydrogen evolution potential of −0.60 V, an oxygen evolution potential of 1.50 V and a potential window of 2.10 V.

[0112] Comparison with Example 1-2 revealed that the presence of urea allowed the electrolyte salt to dissolve beyond the saturation concentration, up to 3.5 times the saturation concentration, and moreover, allowed the hydrogen evolution potential to shift to extend the potential window.

Comparative Example 1-3

[0113] At room temperature, magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI).sub.2) was dissolved in water to prepare an aqueous electrolyte solution having a Mg(TFSI).sub.2 concentration of 4 mol/kg.

[0114] The resulting aqueous electrolyte solution had a hydrogen evolution potential of −0.9 V, an oxygen evolution potential of 1.1 V and a potential window of 2.0 V.

[0115] Comparison with Example 1-2 revealed that Mg(TFSI).sub.2 did not increase the oxygen evolution potential.

Comparative Example 1-4

[0116] At room temperature, magnesium sulfate (MgSO.sub.4) was dissolved in water to prepare an aqueous electrolyte solution having a MgSO.sub.4 concentration of 1 mol/kg.

[0117] The resulting aqueous electrolyte solution had a hydrogen evolution potential of −0.4 V, an oxygen evolution potential of 0.9 V and a potential window of 1.3 V.

Evaluation of Electrochemical Device Including Aqueous Electrolyte Solution

Example 1-3

[0118] A sodium secondary battery was produced using an aqueous electrolyte solution prepared in the same manner as in Example 1-1 as an electrolyte solution, a hydrated product (hereinafter, also referred to as “NMHCF”) of a Prussian blue analogue represented by the general formula Na.sub.2Mn[Fe(CN).sub.6] synthesized by a coprecipitation method as a positive electrode active material and a Prussian blue analogue (hereinafter, also referred to as “KMHCC”) represented by the general formula KMn[Cr(CN).sub.6] synthesized by a coprecipitation method as a negative electrode active material. The configuration of the produced sodium secondary battery is described below.

[0119] Positive electrode active material: NMHCF

[0120] Negative electrode active material: KMHCC

[0121] Ratio by mass: NMHCF/KMHCC=2/3

[0122] Current collector: Ti mesh

[0123] The sodium secondary battery including the aqueous electrolyte solution was connected to a charge-discharge apparatus (apparatus name: Charge-discharge apparatus BTS2004H, available from Nagano Co., Ltd.), and a charge-discharge test was performed under the following conditions. FIG. 1 illustrates the results.

[0124] Measurement mode: charging and discharging at constant current

[0125] Current value: 5 mA/cm.sup.2

[0126] Measurement voltage: 0 V to 2 V

Comparative Example 1-5

[0127] A sodium secondary battery was produced in the same manner as in Example 1-3, except that an aqueous electrolyte solution prepared by the same method as in Comparative example 1-1 was used as the electrolyte solution. A charge-discharge test was performed. FIG. 1 illustrates the results.

[0128] FIG. 1 indicated that the discharge capacity of the sodium secondary battery of Example 1-3 was higher than that of the sodium secondary battery of Comparative example 1-5, after 300 charge-discharge cycles. This demonstrated that the aqueous electrolyte solution of the present embodiment can also contribute to improving the cycle characteristics.

Example 1-4

[0129] Ethylene glycol was dissolved in water to prepare an aqueous ethylene glycol solution having an ethylene glycol concentration of 20 mol/kg. At room temperature, NaClO.sub.4 was dissolved in the aqueous ethylene glycol solution until the solution was saturated therewith, thereby preparing an aqueous electrolyte solution having an ethylene glycol concentration of 25 mol/kg and a NaClO.sub.4 concentration of 27 mol/kg.

[0130] The resulting aqueous electrolyte solution had a hydrogen evolution potential of −0.62 V, an oxygen evolution potential of 1.86 V and a potential window of 2.48 V.

[0131] A sodium secondary battery was produced in the same manner as in Example 1-3, except that the resulting electrolyte solution was used and the ratio by mass of the positive electrode to the negative electrode was NMHCF/KMHCC=1/2. A charge-discharge test was performed.

[0132] The aqueous electrolyte solution of this Example had the same potential window as the aqueous electrolyte solution of Comparative example 1-1. Nevertheless, the discharge capacity of the sodium battery of this Example was 1.5 times the discharge capacity of the sodium secondary battery of Comparative example 1-1, after 300 charge-discharge cycles. This indicated that ethylene glycol was effective in improving the cycle characteristics.

Example 1-5

[0133] An aqueous electrolyte solution having an ethylene glycol concentration of 25 mol/kg and a NaClO.sub.4 concentration of 29 mol/kg was prepared in the same manner as in Example 1-4, except that an aqueous ethylene glycol solution having an ethylene glycol concentration of 25 mol/kg was used.

[0134] A sodium secondary battery was produced in the same manner as in Example 1-4, except that the resulting electrolyte solution was used. A charge-discharge test was performed.

[0135] The aqueous electrolyte solution of this example had the same potential window as the aqueous electrolyte solution of Comparative example 1-1. Nevertheless, the discharge capacity of the sodium battery of this Example was 1.6 times the discharge capacity of the sodium secondary battery of Comparative example 1-1, after 100 charge-discharge cycles. This indicated that ethylene glycol was effective in improving the cycle characteristics.

Example 1-6

[0136] Urea was dissolved in water to prepare an aqueous urea solution having a urea concentration of 5 mol/kg. At room temperature, potassium nitrate (KNO.sub.3) was dissolved in the aqueous urea solution until the solution was saturated therewith, thereby preparing an aqueous electrolyte solution having a urea concentration of 5 mol/kg and a KNO.sub.3 concentration of 4 mol/kg. The aqueous electrolyte solution had a hydrogen evolution potential of −0.40 V, an oxygen evolution potential of 1.50 V and a potential window of 1.90 V.

Comparative Example 1-6

[0137] At room temperature, KNO.sub.3 was dissolved in water until the solution was saturated therewith, thereby preparing an aqueous electrolyte solution having a KNO.sub.3 concentration of 3 mol/kg. The resulting aqueous electrolyte solution had a hydrogen evolution potential of −0.40 V, an oxygen evolution potential of 1.40 V and a potential window of 1.80 V.

[0138] Comparison of Example 1-6 and Comparative example 1-6 revealed that, even when the potassium salt was used as an electrolyte salt, the presence of urea allowed the electrolyte salt to dissolve beyond the saturation concentration, up to 1.67 times the saturation concentration, and increased the oxygen evolution potential to extend the potential window.

Evaluation of Non-Aqueous Electrolyte Solution

Example 2-1

[0139] Urea was dissolved in acetonitrile to prepare a non-aqueous solution (non-aqueous urea solution) having a urea concentration of 5 mol/kg. Mg(ClO.sub.4).sub.2 was dissolved in the resulting non-aqueous solution at room temperature until the solution was saturated therewith, thereby preparing a non-aqueous electrolyte solution having a urea concentration of 5 mol/kg and a Mg(ClO.sub.4).sub.2 concentration of 5 mol/kg.

[0140] A model non-aqueous magnesium battery was produced using the resulting non-aqueous electrolyte solution. The non-aqueous electrolyte solution was evaluated using the model battery by linear sweep voltammetry. The evaluation conditions are described below.

[0141] Working electrode, counter electrode: Ti mesh (trade name: Expanded Metal, available from Thank Metal Co., Ltd.)

[0142] Reference electrode: silver/silver chloride electrode (trade name: RE-1CP, available from BAS Inc.)

[0143] Potential scan rate: 0.5 mV/sec

[0144] Scanning potential: −2.5 V to 2.5 V

[0145] Measurement temperature: 2.5° C.

[0146] Measurement device: potentio-galvanostat (device name: Versastat 3, available from AMETEC Inc.)

[0147] The measurement results indicated that neither a reduction current nor an oxidation current occurred in the scanning potential range.

[0148] FIG. 2 illustrates the cyclic voltammograms of Example 2-1 and Comparative example 2-1 below.

Comparative Example 2-1

[0149] Mg(ClO.sub.4).sub.2 was dissolved in acetonitrile at room temperature until the solutions was saturated therewith, thereby preparing a non-aqueous electrolyte solution having a Mg(ClO.sub.4).sub.2 concentration of 2 mol/kg.

[0150] The evaluation was performed in the same manner as in Example 2-1, except that the resulting electrolyte solution was used. An oxidation current and a rapid increase in potential were observed at 1.7 V or higher, suggesting that an oxidation reaction of the solvent occurred.

[0151] Comparison of Example 2-1 and Comparative example 2-1 revealed that the presence of urea allowed the electrolyte salt to dissolve beyond the saturation concentration, up to 2.5 times the saturation concentration. In Example 2-1, no abnormal currents were observed in the reduction current or oxidation current, indicating that no gas generation occurred in the scanning potential range. Accordingly, the comparison of Example 2-1 and Comparative example 2-1 indicated that the oxidation reaction was suppressed in the non-aqueous electrolyte solution containing urea. The incorporation of the chaotropic additive can improve the characteristics of the non-aqueous electrolyte solution, so that it is possible to provide the non-aqueous electrolyte solution that can achieve a higher energy density.

[0152] This application claims priority to Japanese Patent Application No. 2019-200700 filed Nov. 5, 2019 and No. 2019-200778 filed Nov. 5, 2019, the entire content of which are incorporated herein by reference.

Reference Signs List

[0153] 1 Example 1-3 [0154] 2 Comparative example 1-5 [0155] 3 Example 2-1 [0156] 4 Comparative example 2-1