JP2020155279A - Aqueous electrolyte solution for power storage device, and power storage device including the same - Google Patents

Abstract

To provide an aqueous electrolyte solution for a power storage device, which can increase the coulomb efficiency.SOLUTION: In the aqueous electrolyte solution, an electrolyte is one of: a K salt of an asymmetric imide having an Rf group (perfluoroalkyl group); a mixed salt of a K salt of an asymmetric imide and a K salt of a symmetric imide having an Rf group; and a mixed salt of a K salt of an asymmetric imide, a K salt of a symmetric imide and a sulfonic acid K salt having Rf group. The proportion of the K salt is as follows: (Asymmetric imide K salt)x(Symmetric imide K salt)y(Sulfonic acid K salt)1-(x+y) (where x=0.1-1.0, y=0-0.9, and x+y≤1.0) in terms of mole ratio. The asymmetric imide K salt is (C2F5SO2)(CF3SO2)NK, (C3F7SO2)(CF3SO2)NK or (C4F9SO2)(CF3SO2)NK. The symmetric imide K salt is (CF3SO2)2 NK or (C2F5SO2)2 NK. The sulfonic acid K salt is CF3SO3K. As to the composition of the electrolyte solution, a quantity of solvent is 2 mol or more and less than 40 mole to 1 mol of the K salt or K salt of the mixed salt.SELECTED DRAWING: Figure 1

Description

The present invention relates to a nonflammable water-based electrolytic solution for a power storage device and a power storage device containing the water-based electrolytic solution.

Conventionally, a potassium ion battery has been proposed as a power storage device in place of the lithium ion battery (see, for example, Patent Document 1 (claims 1 to 3, paragraphs [0010] and 0036]). The potassium ion battery during comprises a _{K m Fe x Mn y (CN} ) 6 · zH compounds represented by ₂ O of formula (1) (Formula (1), m is a number of 0.5 to 2 Represented, x represents a number of 0.5 or more and 1.5 or less, y represents a number of 0.5 or more and 1.5 or less, and z represents 0 or a positive number.) A positive electrode active material for a potassium ion battery. It includes a positive electrode for a potassium ion battery.

Since this potassium ion battery includes a positive electrode for a potassium ion battery containing the positive electrode active material for the potassium ion battery, it is said to have a feature of high energy density. In this potassium ion battery, an aqueous electrolyte and a non-aqueous electrolyte are exemplified as the electrolyte as the electrolyte, and in the case of the aqueous electrolyte, KClO ₄ , KPF ₆ , KNO ₃ , KOH, and KCl are as potassium salts. , K ₂ SO ₄ , K ₂ S, etc.

On the other hand, an electrolytic solution for a power storage device containing water as a solvent, wherein the composition of the electrolytic solution is 4 mol or less with respect to 1 mol of the alkali metal salt, and the electrolytic solution and the alkali metal salt are characterized. It is disclosed that it is a lithium salt or a sodium salt (see Patent Document 2 (claim 1, paragraph [0028], paragraph [0087], claim 6)). By using such a high-concentration alkali metal salt, the electrolytic solution for a power storage device has a potential window that exceeds the potential window (stable potential region) of pure water, preferably a potential window of 2 V or more, and this implementation. In the example, Patent Document 2 shows that the potential window of the electrolytic solution No. 4 is calculated to be 3.2 V.

JP-A-2018-106911 International Publication No. 2016/114141

However, in the potassium ion battery using the aqueous electrolyte solution containing the potassium salt shown in Patent Document 1, since the decomposition voltage of water is limited to 1.2 V, there is a problem that a power storage device having a high voltage and capacity cannot be realized. there were. Further, in Patent Document 1, in the examples and comparative examples, the aqueous electrolytic solution is not shown, and only the non-aqueous electrolytic solution is shown. Further, Patent Document 2 discloses an alkali metal salt which is a lithium salt or a sodium salt as the composition of the electrolytic solution, but does not describe the potassium salt.

An object of the present invention is to provide a water-based electrolytic solution for a power storage device capable of increasing the Coulomb efficiency of the power storage device, and a power storage device containing the water-based electrolytic solution.

The present inventors specify one or more potassium salts of asymmetric imide having a specific perfluoroalkyl group, and potassium salt of asymmetric imide having a specific perfluoroalkyl group as an electrolyte of an aqueous electrolyte solution. When a mixed salt of a symmetric imide having a perfluoroalkyl group or a mixed salt of these potassium salts and a specific potassium sulfonic acid salt is used, 1 mol of the potassium salt is used even in potassium having a relatively large ionic radius. On the other hand, when the amount of the solvent is 2 mol or more and less than 40 mol, the above-mentioned one or more kinds of potassium salt or mixed salt becomes an aqueous solution, and the electrolysis of water is suppressed even if water is used. And arrived at the present invention. In the present specification, "asymmetric imide" means an imide whose composition is not symmetric in structure, and "symmetric imide" means an imide whose composition is symmetric in structure.

The first aspect of the present invention is that in an aqueous electrolyte for a power storage device containing water as a solvent, the electrolyte is one or more potassium salts of an asymmetric imide having a perfluoroalkyl group, or the asymmetry. A mixed salt of a potassium salt of an imide and a potassium salt of a symmetric imide having a perfluoroalkyl group, or a potassium sulfonate having a potassium salt of the asymmetric imide, a potassium salt of the symmetric imide and a perfluoroalkyl group. The composition ratio of the one or more potassium salts and the mixed salt is (asymmetric imide potassium salt) _x (symmetric imide potassium salt) _y (sulfone). Potassium acid salt) _{1- (x + y)} (where x is 0.1 to 1.0, y is 0 to 0.9, and x + y ≦ 1.0), and the asymmetry. The sex imide potassium salt is potassium (pentafluoroethanesulfonyl) (trifluoromethanesulfonyl) imide ((C ₂ F ₅ SO ₂ ) (CF ₃ SO ₂ ) NK, hereinafter referred to as “Compound 1”), potassium (heptafluoro). Propanesulfonyl) (trifluoromethanesulfonyl) imide ((C ₃ F ₇ SO ₂ ) (CF ₃ SO ₂ ) NK, hereinafter referred to as "Compound 2") or potassium (nonafluorobutanesulfonyl) (trifluoromethanesulfonyl) imide ( (C ₄ F ₉ SO ₂ ) (CF ₃ SO ₂ ) NK, hereinafter referred to as “Compound 4”), and the symmetric imide potassium salt is potassium bis (trifluoromethanesulfonyl) imide ((CF ₃ SO _2). ) ₂ NK, hereinafter referred to as “Compound 3”) or potassium bis (pentafluoroethanesulfonyl) imide ((C ₂ F ₅ SO ₂ ) ₂ NK, hereinafter referred to as “Compound 5”), which is the sulfonic acid. The potassium salt is potassium trifluoromethanesulfonate (CF ₃ SO ₃ K, hereinafter referred to as “Compound 8”), and the composition of the electrolytic solution is one or more of the above potassium salts or the mixed salts. The amount of the solvent is 2 mol or more and less than 40 mol with respect to 1 mol of the potassium salt.

A second aspect of the present invention is an invention based on the first aspect, which is an aqueous electrolytic solution for a power storage device in which the melting point of one or more potassium salts is 230 ° C. or lower.

The third aspect of the present invention is the invention based on the first or second aspect, in which the fluorine ion content in the aqueous electrolytic solution is 10 ppm or less and the hydrogen content in the aqueous electrolytic solution is 10 ppm or less. It is an aqueous electrolyte for a power storage device. Here, the fluorine ions in the aqueous electrolyte are the fluorine ions remaining without completely removing the side reaction products generated during the production of the potassium salt, and / or the fluorine ions mixed by the decomposition of the reaction products. And is derived from the imide potassium salt. Further, hydrogen in the aqueous electrolytic solution is a hydrogen atom in which the potassium salt is not completely fluorinated and remains in the molecule of the imide potassium salt.

A fourth aspect of the present invention is an invention based on any one of the first to third aspects, wherein the electricity storage device is a potassium ion secondary battery, an electric double layer capacitor, or a potassium ion capacitor. It is an aqueous electrolyte for devices.

The fifth aspect of the present invention is used as an electrolyte of an aqueous electrolyte solution for a power storage device according to any one of the first to fourth aspects, and has a fluorine ion content of 10 ppm or less and a hydrogen content of 10 ppm or less. A potassium salt of one or more asymmetric imides having a perfluoroalkyl group of the first aspect.

The sixth aspect of the present invention is used as an electrolyte of an aqueous electrolyte solution for a power storage device according to any one of the first to fourth aspects, and has a fluorine ion content of 10 ppm or less and a hydrogen content of 10 ppm or less. It is a mixed salt of an asymmetric imide potassium salt having a perfluoroalkyl group of the first aspect and a symmetric imide potassium salt having a perfluoroalkyl group of the first aspect.

The seventh aspect of the present invention is used as an electrolyte of an aqueous electrolyte solution for a power storage device according to any one of the first to fourth aspects, and has a fluorine ion content of 10 ppm or less and a hydrogen content of 10 ppm or less. The potassium salt of an asymmetric imide having a perfluoroalkyl group of the first aspect, the potassium salt of a symmetric imide having a perfluoroalkyl group of the first aspect, and the potassium sulfonate having a perfluoroalkyl group of the first aspect. It is a mixed salt with salt.

In the aqueous electrolyte solution for a power storage device according to the first aspect of the present invention, one or more potassium salts of an asymmetric imide having a specific perfluoroalkyl group and potassium of the asymmetric imide as an electrolyte of the aqueous electrolyte solution. A mixed salt of a salt and a potassium salt of a symmetric imide having a specific perfluoroalkyl group, or a mixed salt of the potassium salt of the asymmetric imide, the potassium salt of the symmetric imide, and a potassium sulfonic acid having a perfluoroalkyl group. The composition ratio of one or more potassium salts and mixed salts is determined by molar ratio of (potassium salt of asymmetric imide) _x (potassium salt of symmetric imide) _y (potassium sulfonate). _It is specified as _{1- (x + y)} (where x is 0.1 to 1.0, y is 0 to 0.9, and x + y ≦ 1.0). By doing so, the solubility of the one or more potassium salts or the mixed salts is increased, and even in potassium having a relatively large ionic radius, the amount of solvent is 2 mol or more and 40 mol per 1 mol of potassium salt. In the range of less than, the potassium salt of one or more of the above or two or more kinds of potassium salts or mixed salts becomes an aqueous solution, and the electrolysis of water is suppressed, which is suggested from the examples shown in Patent Document 2. The Coulomb efficiency can be higher than the Coulomb efficiency of the potassium battery.

In the aqueous electrolyte solution for a power storage device according to the second aspect of the present invention, the melting point of one or more potassium salts having the asymmetric imide structure is as low as 230 ° C. or lower. This is because a potassium salt having an asymmetric imide structure has a lower crystallinity due to its asymmetric molecular structure, has a lower melting point than a potassium salt having a symmetric imide structure, and has higher solubility in water. by. In particular, a potassium salt having an asymmetric imide having a melting point of 230 ° C. or lower is highly soluble in water and is useful as an aqueous electrolytic solution.

In the aqueous electrolytic solution for a power storage device according to the third aspect of the present invention, when the fluorine ion content in the aqueous electrolytic solution is 10 ppm or less and the hydrogen content in the aqueous electrolytic solution is 10 ppm or less, good melting is performed. Since the salt state can be formed and side reactions derived from impurities such as fluorine ions and hydrogen are reduced, the Coulomb efficiency can be further improved when the potassium battery is used.

The aqueous electrolyte solution for a power storage device according to the fourth aspect of the present invention can be suitably used for a potassium ion secondary battery, an electric double layer capacitor or a potassium ion capacitor.

One or more potassium salts of the specific asymmetric imide of the fifth aspect of the present invention or the potassium salt of the specific asymmetric imide of the sixth aspect of the present invention and the potassium salt of the specific symmetric imide. A mixed salt of a specific asymmetric imide potassium salt of a seventh aspect, a specific symmetric imide potassium salt, and a sulfonic acid potassium salt having a perfluoroalkyl group are impurities such as fluorine ions and hydrogen. Therefore, it can be suitably used as an electrolyte for an aqueous electrolyte solution for a power storage device.

Each of the potassium salts of Examples 1-2-1-5 which is a mixed salt of Compound 1 and Compound 3, Example 1-1 which is a potassium salt of Compound 1, and Comparative Example 1 which is a potassium salt of Compound 3. It is a figure which showed the composition ratio and the ratio of the number of moles of water to the number of moles of potassium salt required for dissolution. Each of the potassium salts of Examples 3-2-3-5, which is a mixed salt of Compound 2 and Compound 3, Example 3-1 which is a potassium salt of Compound 2, and Comparative Example 1 which is a potassium salt of Compound 3. It is a figure which showed the composition ratio and the ratio of the number of moles of water to the number of moles of potassium salt required for dissolution. It is a figure which performed the linear soup voltum gram measurement with respect to the electrolytic solution of Example 2 using a tripolar electrochemical cell, and confirmed the potential window.

Next, a mode for carrying out the present invention will be described.

[Electrolytic solution]
(1) Solvent The electrolytic solution for a power storage device of the present embodiment (hereinafter, may be simply referred to as “electrolytic solution”) is characterized by being an aqueous electrolytic solution. Therefore, the solvent used in the aqueous electrolytic solution for the power storage device of the present embodiment is water. However, the solvent can be a mixed solvent containing water and other non-aqueous solvents. Such non-aqueous solvents are soluble in water and are, for example, alcohols such as methanol, as well as acetone, acetonitrile, dimethyl sulfoxide, or dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, ethylene carbonate, propylene. Examples thereof include aprotic polar solvents such as carbonates such as carbonates. Even in the case of such a mixed solvent, the ratio of water is preferably 90% or more by volume.

(2) Potassium salt as an electrolyte The potassium salt used as an electrolyte in the aqueous electrolyte solution for a power storage device of the present embodiment is (i) one or more potassium salts of an asymmetric imide having a perfluoroalkyl group. Or (ii) a mixed salt of the potassium salt of the asymmetric imide and a potassium salt of the symmetric imide having a perfluoroalkyl group, or (iii) the potassium salt of the asymmetric imide and the symmetric imide. It is a mixed salt of a potassium salt and a potassium sulfonic acid salt having a perfluoroalkyl group. When the composition ratio of one or more potassium salts and mixed salts is expressed in molar ratio, (potassium salt of asymmetric imide) _x (potassium salt of symmetric imide) _y (potassium sulfonate) _{1-( x + y)} (where x is 0.1 to 1.0, y is 0 to 0.9, and x + y ≦ 1.0). That is, the composition ratio of the one or more potassium salts and the mixed salt is asymmetric imide when the total number of moles of the symmetric imide potassium salt and the asymmetric imide potassium salt is 1 mol. The potassium salt of is 0.1 mol to 1.0 mol. The composition is preferably in molar ratio, potassium salt of asymmetric imide: potassium salt of symmetric imide = 0.2: 0.8 to 1.0: 0, and more preferably 0.4: 0. 6 to 0.8: 0.2. If the potassium salt of the asymmetric imide is less than the lower limit of the molar ratio, it does not become an aqueous solution, so that the amount of solvent is 40 mol or more per 1 mol of the potassium salt.

The asymmetric imide potassium salt is either the compound 1 potassium (pentafluoroethanesulfonyl) (trifluoromethanesulfonyl) imide ((C ₂ F ₅ SO ₂ ) (CF ₃ SO ₂ ) NK) or compound 2. There is a potassium (heptafluoropropanesulfonyl) (trifluoromethanesulfonyl) imide ((C ₃ F ₇ SO ₂ ) (CF ₃ SO ₂ ) NK) or the compound 4 potassium (nonafluorobutane sulfonyl) (trifluoromethane). It is a sulfonyl) imide ((C ₄ F ₉ SO ₂ ) (CF ₃ SO ₂ ) NK). The symmetric imide anion is either potassium bis (trifluoromethanesulfonyl) imide ((CF ₃ SO ₂ ) ₂ NK), which is compound 3, or potassium bis (pentafluoroethanesulfonyl) imide ((CF ₃ SO ₂ ) ₂ NK), which is compound 5. C ₂ F ₅ SO ₂ ) ₂ NK).

Asymmetric imide potassium salts and symmetric imide potassium salts other than the above compounds 1 to 5 include potassium salts shown in Table 1 below. Table 1 shows the potassium salts of compounds 1 to 5, the potassium salts of compounds 6 to 7, and the potassium sulfonic acid salt of the compound 8 together with their respective melting points and their respective names (compounds 1 to 8). .. The melting point of the potassium salt is a value measured by TG-DTA (TG-DTA2000SA manufactured by Netch Japan). As is clear from Table 1, the melting points of the potassium salts of compounds 1 to 4 are 230 ° C. or lower, and the melting points of the imide potassium salts of the other compounds 5 to 7 exceed 230 ° C.

Further, the aqueous electrolytic solution for a power storage device of the present embodiment is characterized by containing a high concentration of potassium salt. As a result, it is possible to realize a power storage device such as a secondary battery that generates a high voltage even in an electrode configuration that could not operate reversibly with an aqueous electrolytic solution in the past. The mixing ratio of the potassium salt and the solvent in the electrolytic solution is such that the solvent is 2 mol or more and less than 40 mol with respect to 1 mol of the potassium salt of one or more kinds or the mixed salt, and preferably. The solvent is 4 mol or more and 34 mol or less. The electrolytic solution for a power storage device of the present embodiment has a potential window that exceeds the potential window (stable potential region) of pure water by using such a high concentration potassium salt, and preferably has a potential window of 2.0 V or higher. Has.

Further, it is preferable that the potassium salt of the mixed salt in the aqueous electrolytic solution for the power storage device of the present embodiment has a low melting point. A potassium salt having an asymmetric imide structure has a lower crystallinity due to its asymmetric molecular structure, has a lower melting point than a potassium salt having a symmetric imide structure, and has higher solubility in water. In particular, a potassium salt having an asymmetric imide having a melting point of 230 ° C. or lower is highly soluble in water and is useful as an aqueous electrolytic solution. Further, it is preferable that the fluorine ion content in the water-based electrolytic solution is 10 ppm or less and the hydrogen content in the water-based electrolytic solution is 10 ppm or less. By doing so, a good molten salt state can be formed, and side reactions derived from impurities such as fluorine ions and hydrogen are reduced, so that the Coulomb efficiency of the potassium battery is further enhanced. When the fluorine ion content exceeds 10 ppm, and when the hydrogen content exceeds 10 ppm, the formation of a good molten salt is inhibited and side reactions derived from impurities such as fluorine ions and hydrogen increase. , The Coulomb efficiency of potassium batteries tends to decrease.

In addition to the potassium salt described above, supporting electrolytes known in the art can be included. Such supporting electrolytes include, for example, KPF ₆ , KBF ₄ , KClO ₄ , KNO ₃ , KCl, K ₂ SO ₄ and K ₂ S, etc., and these, when the secondary battery is a potassium ion secondary battery. The one selected from any combination can be mentioned.

(3) Other Components The electrolytic solution for a power storage device of the present embodiment may contain other components as necessary for the purpose of improving its function. Examples of other components include conventionally known overcharge inhibitors, deoxidizers, and property improving aids for improving capacity retention characteristics and cycle characteristics after high temperature storage.

When the electrolytic solution contains an overcharge inhibitor, the content of the overcharge inhibitor in the electrolytic solution is preferably 0.01% by mass to 5% by mass. By containing 0.1% by mass or more of the overcharge inhibitor in the electrolytic solution, it becomes easier to suppress the bursting and ignition of the power storage device due to overcharge, and the power storage device can be used more stably.

[Power storage device]
The power storage device of the present embodiment includes a positive electrode and a negative electrode, and the aqueous electrolytic solution of the present embodiment. Examples of the power storage device include a potassium ion secondary battery, an electric double layer capacitor, and a potassium ion capacitor.

(1) Negative electrode As the negative electrode in the power storage device of the present embodiment, an electrode configuration known in the art can be used. For example, when the power storage device is a potassium ion secondary battery, an electrode containing a negative electrode active material capable of electrochemically storing and releasing potassium ions can be mentioned. As such a negative electrode active material, a known negative electrode active material for a potassium ion secondary battery can be used, and examples thereof include carbonaceous materials such as graphitizable carbon and non-graphitizable carbon (hard carbon). Be done. Still other examples include metal compounds such as alloys having potassium elements and metal oxides. Examples of alloys having a potassium element include potassium aluminum alloys, potassium tin alloys, potassium lead alloys, potassium silicon alloys and the like. Examples of the metal compound having a potassium element include potassium-containing titanium oxides such as potassium titanate (K ₂ Ti ₃ O ₇ or K ₄ Ti ₅ O ₁₂ ). These negative electrode active materials may be used alone or in combination of two or more. Of these, potassium titanate is preferable as the negative electrode active material in the case of a potassium ion secondary battery.

When the power storage device is an electric double layer capacitor, the negative electrode material contains a polarizable electrode material. The polar electrode material may be any material used for ordinary electric double layer capacitors, and activated carbon produced from various raw materials can be exemplified. The activated carbon preferably has a large specific surface area.

When the power storage device is a potassium ion capacitor, the negative electrode contains a material capable of occluding and releasing potassium ions. Examples of the material include graphite-containing materials such as natural graphite and artificial graphite. Further, a material such as potassium titanate, which exhibits a redox capacity at a constant potential by inserting and desorbing a cation such as potassium ion, may be used. When a potassium-free material is used as the negative electrode active material, a metal potassium or a compound containing a large amount of potassium may be added to the negative electrode or the positive electrode, and potassium may be pre-doped into the negative electrode active material.

When the power storage device is a secondary battery, the negative electrode may contain only the negative electrode active material, and contains at least one of a conductive material and a binder (binder) in addition to the negative electrode active material. , The negative electrode mixture may be attached to the negative electrode current collector. For example, when the negative electrode active material is in the form of a foil, it can be a negative electrode containing only the negative electrode active material. On the other hand, when the negative electrode active material is in the form of powder, it can be a negative electrode having a negative electrode active material and a binder. As a method for forming the negative electrode using the powdered negative electrode active material, a doctor blade method, a molding method by a crimp press, or the like can be used. The same applies when the power storage device is a capacitor.

As the conductive material, for example, a carbon material, a conductive fiber such as a metal fiber, a metal powder such as copper, silver, nickel or aluminum, or an organic conductive material such as a polyphenylene derivative can be used. As the carbon material, graphite, soft carbon, hard carbon, carbon black, Ketjen black, acetylene black, graphite, activated carbon, carbon nanotubes, carbon fiber and the like can be used. It is also possible to use mesoporous carbon obtained by firing a synthetic resin containing an aromatic ring, petroleum pitch, or the like.

As the binder, for example, a fluororesin such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), or ethylene tetrafluoroethylene (ETFE), or polyethylene or polypropylene can be preferably used. As the negative electrode current collector, a rod-shaped body, a plate-shaped body, a foil-shaped body, a net-like body or the like mainly made of a metal such as copper, nickel, aluminum, zinc, titanium, platinum or stainless steel can be used.

(2) Positive Electrode As the positive electrode of the power storage device of the present embodiment, an electrode configuration known in the art can be used. For example, when the power storage device is a potassium ion secondary battery, the positive electrode active material includes potassium cobaltate (KCoO ₂ ), potassium manganate (KMnO ₂ ), potassium nickelate (KNiO ₂ ), and potassium vanadate (KVO). ₂ ). Potassium-containing transition metal oxides containing one or more transition metals such as potassium ironate (KFeO ₂ ), transition metal sulfides, metal oxides, potassium iron phosphate (KFePO ₄ ), or potassium cobalt phosphate (K _2). CoPO ₄₎ and potassium-containing phosphate compounds containing one or more transition metals such as fluoride cobalt phosphate potassium (KCoPO ₄ F) of potassium fluoride iron phosphate (K ₂ FePO ₄ F) is. The positive electrode may contain a conductive material or a binder.

Further, an oxygen-containing metal salt such as oxygen or potassium oxide may be used as the positive electrode active material. Then, the positive electrode including the positive electrode active material may contain a catalyst that promotes the redox reaction of oxygen in the positive electrode active material. As a preferable positive electrode active material, a transition metal oxide containing an excess of potassium (the transition metal is, for example, manganese, cobalt, iron, nickel, copper) can be exemplified. In addition, a high specific surface area material such as activated carbon can be used in the positive electrode in order to efficiently redox oxygen in the atmosphere and create a reaction field for extracting the capacity.

When the power storage device is a capacitor, the polar electrode material is contained in the positive electrode. As the partial polarity electrode material, the material described for the negative electrode may be adopted. In addition, polar electrode materials include conductive polymers such as polyacene and redox capacitors such as 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) whose capacitance increases due to the adsorption and desorption of anions. You may use the material used for. Further, a material such as potassium manganate having a spinel structure or potassium iron phosphate having an olivine structure, which exhibits a redox capacity at a constant potential of 3 V or more by inserting and removing a cation such as potassium ion, may be contained.

As the conductive material and the binder, the same materials as those for the negative electrode can be used.

As the positive electrode current collector, a rod-shaped body, a plate-shaped body, a foil-shaped body, a net-like body, or the like mainly made of a metal such as nickel, aluminum, titanium, platinum, or stainless steel can be used. The metal is, for example, copper, nickel, aluminum, stainless steel and the like.

(3) Separator The separator used in the power storage device of the present embodiment is not particularly limited as long as it has a function of electrically separating the positive electrode layer and the negative electrode layer, but for example, polyethylene (PE). , Polypropylene (PP), polyester, cellulose, polyamide and other resin-based porous sheets, and non-woven fabrics such as non-woven fabrics and glass fiber non-woven fabrics and other porous insulating materials can be mentioned.

(4) Shape, etc. The shape of the power storage device of the present embodiment is not particularly limited as long as it can store the positive electrode, the negative electrode, and the electrolytic solution, but for example, it is a cylindrical type, a coin type, a flat plate type, or a laminated type. And so on.

Further, the case for storing the power storage device may be an open-air case or a closed-type case.

Although the electrolytic solution and the secondary battery of the present embodiment are suitable for use as a secondary battery, the use as a primary battery is not excluded.

Next, examples of the present invention will be described in detail together with comparative examples.

<Example 1-1>
As an asymmetric imide, 16.0 mol of water was added to 1.0 mol of a single salt of "Compound 1" containing 9 ppm of fluorine ions and 8 ppm of hydrogen, respectively, and heated and melted at 50 ° C. to obtain a saturated aqueous solution of potassium salt at room temperature. The electrolytic solution of was prepared.

<Example 1-2>
As the symmetric imide, potassium bis (trifluoromethanesulfonyl) imide ((CF ₃ SO ₂ ) ₂ NK) shown as “Compound 3” in Table 1 was used. 13.5 mol of water was added to a mixed salt in which 0.8 mol of "Compound 1" which was the same as in Example 1-1 and 0.2 mol of "Compound 3" containing 8 ppm of fluorine ions and 4 ppm of hydrogen were mixed. It was heated and melted at 50 ° C. to prepare an electrolytic solution as a saturated aqueous solution of potassium salt at room temperature.

<Example 1-3>
Add 11.0 mol of water to a mixed salt obtained by mixing 0.6 mol of “Compound 1” same as Example 1-1 and 0.4 mol of “Compound 3” same as Example 1-2, and at 50 ° C. It was heated and melted to prepare an electrolytic solution as a saturated aqueous solution of potassium salt at room temperature.

<Example 1-4>
Add 22.0 mol of water to a mixed salt in which 0.4 mol of "Compound 1" same as Example 1-1 and 0.6 mol of "Compound 3" same as Example 1-2 are mixed, and at 50 ° C. It was heated and melted to prepare an electrolytic solution as a saturated aqueous solution of potassium salt at room temperature.

<Example 1-5>
Add 34.0 mol of water to a mixed salt in which 0.2 mol of "Compound 1" same as Example 1-1 and 0.8 mol of "Compound 3" same as Example 1-2 are mixed, and at 50 ° C. It was heated and melted to prepare an electrolytic solution as a saturated aqueous solution of potassium salt at room temperature.

<Example 1-6>
13.5 mol of water in a mixed salt containing 0.8 mol of "Compound 1" containing 26 ppm of fluorine ions and 11 ppm of hydrogen, and 0.2 mol of "Compound 3" containing 12 ppm of fluorine ions and 30 ppm of hydrogen, respectively. Was added and melted by heating at 50 ° C. to prepare an electrolytic solution as a saturated aqueous solution of potassium salt at room temperature.

<Example 1-7>
Add 11.0 mol of water to a mixed salt in which 0.6 mol of "Compound 1" same as in Example 1-6 and 0.4 mol of "Compound 3" same as in Example 1-6 are mixed, and at 50 ° C. It was heated and melted to prepare an electrolytic solution as a saturated aqueous solution of potassium salt at room temperature.

<Example 1-8>
Add 22.0 mol of water to a mixed salt in which 0.4 mol of "Compound 1" same as in Example 1-6 and 0.6 mol of "Compound 3" same as in Example 1-6 are mixed, and at 50 ° C. It was heated and melted to prepare an electrolytic solution as a saturated aqueous solution of potassium salt at room temperature.

<Comparative example 1>
42.0 mol of water was added to 1 mol of a single salt of "Compound 3" which is a potassium salt having the same symmetric imide structure as in Example 1-2, and the mixture was heated and melted at 50 ° C. to prepare a saturated aqueous solution of the potassium salt at room temperature. An electrolyte was prepared.

FIG. 1 shows the potassium of Examples 1-2-1-5, which is a mixed salt of Compound 1 and Compound 3, Example 1-1, which is a single salt of Compound 1, and Comparative Example 1, which is a single salt of Compound 3. The composition ratio of each salt and the amount of water required for dissolution are shown. In FIG. 1, the vertical axis represents (the number of moles of water) / (the number of moles of total potassium salt), and the horizontal axis represents the molar ratio of compound 1 to the total amount of compound 1 and compound 3. From FIG. 1, in the mixed salt of Examples 1-2 to 1-5 or the single salt having an asymmetric imide structure of Example 1-1, (number of moles of water) / (number of moles of total potassium salt) is 11. It was demonstrated that 0-34.0 became an aqueous solution. On the other hand, it was demonstrated that the single salt having a symmetric imide structure of Comparative Example 1 did not become an aqueous solution until (the number of moles of water) / (the number of moles of total potassium salt) reached 42.0.

<Example 2>
0.12 mol of "Compound 1" same as Example 1-1, 0.08 mol of "Compound 3" same as Example 1-2, and 8 ppm of fluorine ion and 3 ppm of hydrogen as potassium sulfonate, respectively. 2.0 mol of water was added to a mixed salt mixed with 0.8 mol of "Compound 8" and heated and melted at 50 ° C. to prepare an electrolytic solution as a saturated aqueous solution of potassium salt at room temperature.

<Example 3-1>
As an asymmetric imide, 6.0 mol of water was added to 1 mol of a single salt of "Compound 2" containing 7 ppm of fluorine ions and 7 ppm of hydrogen, and the mixture was heated and melted at 50 ° C. and electrolyzed as a saturated aqueous solution of potassium salt at room temperature. The liquid was prepared.

<Example 3-2>
4.0 mol of water was added to a mixed salt obtained by mixing 0.8 mol of the same "Compound 2" as in Example 3-1 and 0.2 mol of the same "Compound 3" as in Example 1-2, and at 50 ° C. It was heated and melted to prepare an electrolytic solution as a saturated aqueous solution of potassium salt at room temperature.

<Example 3-3>
Add 3.0 mol of water to a mixed salt in which 0.6 mol of "Compound 2" same as Example 3-1 and 0.4 mol of "Compound 3" same as Example 1-2 are mixed, and heat at 50 ° C. It was melted to prepare an electrolytic solution as a saturated aqueous solution of potassium salt at room temperature.

<Example 3-4>
9.0 mol of water was added to a mixed salt obtained by mixing 0.4 mol of "Compound 2" same as Example 3-1 and 0.6 mol of "Compound 3" same as Example 1-2, and heated at 50 ° C. It was melted to prepare an electrolytic solution as a saturated aqueous solution of potassium salt at room temperature.

<Example 3-5>
Add 18.0 mol of water to a mixed salt obtained by mixing 0.2 mol of "Compound 2" same as Example 3-1 and 0.8 mol of "Compound 3" same as Example 1-2, and heat at 50 ° C. It was melted to prepare an electrolytic solution as a saturated aqueous solution of potassium salt at room temperature.

FIG. 2 shows Examples 3-2-3-5, which is a mixed salt of Compound 2 and Compound 3, Example 3-1 which is a single salt of the asymmetric imide structure of Compound 2, and the above-mentioned symmetry of Compound 3. The composition ratio of the potassium salt of Comparative Example 1, which is a single salt having an imide structure, and the amount of water required for dissolution are shown. The vertical axis of FIG. 2 is the same as that of FIG. The horizontal axis shows the molar ratio of compound 2 to the total amount of compound 2 and compound 3. From FIG. 2, in the mixed salt of Examples 3-2-3-5 or the single salt having an asymmetric imide structure of Example 3-1 (number of moles of water) / (number of moles of total potassium salt) is 3. It was demonstrated that 0 to 18.0 became an aqueous solution. On the other hand, it was demonstrated that the single salt having a symmetric imide structure of Comparative Example 1 did not become an aqueous solution until (the number of moles of water) / (the number of moles of total potassium salt) reached 42.0.

<Example 4>
Potassium (nonafluorobutanesulfonyl) shown as "Compound 4" in Table 1 containing 0.2 mol of "Compound 1" which is the same as in Example 1-1 and 8 ppm of fluorine ions and 9 ppm of hydrogen as asymmetric imides. Table 1 shows "Compounds" containing 0.2 mol of (trifluoromethanesulfonyl) imide ((C ₄ F ₉ SO ₂ ) (CF ₃ SO ₂ ) NK) and 6 ppm of fluorine ions and 1 ppm of hydrogen as symmetric imides. Add 38.0 mol of water to a mixed salt mixed with 0.6 mol of potassium bis (pentafluoroethanesulfonyl) imide ((C ₂ F ₅ SO ₂ ) ₂ NK) "Compound 5" shown as "5", and add 50. It was heated and melted at ° C. to prepare an electrolytic solution as a saturated aqueous solution of potassium salt at room temperature.

<Example 5>
Add 2.5 mol of water to a mixed salt in which 0.5 mol of "Compound 1" same as Example 1-1 and 0.5 mol of "Compound 2" same as Example 3-1 are mixed, and at 50 ° C. It was heated and melted to prepare an electrolytic solution as a saturated aqueous solution of potassium salt at room temperature.

<Comparative example 2>
The potassium bis (pentafluoroethane) shown as "Compound 6" in Table 1 contains the same "Compound 3" as in Example 3-1 as an asymmetric imide and 8 ppm of fluorine ions and 4 ppm of hydrogen as a symmetric imide. A sulfonyl) imide ((C ₃ F ₇ SO ₂ ) ₂ NK) was used. 120.0 mol of water was added to a mixed salt in which 0.3 mol of "Compound 3" and 0.7 mol of "Compound 6" were mixed, and the mixture was heated and melted at 50 ° C. and electrolyzed as a saturated aqueous solution of potassium salt at room temperature. The liquid was prepared.

<Evaluation test>
18 kinds of electrolytic solutions of Examples 1-1 to 1-8, Comparative Example 1, Example 2, Example 3-1 to Example 3-5, Example 4, Example 5 and Comparative Example 2. The fluorine ion content, the hydrogen content, and the coulombic efficiency (charge / discharge efficiency) of the coin-type potassium ion secondary battery using these electrolytic solutions were measured. Hereinafter, each measurement method will be described.

(1) Fluorine ion content The fluorine ion content of each of compounds 1 to 6 and 8 and the fluorine ion content in the electrolytic solution were measured by an ion chromatograph (ICS-2100 manufactured by Thermo).

(2) Hydrogen content The hydrogen content of each of compounds 1 to 6 and 8 and the hydrogen content in the electrolytic solution were calculated from the measurement results of ¹ H-NMR (AV400M manufactured by Bruker).

(3) The configuration of the secondary battery used for the Coulomb efficiency measurement is as follows. The positive electrode was composed of a positive electrode mixture layer containing 85% by mass of KCoO ₂ , 9% by mass of PVDF and 6% by mass of acetylene black, and a current collector made of titanium. The negative electrode was composed of a negative electrode mixture layer containing 85% by mass of K ₄ Ti ₅ O ₁₂ , 5% by mass of PVDF and 10% by mass of acetylene black, and an aluminum current collector. A glass fiber non-woven fabric filter was used as the separator. The secondary battery was charged and discharged for 10 cycles in the range of 1.7 V to 2.8 V at a temperature of 25 ° C., and the value at the 10th cycle was taken as the Coulomb efficiency.

Table 2 below shows the types and ratios of potassium salts constituting mixed salts or potassium salts having an asymmetric imide structure, molar amounts of water per 1 mol of potassium salts, fluorine ion content, and hydrogen content for 18 types of electrolytic solutions. The quantity and the Coulomb efficiency of the secondary battery using 18 kinds of electrolytic solutions are shown. The types and ratios of potassium salts constituting the mixed salt or potassium salt having an asymmetric imide structure are shown as potassium salt 1 and potassium salt 2, and the types and ratios of sulfonic acid potassium salt are shown as potassium salt 3, respectively.

As is clear from Table 2, in Comparative Example 1, since the potassium salt of the asymmetric imide was not used and only the potassium salt of the symmetric imide was used (1 mol), the molar amount of water per 1 mol of the potassium salt was different. 42.0 mol was required. The Coulomb efficiency of the secondary battery using these electrolytes was not as high as 90.3%.

In Comparative Example 2, since potassium bis (pentafluoroethanesulfonyl) imide ((C ₃ F ₇ SO ₂ ) ₂ NK) of compound 6 was used as the potassium salt of the symmetric imide, the molar amount of water per 1 mol of the potassium salt was used. Needed 120.0 mol. The Coulomb efficiency of the secondary battery using these electrolytes was not so high as 76.8%.

On the other hand, in Examples 1-1 to 1-8, Example 2, Example 3-1 to Example 3-5, Example 4 and Example 5, the potassium salt is the first of the present invention. A single salt or a mixed salt having the requirements of the viewpoint, and the composition ratio thereof meets the requirements of the first viewpoint of the present invention, the molar amount of water per 1 mol of the potassium salt is 2 mol or more and less than 40 mol. It was. Therefore, the Coulomb efficiency of the secondary battery using these electrolytic solutions was as high as 94.2% to 98.5%.

(4) Confirmation of Electrochemical Window The electrolytic solution of Example 2 was subjected to linear soup voltamgram measurement using the following three-pole electrochemical cell, and the electrochemical window was confirmed. The measurement temperature was 25 ° C. and the sweep rate was 0.1 mV / sec.
Working electrode and counter electrode: Platinum or aluminum Reference electrode: Ag / AgCl (saturated KCl)
The above results are shown in FIG. From the results of FIG. 3, when the potential window of the electrolytic solution of Example 2 was a platinum electrode, it was calculated to be 2.4 V, and when the combination of the platinum electrode and the aluminum electrode was calculated to be 3.1 V.

The aqueous electrolytic solution of the present invention can be used for a power storage device such as a potassium ion secondary battery, an electric double layer capacitor or a potassium ion capacitor.

Claims (7)

In an aqueous electrolyte for a power storage device containing water as a solvent
The electrolyte is one or more potassium salts of the asymmetric imide having a perfluoroalkyl group, or a mixed salt of the potassium salt of the asymmetric imide and the potassium salt of the symmetric imide having a perfluoroalkyl group. Or, it is a mixed salt of the potassium salt of the asymmetric imide, the potassium salt of the symmetric imide, and the sulfonic acid potassium salt having a perfluoroalkyl group.
The composition ratio of the one or more potassium salts and the mixed salt is, in molar ratio, (potassium salt of asymmetric imide) _x (potassium salt of symmetric imide) _y (potassium sulfonate) _{1-( x + y)} (where x is 0.1 to 1.0, y is 0 to 0.9, and x + y ≦ 1.0).
The asymmetric imide potassium salt is potassium (pentafluoroethanesulfonyl) (trifluoromethanesulfonyl) imide ((C ₂ F ₅ SO ₂ ) (CF ₃ SO ₂ ) NK), potassium (heptafluoropropanesulfonyl) (trifluoromethanesulfonyl). ) Imide ((C ₃ F ₇ SO ₂ ) (CF ₃ SO ₂ ) NK) or potassium (nonafluorobutane sulfonyl) (trifluoromethanesulfonyl) imide ((C ₄ F ₉ SO ₂ ) (CF ₃ SO ₂ ) NK) And
The symmetric imide potassium salt is potassium bis (trifluoromethanesulfonyl) imide ((CF ₃ SO ₂ ) ₂ NK) or potassium bis (pentafluoroethane sulfonyl) imide ((C ₂ F ₅ SO ₂ ) ₂ NK). ,
The potassium sulfonic acid salt is potassium trifluoromethanesulfonate (CF ₃ SO ₃ K).
The composition of the electrolytic solution is 2 mol or more and less than 40 mol of the solvent amount with respect to 1 mol of the potassium salt of the one or more kinds or the mixed salt. liquid. The aqueous electrolyte solution for a power storage device according to claim 1, wherein the melting point of one or more potassium salts is 230 ° C. or lower. The water-based electrolyte for a power storage device according to claim 1 or 2, wherein the fluorine ion content in the water-based electrolyte is 10 ppm or less, and the hydrogen content in the water-based electrolyte is 10 ppm or less. The aqueous electrolyte solution for a power storage device according to any one of claims 1 to 3, wherein the power storage device is a potassium ion secondary battery, an electric double layer capacitor, or a potassium ion capacitor. The perfluoroalkyl group according to claim 1, which is used as an electrolyte for the aqueous electrolyte solution for a power storage device according to any one of claims 1 to 4, and has a fluorine ion content of 10 ppm or less and a hydrogen content of 10 ppm or less. One or more potassium salts of asymmetric imides having. The perfluoroalkyl according to claim 1, which is used as an electrolyte for the aqueous electrolyte solution for a power storage device according to any one of claims 1 to 4, and has a fluorine ion content of 10 ppm or less and a hydrogen content of 10 ppm or less. A mixed salt of a potassium salt of an asymmetric imide having a group and a potassium salt of a symmetric imide having a perfluoroalkyl group according to claim 1. The perfluoroalkyl according to claim 1, which is used as an electrolyte for the aqueous electrolyte solution for a power storage device according to any one of claims 1 to 4, and has a fluorine ion content of 10 ppm or less and a hydrogen content of 10 ppm or less. A mixed salt of a potassium salt of an asymmetric imide having a group, a potassium salt of a symmetric imide having a perfluoroalkyl group according to claim 1, and a sulfonic acid potassium salt having a perfluoroalkyl group according to claim 1.

Images