Positive electrode active material for aqueous potassium ion batteries and aqueous potassium ion secondary batteries

Abstract

To provide a novel positive electrode active material for an aqueous potassium ion battery and a novel potassium ion secondary battery.SOLUTION: A positive electrode active material for an aqueous potassium ion battery according to the present disclosure is represented by the general formula LiNixMnyCo1-x-yO2, where 0≤x<1, 0≤y<1, and x+y<1. The aqueous potassium ion secondary battery according to the present disclosure includes a positive electrode active material which is represented by the general formula LiNixMnyCo1-x-yO2, where 0≤x<1, 0≤y<1, and x+y<1. The aqueous potassium ion secondary battery according to the present disclosure includes an aqueous electrolyte, and the pH of the aqueous electrolyte is 4.0 to 13.0, and the aqueous electrolyte may include an aqueous solvent and potassium pyrophosphate dissolved in the aqueous solvent.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to a positive electrode active material for an aqueous potassium ion battery and an aqueous potassium ion secondary battery. [Background technology]

[0002] Patent Document 1 discloses an aqueous electrolyte solution containing water and potassium pyrophosphate dissolved in water as an electrolyte solution for use in an aqueous potassium ion battery. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-220294 A Summary of the Invention [Problem to be solved by the invention]

[0004] For example, a cyanide-based compound called Prussian blue has been used as a positive electrode active material for potassium ion secondary batteries. Cyanide-based compounds used as positive electrode active materials have the risk of generating cyanide when the battery malfunctions, and their large molecular structure means that their energy density is insufficient.

[0005] An object of the present disclosure is to provide a novel positive electrode active material for an aqueous potassium ion battery, and a novel potassium ion secondary battery. [Means for solving the problem]

[0006] The present inventors have found that the above object can be achieved by the following means: Aspect 1 General formula LiNi x Mn y Co 1-x-y A positive electrode active material for an aqueous potassium ion battery, represented by O2, where 0≦x<1, 0≦y<1, and x+y<1. Aspect 2 General formula LiNi x Mn y Co 1-x-y O2, where 0≦x<1, 0≦y<1, and x+y<1; Aqueous potassium-ion secondary battery. Aspect 3 Contains an aqueous electrolyte, and The aqueous electrolyte has a pH of 7.0 to 13.0; and The aqueous electrolyte contains an aqueous solvent and potassium pyrophosphate dissolved in the aqueous solvent. 3. The water-based potassium ion secondary battery according to embodiment 2. Aspect 4 4. The water-based potassium ion secondary battery according to aspect 3, wherein the potassium pyrophosphate is dissolved in the water-based solvent at a concentration of 2.0 mol / kg or more. Aspect 5 5. The water-based potassium ion secondary battery according to aspect 4, wherein the potassium pyrophosphate is dissolved in the water-based solvent at a concentration of 5.0 mol / kg or more. Effect of the Invention

[0007] According to the present disclosure, it is possible to provide a novel positive electrode active material for an aqueous potassium ion battery, and a novel potassium ion secondary battery. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic diagram illustrating a potassium ion secondary battery according to one embodiment of the present disclosure. [Diagram 2] 2 is a graph showing charge / discharge curves of an evaluation cell of Example 1. [Diagram 3] 1 is a graph showing charge / discharge curves of an evaluation cell of Example 2. [Figure 4] 1 is a graph showing charge / discharge curves of an evaluation cell of Comparative Example 1. [Diagram 5] 1 is a graph showing the relationship between potassium pyrophosphate concentration and ionic conductivity. [Figure 6]1 is a graph showing the relationship between potassium pyrophosphate concentration and pH. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, embodiments of the present disclosure will be described in detail. Note that the present disclosure is not limited to the following embodiments, and various modifications can be made within the scope of the present disclosure.

[0010] 1. Positive electrode active material for aqueous potassium ion batteries The positive electrode active material for an aqueous potassium ion battery according to the present disclosure (hereinafter referred to as the "positive electrode active material according to the present disclosure") has the general formula LiNi x Mn y Co 1-x-y O2, where 0≦x<1, 0≦y<1, and x+y<1.

[0011] Here, x and y are preferably 0 or 0.30 to 0.36. x and y may be 0.30 or more, 0.31 or more, 0.32 or more, or 0.33 or more, and 0.36 or less, 0.35 or less, or 0.34 or less. x and y are particularly preferably 0 or 1 / 3.

[0012] The shape of the positive electrode active material of the present disclosure may be any shape that is common to positive electrode active materials for batteries. The positive electrode active material of the present disclosure may be, for example, particulate. In this case, the particle size is not particularly limited, and an appropriate size may be selected according to the design of the battery.

[0013] The positive electrode active material of the present disclosure may have a primary particle diameter of 1 nm or more, 5 nm or more, 10 nm or more, or 50 nm or more, and may be 500 μm or less, 100 μm or less, 50 μm or less, 30 μm or less, or 10 μm or less. In addition, the positive electrode active material of the present disclosure may have primary particles aggregated together to form secondary particles. In this case, the particle diameter of the secondary particles is not particularly limited, and may be, for example, 100 nm or more, 500 nm or more, or 1 μm or more, and may be 1000 μm or less, 500 μm or less, 100 μm or less, 50 μm or less, 30 μm or less, or 20 μm or less.

[0014] Conventionally, almost no positive electrode active material is known that can be charged and discharged in an electrolyte solution in which a potassium compound is dissolved, whether the electrolyte solution is non-aqueous or aqueous.

[0015] The present disclosure has an aspect of providing a novel material as a positive electrode active material in an aqueous potassium ion secondary battery. x Mn y Co 1-x-y The compound represented by the formula O2, where 0≦x<1, 0≦y<1, and x+y<1, has been found to be useful as a positive electrode active material for aqueous potassium ion secondary batteries.

[0016] 2. Water-based potassium-ion secondary battery The aqueous potassium ion secondary battery disclosed herein is a battery having the general formula LiNi x Mn y Co 1-x-y O2, where 0≦x<1, 0≦y<1, and x+y<1.

[0017] In addition, the aqueous potassium ion secondary battery of the present disclosure preferably contains an aqueous electrolyte solution, the aqueous electrolyte solution has a pH of 7.0 to 13.0, and the aqueous electrolyte solution preferably contains an aqueous solvent and potassium pyrophosphate dissolved in the aqueous solvent.

[0018] As mentioned in "1. Positive electrode active material for aqueous potassium ion batteries" above, almost no positive electrode active material is known that can be charged and discharged in an electrolyte solution in which a potassium compound is dissolved. In particular, the optimal combinations of electrolyte solutions and positive electrode active materials that allow charging and discharging are limited to only a very small portion of the infinite number of combinations of electrolyte solutions and positive electrode active materials, and even a slight change in the crystal structure of the active material, the components of the electrolyte solution, the potential window, etc. makes charging and discharging impossible. In other words, the optimal combination cannot be found unless the electrolyte solution and the positive electrode active material are actually combined and evaluated.

[0019] The technology disclosed herein was developed by finding, through trial and error, a new combination that enables charging and discharging from the infinite number of combinations of electrolytes and positive electrode active materials, and is not something that could easily be conceived of using conventional technology.

[0020] 2-1.Aqueous electrolyte The aqueous potassium ion secondary battery of the present disclosure contains an aqueous electrolyte solution, and the aqueous electrolyte solution has a pH of 7.0 to 13.0. The aqueous electrolyte solution can contain an aqueous solvent and potassium pyrophosphate dissolved in the aqueous solvent.

[0021] The pH of the aqueous electrolyte may be 7.0 or more, 8.0 or more, 9.0 or more, or 10.0 or more, and may be 13.0 or less, 12.5 or less, 12.0 or less, or 11.5 or less.

[0022] 2-1-1. Water-based solvents In the present disclosure, an "aqueous solvent" refers to a solvent that contains water. The aqueous solvent may contain water as a main component. That is, based on the total amount of the solvent constituting the electrolytic solution (100 mol%), water may account for 50 mol% or more, 70 mol% or more, 90 mol% or more, or 95 mol% or more. There is no particular upper limit to the proportion of water in the aqueous solvent, and the aqueous solvent may be 100 mol%, that is, the entire amount of the aqueous solvent may be water.

[0023] The aqueous solvent may consist of only water, but may further contain other components, such as one or more organic solvents selected from ethers, carbonates, nitriles, alcohols, ketones, amines, amides, sulfur compounds, and hydrocarbons. The solvent other than water may account for 50 mol% or less, 30 mol% or less, 10 mol% or less, or 5 mol% or less based on the total amount of the aqueous solvent (100 mol%).

[0024] 2-1-2. Potassium pyrophosphate The term "dissolved" in a solvent does not mean that the potassium pyrophosphate is not completely ionized into potassium ions and pyrophosphate ions in an aqueous electrolyte. + , P2O7 4- , KP2O7 3- , K2P2O7 2- , K3P2O7 - or may exist as an association of these ions. In addition, in the aqueous electrolyte, the "dissolved potassium pyrophosphate" does not have to be derived from a salt of potassium and pyrophosphate (K4P2O7) (obtained by adding K4P2O7 to water). For example, the aqueous electrolyte also includes a solution in which a potassium ion source (KOH, CH3COOK, etc.) and a pyrophosphate ion source (H4P2O7, etc.) are separately added to water and dissolved therein, resulting in the formation of the above ions or associations in the water.

[0025] The concentration of potassium pyrophosphate in the aqueous electrolyte is not particularly limited and may be appropriately selected depending on the intended performance of the battery.

[0026] In the aqueous electrolyte, potassium pyrophosphate may be dissolved in the aqueous solvent at a concentration of 2.0 mol / kg or more, i.e., 2.0 mol or more per 1.0 kg of aqueous solvent, or at a concentration of 5.0 mol / kg or more, i.e., 5.0 mol or more per 1.0 kg of aqueous solvent.

[0027] According to the new findings of the present inventor, the higher the concentration of potassium pyrophosphate in the aqueous electrolyte, the smaller the hysteresis of the positive electrode active material during charging and discharging, and the more likely it is to obtain high performance as a potassium ion secondary battery. In addition, the higher the concentration of potassium pyrophosphate in the aqueous electrolyte, the lower the overvoltage, and the more likely it is to develop a good charge and discharge plateau. Furthermore, it is considered that the higher the concentration of potassium pyrophosphate in the aqueous electrolyte, the more likely it is that pyrophosphate ions and potassium ions approach each other to form an association. Therefore, for example, it is considered that pyrophosphate ions are easily dragged by potassium ions to move to the negative electrode side when charging a potassium ion secondary battery. It is considered that the pyrophosphate ions that reach the negative electrode are decomposed at the high work function site on the surface of the negative electrode, and a coating is formed on the surface of the negative electrode, and as a result, direct contact between the aqueous electrolyte and the high work function site on the surface of the negative electrode is suppressed, and electrolysis of the aqueous electrolyte is easily suppressed.

[0028] The concentration of "dissolved potassium pyrophosphate" in the aqueous electrolyte can be determined as follows. For example, elements and ions contained in the aqueous electrolyte are determined by elemental analysis or ion analysis, and the potassium ion concentration and pyrophosphate ion concentration in the aqueous electrolyte are determined, and the determined ion concentration is converted into the potassium pyrophosphate concentration. Alternatively, the solvent is removed from the aqueous electrolyte, and the solid content is chemically analyzed and converted into the potassium pyrophosphate concentration.

[0029] In the aqueous electrolyte, the entire amount of potassium ions contained in the electrolyte does not have to be calculated as "dissolved potassium pyrophosphate". That is, the aqueous electrolyte may contain more potassium ions than the concentration that can be calculated as potassium pyrophosphate. For example, when producing an aqueous electrolyte, if a potassium ion source other than the potassium pyrophosphate source (e.g., KOH, CH3COOK, K3PO4, etc.) is added and dissolved in water together with the potassium pyrophosphate source, the aqueous electrolyte will contain more potassium ions than the concentration that can be calculated as potassium pyrophosphate.

[0030] The aqueous electrolyte may contain cations other than potassium ions. For example, alkali metal ions, alkaline earth metal ions, transition metal ions, etc. other than potassium ions may be contained. In addition, the aqueous electrolyte may contain pyrophosphate ions (as described above, P2O7 4- In addition, KP2O7 3- , K2P2O7 2- , K3P2O7 - The anion may contain anions other than those mentioned above (which may be present in a state combined with a cation, such as anions derived from other electrolytes, which will be described later).

[0031] Other electrolytes may be dissolved in the aqueous electrolyte solution of the present disclosure. For example, KPF6, KBF4, K2SO4, KNO3, CH3COOK, (CF3SO2)2NK, KCF3SO3, (FSO2)2NK, K2HPO4, KH2PO4, etc. may be dissolved. The other electrolytes may account for 50 mol% or less, 30 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less based on the total amount of electrolytes dissolved in the electrolyte solution (100 mol%).

[0032] 2-1-3.Other ingredients The aqueous electrolyte may contain, in addition to the aqueous solvent and electrolyte described above, an acid or hydroxide for adjusting the pH of the aqueous electrolyte, and may also contain various additives.

[0033] 2-2.Other configurations The potassium ion secondary battery of the present disclosure is not particularly limited as long as it has the above-mentioned positive electrode active material and aqueous electrolyte solution. The potassium ion secondary battery of the present disclosure is configured so that the above-mentioned positive electrode active material is in contact with the aqueous electrolyte solution.

[0034] FIG. 1 is a schematic diagram showing a configuration of a potassium ion secondary battery 100 according to a first embodiment of the present disclosure. As shown in FIG. 1, the potassium ion secondary battery 100 may include a positive electrode 10, an electrolyte layer 20, and a negative electrode 30. The positive electrode 10 may include a positive electrode active material layer 11 and a positive electrode current collector 12, and the negative electrode 30 may include a negative electrode active material layer 31 and a negative electrode current collector 32. In this case, the positive electrode active material layer 11 may contain the above positive electrode active material. The positive electrode 10, the electrolyte layer 20, and the negative electrode 30 may all contain the above aqueous electrolyte solution.

[0035] 2-2-1. Positive electrode The positive electrode may have a known configuration, except that it contains the above-mentioned positive electrode active material and aqueous electrolyte solution. For example, the positive electrode may include a positive electrode active material layer and a positive electrode current collector.

[0036] 2-2-1-1. Positive electrode active material layer The positive electrode active material layer contains the positive electrode active material described above, and may further contain an optional conductive assistant, binder, etc. The thickness of the positive electrode active material layer is not particularly limited, and may be, for example, 0.1 μm or more or 1 μm or more, and may be 1 mm or less or 100 μm or less.

[0037] The amount of the positive electrode active material contained in the positive electrode active material layer is not particularly limited. For example, the positive electrode active material may be contained in an amount of 20 mass% or more, 40 mass% or more, 60 mass% or more, or 70 mass% or more, or 99 mass% or less, 97 mass% or less, or 95 mass% or less, based on the entire positive electrode active material layer (100 mass%).

[0038] The conductive assistant optionally contained in the positive electrode active material layer can be any of those known as conductive assistants used in potassium ion secondary batteries. For example, carbon materials can be used. Specifically, they are ketjen black (KB), vapor grown carbon fiber (VGCF), acetylene black (AB), carbon nanotube (CNT), carbon nanofiber (CNF), carbon black, coke, graphite, etc. Alternatively, they may be metal materials that can withstand the environment during use of the battery. Only one type of conductive assistant may be used alone, or two or more types may be used in combination. The conductive assistant may be in various shapes, such as powder and fiber. The amount of the conductive assistant contained in the positive electrode active material layer is not particularly limited.

[0039] The binder optionally contained in the positive electrode active material layer may be any binder known to be used in potassium ion secondary batteries. For example, styrene butadiene rubber (SBR)-based binders, carboxymethyl cellulose (CMC)-based binders, acrylonitrile butadiene rubber (ABR)-based binders, butadiene rubber (BR)-based binders, polyvinylidene fluoride (PVDF)-based binders, polytetrafluoroethylene (PTFE)-based binders, etc. Only one type of binder may be used alone, or two or more types may be used in combination. The amount of binder contained in the positive electrode active material layer is not particularly limited.

[0040] 2-2-1-2. Positive electrode current collector The positive electrode current collector may be made of a known metal that can be used as a positive electrode current collector for a potassium ion secondary battery. Examples of such metals include metal materials containing at least one element selected from the group consisting of Cu, Ni, Al, V, Au, Pt, Mg, Fe, Ti, Pb, Co, Cr, Zn, Ge, In, Sn, and Zr. The form of the positive electrode current collector is not particularly limited. It may take various forms such as a foil, a mesh, or a porous form. The above metal may be vapor-deposited or plated on the surface of a substrate.

[0041] 2-2-2. Electrolyte layer In the potassium ion secondary battery, for example, an electrolyte layer may be disposed between the positive electrode active material layer and the negative electrode active material layer. The electrolyte layer may be composed of a separator and the above-mentioned aqueous electrolyte solution. As the separator, a separator known to be used in secondary batteries (for example, nickel-metal hydride batteries, zinc-air batteries, etc.) may be adopted. For example, the separator may be a hydrophilic one such as a nonwoven fabric made of cellulose. The thickness of the separator is not particularly limited, and may be, for example, 5 μm or more and 1 mm or less.

[0042] 2-2-3. Negative electrode The negative electrode may have a known structure as a negative electrode for a potassium ion secondary battery, for example, the negative electrode may include a negative electrode active material layer and a negative electrode current collector.

[0043] 2-2-3-1. Negative electrode active material layer The negative electrode active material layer includes a negative electrode active material. The negative electrode active material layer may include a conductive assistant and a binder in addition to the negative electrode active material. The thickness of the negative electrode active material layer is not particularly limited, but may be, for example, 0.1 μm or more or 1 μm or more, and may be 1 mm or less or 100 μm or less.

[0044] The negative electrode active material contained in the negative electrode active material layer may be selected from active materials having a charge / discharge potential of carrier ions lower than that of the positive electrode active material, taking into consideration the potential window of the aqueous electrolyte. For example, potassium-transition metal complex oxides, titanium oxide, metal sulfides such as Mo6S8, elemental sulfur, KTi2(PO4)3, NASICON type compounds, etc. Only one type of negative electrode active material may be used alone, or two or more types may be used in combination.

[0045] The shape of the negative electrode active material is not particularly limited, and may be, for example, particulate. In this case, the particle size is not particularly limited, and an appropriate size may be selected depending on the design of the battery.

[0046] The negative electrode active material may have a primary particle diameter of 1 nm or more, 5 nm or more, 10 nm or more, 50 nm or more, or 100 nm or more, and may be 500 μm or less, 100 μm or less, 50 μm or less, 30 μm or less, or 10 μm or less. The negative electrode active material may also have primary particles aggregated together to form secondary particles. In this case, the particle diameter of the secondary particles is not particularly limited, and may be, for example, 100 nm or more, 500 nm or more, or 1 μm or more, and may be 1000 μm or less, 500 μm or less, 100 μm or less, 50 μm or less, 30 μm or less, or 20 μm or less.

[0047] The amount of the negative electrode active material contained in the negative electrode active material layer is not particularly limited. For example, the negative electrode active material may be contained in an amount of 20 mass% or more, 40 mass% or more, 60 mass% or more, or 70 mass% or more, or 99 mass% or less, 97 mass% or less, or 95 mass% or less, based on the entire negative electrode active material layer (100 mass%).

[0048] The type of conductive assistant and binder optionally contained in the negative electrode active material layer is not particularly limited, and may be appropriately selected from those exemplified as the conductive assistant and binder optionally contained in the positive electrode active material layer. The amount of conductive assistant and binder contained in the negative electrode active material layer is not particularly limited.

[0049] 2-2-3-2. Negative electrode current collector The negative electrode current collector may be made of a known metal that can be used as a negative electrode current collector for a potassium ion secondary battery. Examples of such metals include metal materials containing at least one element selected from the group consisting of Cu, Ni, Al, V, Au, Pt, Mg, Fe, Ti, Pb, Co, Cr, Zn, Ge, In, Sn, and Zr. In particular, when considering stability in an aqueous electrolyte, the negative electrode current collector may contain at least one element selected from the group consisting of Al, Ti, Pb, Zn, Sn, Mg, Zr, and In, may contain at least one element selected from the group consisting of Ti, Pb, Zn, Sn, Mg, Zr, and In, or may contain Ti. All of Al, Ti, Pb, Zn, Sn, Mg, Zr, and In have low work functions, and it is considered that electrolysis of the aqueous electrolyte is unlikely to occur even if they come into contact with the aqueous electrolyte.

[0050] The form of the negative electrode current collector is not particularly limited. It may be in various forms such as foil, mesh, porous, etc. It may be a substrate having the above-mentioned metal plated or vapor-deposited on its surface.

[0051] The surface of the negative electrode current collector may be coated with a carbon material. That is, the negative electrode may further include a negative electrode current collector and a coating layer provided on the surface of the negative electrode current collector on which the aqueous electrolyte is disposed (between the negative electrode current collector and the negative electrode active material layer), and the coating layer may contain a carbon material. Examples of the carbon material include Ketjen Black (KB), vapor grown carbon fiber (VGCF), acetylene black (AB), carbon nanotubes (CNT), carbon nanofibers (CNF), carbon black, coke, graphite, etc.

[0052] The thickness of the coating layer is not particularly limited. The coating layer may be provided on the entire surface of the negative electrode current collector, or may be provided on only a part of the surface.

[0053] The coating layer may contain a binder for binding the carbon materials together and between the carbon materials and the negative electrode current collector.

[0054] When a coating layer containing a carbon material is provided on the surface of a negative electrode current collector, the withstand voltage of the reduction side of the aqueous electrolyte is likely to be improved. Since the edge portion of the carbon material has high reaction activity, it is considered that the adsorption and decomposition of anions contained in the aqueous electrolyte, such as pyrophosphate ions, are likely to occur, and a coating is likely to accumulate. Therefore, when the above-mentioned aqueous electrolyte is used in a potassium ion secondary battery, the edge portion of the carbon material is inactivated, and the electrolysis of the aqueous electrolyte at the edge portion can be suppressed, and as a result, the reduction side potential window of the aqueous electrolyte is considered to be expanded.

[0055] In addition to the above configuration, the potassium ion secondary battery may have obvious configurations for a battery, such as terminals and a battery case.

[0056] A potassium ion secondary battery having the above configuration can be manufactured, for example, by forming a positive electrode active material layer on the surface of a positive electrode current collector in a dry or wet manner to obtain a positive electrode, forming a negative electrode active material layer on the surface of a negative electrode current collector in a dry or wet manner to obtain a negative electrode, and disposing a separator between the positive electrode and the negative electrode and impregnating them with an aqueous electrolyte solution. EXAMPLES

[0057] Examples 1 and 2, and Comparative Example 1 Example 1 (Preparation of positive electrode) LiNi as a positive electrode active material 1 / 3 Mn 1 / 3 Co 1 / 3 O2, acetylene black as a conductive assistant, and PVDF and CMC as binders were mixed in a mass ratio of 85:10:4.5:0.5 to prepare a positive electrode active material mixture. The positive electrode active material mixture was uniformly coated on the surface of Ni foil using a doctor blade, and then dried to obtain a positive electrode for evaluation. That is, the positive electrode was a positive electrode active material layer containing a positive electrode active material and the like formed on the surface of Ni foil as a positive electrode current collector.

[0058] (Preparation of evaluation cells) An evaluation cell having the following configuration was fabricated. Cell: SBIA (manufactured by EC Frontier) Working electrode: the above positive electrode, opening area 2 cm 2 Counter electrode: Pt foil Reference electrode:Ag / AgCl Aqueous electrolyte: 5.0 mol / kg potassium pyrophosphate (K4P2O7) aqueous solution

[0059] (Cell evaluation) The evaluation cell was charged and discharged under the following conditions to evaluate the charge and discharge characteristics. Sweep speed: 1mV / s Sweep range: -0.4~1.0V vs. Ag / AgCl Measurement temperature: 25℃ Number of cycles: 2

[0060] The evaluation results are shown in Fig. 2. Fig. 2 is a graph showing the charge / discharge curves of the evaluation cell of Example 1.

[0061] As shown in Figure 2, LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 It was confirmed that O2 can electrochemically exchange potassium ions by using a 5.0 mol / kg aqueous solution of potassium pyrophosphate as the electrolyte.

[0062] Example 2 An evaluation cell of Example 2 was produced and evaluated in the same manner as in Example 1, except that LiCoO2 was used as the positive electrode active material.

[0063] The evaluation results are shown in Fig. 3. Fig. 3 is a graph showing the charge / discharge curves of the evaluation cell of Example 2.

[0064] As shown in FIG. 3, it was confirmed that an electrochemical ion exchange reaction of potassium ions was also possible for LiCoO2 by using a 5.0 mol / kg aqueous solution of potassium pyrophosphate as the electrolyte.

[0065] Comparative Example 1 LiNi as the positive electrode active material 0.5 Mn 0.5 An evaluation cell of Comparative Example 1 was produced and evaluated in the same manner as in Example 1, except that O2 was used.

[0066] The evaluation results are shown in Fig. 4. Fig. 4 is a graph showing the charge / discharge curves of the evaluation cell of Comparative Example 1.

[0067] As shown in Figure 4, LiNi 0.5 Mn 0.5 O2 had no charge / discharge activity.

[0068] Reference Examples 1 and 2 <Reference example 1> The relationship between the concentration of the potassium pyrophosphate aqueous solution and the potassium ion conductivity is shown in FIG.

[0069] When the potassium pyrophosphate concentration in the aqueous potassium pyrophosphate solution was in the range of more than 0 mol / kg to 8 mol / kg or less, the aqueous potassium pyrophosphate solution had potassium ion conductivity.

[0070] <Reference example 2> The relationship between the concentration and pH of the aqueous potassium pyrophosphate solution is shown in FIG.

[0071] When the potassium pyrophosphate concentration in the aqueous potassium pyrophosphate solution was more than 0 mol / kg and not more than 8 mol / kg, the pH of the aqueous potassium pyrophosphate solution was 10.0 to 12.0. [Explanation of symbols]

[0072] 10 positive electrode 11 Cathode active material layer 12 Positive electrode current collector 20 Electrolyte layer 30 negative electrode 31 Negative electrode active material layer 32 Negative electrode current collector 100 Potassium ion secondary battery

Claims

1. General formula LiNi ｘ Mn ｙ Co １－ｘ－ｙ O ２ wherein 0≦x≦0.36 and 0≦y≦0.36; and Contains an aqueous electrolyte, and The aqueous electrolyte has a pH of 7.0 to 13.0; and The aqueous electrolyte contains an aqueous solvent and potassium pyrophosphate dissolved in the aqueous solvent. Aqueous potassium-ion secondary battery.

2. 2. The aqueous potassium ion secondary battery according to claim 1, wherein the potassium pyrophosphate is dissolved in the aqueous solvent at a concentration of 2.0 mol / kg or more.

3. 3. The aqueous potassium ion secondary battery according to claim 2, wherein the potassium pyrophosphate is dissolved in the aqueous solvent at a concentration of 5.0 mol / kg or more.

Images