Cathode active material and aqueous battery

Abstract

To provide a novel material that can function as a cathode active material in an aqueous battery having an aqueous electrolyte containing potassium polyphosphate.SOLUTION: A cathode active material according to the present disclosure has a monoclinic crystal structure belonging to the space group C2 / m, and has a composition represented by the formula AxMn1-yBiyO2.zH2O (A: one or both of Na and K, 0<x<1, 0<y<1, 0<z<2).SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] This application discloses a positive electrode active material and an aqueous battery. [Background technology]

[0002] Patent Document 1 describes K as a positive electrode material for non-aqueous potassium ion batteries. x M y Mn 1-y O 2 A layered potassium metal oxide represented by the formula (M: Co, Fe, Ni, Ti, Cr, V, Cu, Zr, Sb or a combination thereof) is disclosed. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2020 / 237386 Summary of the Invention [Problem to be solved by the invention]

[0004] There is a need for new positive electrode active materials that can be used in aqueous batteries having aqueous electrolytes that include potassium polyphosphate. [Means for solving the problem]

[0005] The present application discloses the following aspects as means for solving the above problems. <Aspect 1> A positive electrode active material, It has a monoclinic crystal structure belonging to the space group C2 / m, Composition formula: A x Mn 1-y Bi y O 2 zH 2 O A: Na and / or K 0 <x<1 0 <y<1 0 <z<2 and Used in an aqueous battery having an aqueous electrolyte solution containing water and potassium polyphosphate dissolved in the water, Cathode active material. <Aspect 2> An aqueous battery, A positive electrode active material layer, an aqueous electrolyte solution, and a negative electrode active material layer are included, the positive electrode active material layer comprises the positive electrode active material of aspect 1, The aqueous electrolyte solution contains water and potassium polyphosphate dissolved in the water. water-based battery. <Aspect 3> 3. The aqueous battery of embodiment 2, The aqueous electrolyte solution contains water and potassium polyphosphate dissolved in the water at a concentration of 2.0 mol or more per kg of the water. water-based battery. <Aspect 4> 4. The aqueous battery of embodiment 2 or 3, The aqueous electrolyte solution contains water and potassium pyrophosphate as the potassium polyphosphate dissolved in the water. water-based battery. <Aspect 5> The aqueous battery according to any one of aspects 2 to 4, The pH of the aqueous electrolyte is 3 or more and 13 or less. water-based battery. Effect of the Invention

[0006] The positive electrode active material of the present disclosure can be used as a positive electrode active material in an aqueous battery having an aqueous electrolyte solution containing potassium polyphosphate. [Brief description of the drawings]

[0007] [Figure 1] 1 shows a schematic diagram of an example of the configuration of an aqueous battery. [Diagram 2] 1 shows X-ray diffraction patterns of positive electrode active materials according to examples and comparative examples. [Diagram 3] 4 shows discharge curves of cells according to an embodiment and a comparative example. [Figure 4]It shows the relationship between the potassium pyrophosphate concentration in the aqueous electrolyte and the ionic conductivity of the aqueous electrolyte. [Diagram 5] It shows the relationship between the potassium pyrophosphate concentration in the aqueous electrolyte and the pH of the aqueous electrolyte.

Mode for Carrying Out the Invention

[0008] 1. Positive Electrode Active Material There are few reported examples of inorganic compounds that can function as active materials in an aqueous electrolyte that is not a strong alkali containing potassium ions. There is little knowledge about what inorganic compounds can be used as active materials, and currently, the activity of numerous inorganic compounds is being confirmed one by one. In particular, research on aqueous batteries using an aqueous electrolyte containing potassium polyphosphate is still in the development stage, and there are even fewer reported examples of active materials. In this regard, there is a need for a new positive electrode active material that can be used in an aqueous battery having an aqueous electrolyte containing potassium polyphosphate. For example, the positive electrode active material for an aqueous battery having an aqueous electrolyte containing potassium polyphosphate has room for improvement in terms of discharge capacity. The inventors of the present invention searched for a novel material that functions as a positive electrode active material for an aqueous battery having an aqueous electrolyte containing potassium polyphosphate, and found that the composite oxide disclosed below has a high discharge capacity as a positive electrode active material for an aqueous battery having an aqueous electrolyte containing potassium polyphosphate.

[0009] The positive electrode active material of the present disclosure has a monoclinic crystal structure belonging to the space group C2 / m, and the composition formula: A x Mn 1-y Bi y O 2 ·zH 2 O (A: one or both of Na and K, 0 < x < 1, 0 < y < 1, 0 < z < 2), and is used in an aqueous battery having an aqueous electrolyte containing water and potassium polyphosphate dissolved in the water. The positive electrode active material of the present disclosure can occlude and release ions contained in the aqueous electrolyte as charge compensation ions by having such a crystal structure and composition.

[0010] 1.1 Crystal Structure The positive electrode active material of the present disclosure has a monoclinic crystal structure belonging to the space group C2 / m. The positive electrode active material of the present disclosure may have, for example, diffraction peaks at least at 2θ=12.2°±0.5° and 24.5°±1.0° in an X-ray diffraction pattern using CuKα as a radiation source. The positions of the diffraction peaks may vary depending on the molar ratio of Mn and Bi constituting the positive electrode active material.

[0011] In one aspect, the positive electrode active material of the present disclosure can be said to be a layered manganese oxide having a monoclinic crystal structure in which Bi is present. It is believed that the presence of Bi in the layered manganese oxide having a monoclinic crystal structure stabilizes the crystal structure and / or changes the electronic state within the crystal. This makes it easier for charge compensation ions to enter between the layers of the crystal structure, and is believed to increase the discharge capacity as a positive electrode active material.

[0012] 1.2 Composition The positive electrode active material of the present disclosure has the composition formula: A x Mn 1-y Bi y O 2 zH 2O. Here, A is one or both of Na and K. In the positive electrode active material of the present disclosure, even when A contains Na or K, charge compensation ions can be inserted and removed. x may be greater than 0, 0.01 or more, 0.02 or more, 0.03 or more, 0.04 or more, or 0.05 or more, or may be less than 1, 0.75 or less, 0.50 or less, 0.25 or less, 0.20 or less, 0.15 or less, 0.10 or less, 0.09 or less, 0.08 or less, 0.07 or less, 0.06 or less, or 0.05 or less. y may be greater than 0 and may be 0.01 or greater, 0.02 or greater, 0.03 or greater, or 0.04 or greater, or may be less than 1 and 0.75 or less, 0.50 or less, 0.25 or less, 0.20 or less, 0.15 or less, 0.10 or less, 0.09 or less, 0.08 or less, 0.07 or less, 0.06 or less, 0.05 or less, or 0.04 or less. z may be greater than 0 and may be 0.2 or greater, 0.4 or greater, 0.8 or greater, 1.0 or greater, 1.2 or greater, 1.4 or greater, 1.6 or greater, 1.8 or greater, or may be less than 2 and 1.8 or less, 1.6 or less, 1.4 or less, 1.2 or less, 1.0 or less, 0.8 or less, 0.4 or less, or 0.2 or less.

[0013] 1.3 Supplementary information on element arrangement in crystal structure The arrangement of elements in the positive electrode active material of the present disclosure may be any arrangement as long as it has the above-mentioned composition and the above-mentioned crystal structure. 2 O may be present between layers composed of Mn-O octahedra, for example. In the positive electrode active material of the present disclosure, Bi may be present within the space of the above-mentioned crystal structure, for example. Alternatively, for example, a part of Mn may be substituted with Bi at the Mn site of the positive electrode active material of the present disclosure (the Mn site of the monoclinic layered manganese oxide).

[0014] 1.4 Shape The shape of the positive electrode active material of the present disclosure may be a general shape as a positive electrode active material for a battery. 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. The positive electrode active material of the present disclosure may be a primary particle, or a secondary particle in which a plurality of primary particles are aggregated. The average particle size D50 of the positive electrode active material of the present disclosure may be, for example, 1 nm or more and 1000 μm or less, the lower limit may be 5 nm or more or 10 nm or more, and the upper limit may be 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less. The average particle size D50 in the present application is the particle size (median size) at an integrated value of 50% in the volume-based particle size distribution obtained by a laser diffraction / scattering method.

[0015] 2. Manufacturing method of positive electrode active material A composite oxide having a crystal structure belonging to the space group C2 / m and containing Mn and Bi, such as the positive electrode active material of the present disclosure, can be synthesized, for example, by mixing and firing raw materials to obtain a predetermined precursor, and then subjecting the precursor to post-treatment (soft chemical method). For example, the method for producing the positive electrode active material of the present disclosure is as follows: mixing one or both of a Na source and a K source, a Mn source, and a Bi source to obtain a mixture; calcining the mixture to obtain a precursor; and obtaining a positive electrode active material by subjecting the precursor to an acid treatment; It may include.

[0016] 2.1 Mixing The Na source and / or K source, the Mn source, and the Bi source may be mixed by a known mixing means such as a ball mill. The mixing may be dry mixing or wet mixing using a dispersion medium or the like. The Na source may be, for example, sodium carbonate. The K source may be, for example, potassium carbonate. The Mn source may be, for example, manganese oxide. The Bi source may be, for example, bismuth oxide. One type of compound may serve as two or more of the Na source, the K source, the Mn source, and the Bi source. In the production method of the present disclosure, for example, the crystal structure of the precursor after firing may be α-NaMnO 2 In order to obtain the above-mentioned crystal structure, a Na source and / or a K source, a Mn source, and a Bi source may be mixed in a predetermined ratio.

[0017] 2.2 Firing The mixture may be fired by a known firing means such as a firing furnace. The firing atmosphere of the mixture may be, for example, an inert gas atmosphere such as nitrogen, or an oxygen-containing atmosphere. The temperature rise rate from the firing start temperature (for example, room temperature) to the firing temperature may be, for example, 50°C / h or more and 500°C / h or less. The firing temperature may be, for example, 700°C or more and 800°C or less. The firing time may be, for example, 5 hours or more and 100 hours or less.

[0018] 2.3 Acid treatment The precursor obtained as described above is subjected to an acid treatment to obtain a positive electrode active material having a desired crystal structure and composition. The acid treatment can be performed by contacting the precursor with various acid solutions. The acid constituting the acid solution may be an inorganic acid or an organic acid, but inorganic acids are particularly preferably used. The inorganic acid may be, for example, sulfuric acid. The concentration and temperature of the acid solution may be any concentration and temperature that allow appropriate acid treatment. The time of the acid treatment (contact time between the precursor and the acid solution) may be, for example, 5 hours or more and 100 hours or less. After the acid treatment, washing with water and drying may be performed as desired.

[0019] 3. Water-based batteries As shown in FIG. 1, an aqueous battery 100 according to an embodiment includes a positive electrode active material layer 11, an aqueous electrolyte 20, and a negative electrode active material layer 31. The positive electrode active material layer 11 has a monoclinic crystal structure belonging to the space group C2 / m and having a composition formula: A x Mn 1-y Bi y O 2 ·zH 2 O (A: one or both of Na and K, 0 < x < 1, 0 < y < 1, 0 < z < 2). The aqueous electrolyte 20 includes water and potassium polyphosphate dissolved in the water.

[0020] 3.1 Positive electrode active material layer The positive electrode active material layer 11 includes the positive electrode active material of the present disclosure. Further, the positive electrode active material layer 11 can be impregnated with the aqueous electrolyte 20. Further, the positive electrode active material layer 11 may contain a conductive assistant, a binder, various additives, etc. in addition to the positive electrode active material. The content of each component in the positive electrode active material layer 11 may be appropriately determined according to the intended battery performance. The shape of the positive electrode active material layer 11 is not particularly limited, and for example, it may be a sheet-shaped positive electrode active material layer having a substantially flat surface.

[0021] 3.1.1 Positive electrode active material The positive electrode active material layer 11 may contain only the positive electrode active material of the present disclosure as the positive electrode active material. Alternatively, the positive electrode active material layer 11 may contain other positive electrode active materials together with the positive electrode active material of the present disclosure. The positive electrode active material of the present disclosure may occupy more than 0 mass% and not more than 100 mass%, 10 mass% or more and not more than 100 mass%, 30 mass% or more and not more than 100 mass%, or 50 mass% or more and not more than 100 mass% based on the total amount of the positive electrode active material contained in the positive electrode active material layer 11 (100 mass%). The other positive electrode active material may contain K element. Specifically, oxides and polyanions containing K element may be mentioned. More specifically, the other positive electrode active material may be a composite oxide of potassium and various transition metals. Note that even if the K site in the composite oxide is composed of other alkali metal elements, it may be possible to insert and remove various ions contained in the aqueous electrolyte 20. Alternatively, the other positive electrode active material may be Prussian blue, etc. Alternatively, the other positive electrode active material may be potassium titanate, TiO, which has a charge / discharge potential higher than that of the negative electrode active material described below. 2 , sulfur (S), and the like. The other positive electrode active material may be, for example, particulate. The other positive electrode active material may be primary particles, or secondary particles formed by agglomeration of a plurality of primary particles. The average particle diameter D50 of the other positive electrode active material may be, for example, 1 nm or more and 1000 μm or less, the lower limit may be 5 nm or more or 10 nm or more, and the upper limit may be 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less.

[0022] 3.1.2 Conductive additives Examples of the conductive assistant that may be included in the positive electrode active material layer 11 include carbon materials such as vapor grown carbon fiber (VGCF), acetylene black (AB), ketjen black (KB), carbon nanotubes (CNT), and carbon nanofibers (CNF); and metal materials that are poorly soluble in aqueous electrolytes, such as nickel, titanium, aluminum, and stainless steel. The conductive assistant may be, for example, particulate or fibrous, and its size is not particularly limited. Only one type of conductive assistant may be used alone, or two or more types may be used in combination.

[0023] 3.1.3 Binders Examples of binders that can be included in the positive electrode active material layer 11 include butadiene rubber (BR)-based binders, butylene rubber (IIR)-based binders, acrylate butadiene rubber (ABR)-based binders, styrene butadiene rubber (SBR)-based binders, polyvinylidene fluoride (PVdF)-based binders, polytetrafluoroethylene (PTFE)-based binders, polyimide (PI)-based binders, etc. Only one type of binder may be used alone, or two or more types may be used in combination.

[0024] 3.2 Positive electrode current collector As illustrated in FIG. 1, the positive electrode 10 of the aqueous battery 100 may include a positive electrode current collector 12 in contact with the positive electrode active material layer 11. The positive electrode current collector 12 may be in contact with the aqueous electrolyte 20. Any positive electrode current collector that can function as a positive electrode current collector of an aqueous battery can be used. The positive electrode current collector 12 may be in the form of a foil, a plate, a mesh, a punched metal, a foam, or the like. The positive electrode current collector 12 may be made of a metal foil or a metal mesh. In particular, a metal foil is excellent in terms of ease of handling. The positive electrode current collector 12 may be made of a plurality of foils. Examples of the metal material constituting the positive electrode current collector 12 include those 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 positive electrode current collector 12 may be a metal foil or a substrate plated or vapor-deposited with the above metal. When the positive electrode current collector 12 is made of a plurality of metal foils, some layer may be present between the plurality of metal foils. The thickness of the positive electrode current collector 12 is not particularly limited. For example, it may be 0.1 μm or more or 1 μm or more, and may be 1 mm or less or 100 μm or less.

[0025] 3.3 Aqueous electrolyte The aqueous electrolyte solution 20 includes water and an electrolyte dissolved in the water. The aqueous electrolyte solution 20 may be in contact with the above-mentioned positive electrode current collector 12, may be included in the above-mentioned positive electrode active material layer 11, may be in contact with the below-described negative electrode current collector 32, may be included in the below-described negative electrode active material layer 31, or may be held by the separator 40 between the positive electrode active material layer 11 and the negative electrode active material layer 31.

[0026] 3.3.1 Solvents The aqueous electrolyte 20 contains water as a solvent. The solvent contains water as a main component. That is, based on the total amount of the solvent constituting the aqueous electrolyte (100 mol%), water accounts for 50 mol% or more and 100 mol% or less. Water may account for 70 mol% or more, 90 mol% or more, or 95 mol% or more of the total amount of the solvent. Meanwhile, the upper limit of the proportion of water in the solvent is not particularly limited. The solvent may be composed of only water (100 mol% water). From the viewpoint of forming a solid electrolyte interphase (SEI) on the surface of the active material, the aqueous electrolyte 20 may contain a solvent other than water in addition to water. Examples of the solvent other than water include 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 solvents constituting the aqueous electrolyte solution 20 (100 mol %).

[0027] 3.3.2 Electrolytes At least potassium polyphosphate may be dissolved as an electrolyte in the aqueous electrolyte 20. The electrolyte may be dissociated into cations and anions in the aqueous electrolyte 20. In the aqueous electrolyte 20, the cations may be completely dissociated from the anions, or may form an association by being in close proximity to the anions. For example, the potassium ions contained in the aqueous electrolyte may form an association with the anions. The aqueous electrolyte 20 may contain multiple types of cations. For example, the aqueous electrolyte 20 may contain potassium ions and other cations together with the potassium ions. For example, alkali metal ions, alkaline earth metal ions, transition metal ions, and the like other than potassium ions may be contained. The aqueous electrolyte 20 naturally contains protons. The aqueous electrolyte 20 may also contain one or more types of anions. For example, the aqueous electrolyte 20 contains polyphosphate anions. There is no particular limit to the type of anions other than the polyphosphate anions. The aqueous electrolyte 20 naturally contains hydroxide ions. As described above, the aqueous electrolyte 20 contains protons, hydroxide ions, etc., in addition to cations and anions derived from potassium polyphosphate, and at least one of these ions can be a carrier ion. In this embodiment, the carrier ion may be a proton, a hydroxide ion, a cation derived from potassium polyphosphate, an anion derived from potassium polyphosphate, or a combination thereof. The concentration of the electrolyte in the aqueous electrolyte 20 is not particularly limited. The concentration of the electrolyte in the aqueous electrolyte 20 can be adjusted according to the target performance. According to the findings of the present inventor, the higher the concentration of the electrolyte in the aqueous electrolyte 20, the easier it is to expand the potential window of the aqueous electrolyte 20. In addition, the higher the concentration, the easier it is to increase the capacity of the aqueous battery 100. Furthermore, the higher the concentration, the easier it is to suppress the elution of the current collector into the aqueous electrolyte 20. On the other hand, the lower the concentration, the easier it is to reduce the viscosity of the aqueous electrolyte 20.

[0028] The aqueous electrolyte 20 may contain, as an electrolyte, various potassium compounds that are soluble in water, in addition to potassium polyphosphate. Examples of such electrolytes include KPF 6 , K.B.F. 4 , K 2 SO 4 , KNO 3 , C.H. 3 COOK, (CF 3 SO 2 ) 2 N.K., K.C.F. 3 SO 3 , (FSO 2 ) 2 N.K., K. 2 HPO 4 , K.H. 2 PO 4 , K.P.O. 3 At least one selected from the following may be mentioned.

[0029] According to the findings of the present inventors, when the aqueous electrolyte 20 contains water and a phosphate or acetate of potassium dissolved in the water, particularly when the aqueous electrolyte 20 contains water and a phosphate of potassium dissolved in the water, and particularly when the aqueous electrolyte 20 contains water and potassium polyphosphate dissolved in the water, the performance of the aqueous battery 100 is likely to be improved. "Potassium polyphosphate" refers to a salt in which at least a portion of the hydrogen of polyphosphoric acid is substituted with potassium. In other words, "potassium polyphosphate" is a concept that includes potassium hydrogen polyphosphate. Specific examples of potassium polyphosphate include potassium pyrophosphate (K 4-x H x P 2 O 7 ), potassium tripolyphosphate (K 5-x H x P 3 O 10 ), potassium tetrapolyphosphate (K 6-x H x P 4 O 13 ), potassium heptapolyphosphate (K 7-x H x P 5 O 16 Among them, potassium pyrophosphate (K 4-x H x P2 O 7 ) is employed, higher performance is likely to be ensured. That is, the aqueous electrolyte solution 20 according to one embodiment may contain water and potassium pyrophosphate as the potassium polyphosphate dissolved in the water. The aqueous electrolyte solution 20 may contain other electrolytes together with potassium polyphosphate as electrolytes. In this case, based on the total amount of electrolytes dissolved in the aqueous electrolyte solution 20 (100 mol%), potassium polyphosphate may account for 50 mol% to 100 mol%, 70 mol% to 100 mol%, or 90 mol% to 100 mol%, and other electrolytes may account for 0 mol% to 50 mol%, 0 mol% to 30 mol%, or 0 mol% to 10 mol%.

[0030] In the aqueous electrolyte solution 20, the "potassium polyphosphate dissolved in water" can be dissociated into potassium ions and anions. In the aqueous electrolyte solution 20, the potassium ions may be completely dissociated from the anions, or may form associations by coming close to the anions. For example, when the aqueous electrolyte solution 20 contains potassium pyrophosphate dissolved in water, the "potassium pyrophosphate dissolved in water" in the aqueous electrolyte solution 20 is K + , P 2 O 7 4- , K.P. 2 O 7 3- , K 2 P 2 O 7 2- , K 3 P 2 O 7 - It may exist as an ion such as H 2 O to H + Pull out the HP 2 O 7 3- , H 2 P 2 O 7 2- , H 3 P 2 O 7- Or, H + and K. + and meet in HK 2 P 2 O 7 - , H.K.P. 2 O 7 2- , H 2 KP 2 O 7 - In the aqueous electrolyte solution 20, the "concentration of potassium polyphosphate dissolved in water" is specified by converting the ions and associations contained in the aqueous electrolyte solution 20 into potassium pyrophosphate. In addition, when the aqueous electrolyte solution 20 contains polyphosphate ions (pyrophosphate ions, as described above, P 2 O 7 4- In addition, KP 2 O 7 3- , K 2 P 2 O 7 2- , K 3 P 2 O 7 - , HP 2 O 7 3- , H 2 P 2 O 7 2- , H 3 P 2 O 7 - , H.K. 2 P 2 O 7 - , H.K.P. 2 O 7 2- , H 2 KP 2 O 7 -In the case where the aqueous electrolyte solution 20 contains polyphosphate ions (which may be present in a state bound to a cation, such as potassium polyphosphate), the entire amount of polyphosphate ions contained in the aqueous electrolyte solution 20 does not have to be calculated as "dissolved potassium polyphosphate". In other words, the aqueous electrolyte solution 20 may contain more polyphosphate ions than the concentration that can be calculated as potassium polyphosphate. For example, when the aqueous electrolyte solution 20 is produced, water is added with potassium polyphosphate. 4 P 2 O 7 With K 4 P 2 O 7 A source of pyrophosphate ions other than H 4 P 2 O 7 When a potassium ion source other than a potassium salt (e.g., KOH) and an anion source other than a potassium salt (e.g., H 4 P 2 O 7 ) are dissolved in water and dissociated into potassium ions and anions, the potassium ions and anions are calculated as "potassium salt dissolved in water." For example, in the aqueous electrolyte 20, KOH and H 4 P 2 O 7 When these are dissolved, they can be converted into potassium pyrophosphate.

[0031] The aqueous electrolyte 20 may contain water and potassium polyphosphate dissolved at a certain concentration or more per 1 kg of water. The concentration of potassium polyphosphate may be, for example, 2.0 mol or more, 2.5 mol or more, 3.0 mol or more, 3.5 mol or more, or 4.0 mol or more per 1 kg of water. The upper limit of the concentration of potassium polyphosphate in the aqueous electrolyte 20 is not particularly limited, but if the concentration is too high, the viscosity of the aqueous electrolyte 20 may be excessively increased. In this regard, the concentration of potassium polyphosphate may be 8.0 mol or less, 7.0 mol or less, or 6.0 mol or less per 1 kg of water. When the concentration of potassium polyphosphate is 2.0 mol or more per 1 kg of water, oxidation and reduction resistance specific to a concentrated aqueous solution system, so-called water-in-salt, is expressed, making the aqueous electrolyte more preferable. In addition, various physical properties described below can be ensured.

[0032] 3.3.3 Other ingredients In addition to the above-mentioned solvent and electrolyte, the aqueous electrolyte solution 20 may contain an acid, a hydroxide, or the like for adjusting the pH of the aqueous electrolyte solution 20. In addition, various additives may be included.

[0033] 3.3.4 Physical properties of aqueous electrolytes (1) The pH of the aqueous electrolyte 20 is not particularly limited. However, if the pH is too high, there is a risk that the oxidation side potential window of the aqueous electrolyte may be narrowed. In this regard, the pH of the aqueous electrolyte may be 3 or more and 13 or less. The pH may be 4 or more, 5 or more, 6 or more, 7 or more, 8 or more, 9 or more, or 10 or more, or may be 12 or less. (2) The aqueous electrolyte 20 may not have a freezing point at -60°C or higher. Here, the presence or absence of the "freezing point" of the aqueous electrolyte 20 is confirmed by differential scanning calorimetry (DSC). The sweep rate by DSC is 5°C / min for both the temperature drop and temperature rise, and the sweep range is from room temperature to -120°C, and then to 40°C. The atmosphere in the DSC is an inert gas atmosphere such as Ar, and the pressure is equivalent to atmospheric pressure. However, since a sealed aluminum container is used for the evaluation, the atmosphere inside the container is air sealed under atmospheric pressure. When the aqueous electrolyte is measured under the above conditions, if the crystallization peak temperature (freezing point temperature) is not confirmed at -60°C or higher, the aqueous electrolyte is deemed to "not have a freezing point at -60°C or higher." In the aqueous battery 100 of the present disclosure, in order to achieve the condition that "the aqueous electrolyte 20 does not have a freezing point at -60°C or higher", it is effective to make the concentration of the electrolyte in the aqueous electrolyte 20 high. If the aqueous electrolyte 20 does not have a freezing point at -60°C, the amount of free water molecules in the aqueous electrolyte 20 decreases, and metals contained in the current collector are less likely to dissolve into the aqueous electrolyte 20. In addition, since the aqueous electrolyte 20 does not have a freezing point at -60°C, the aqueous battery 100 can be used even at extremely low temperatures. That is, the aqueous battery 100 can operate appropriately even in cold regions. (3) It is preferable that the aqueous electrolyte 20 does not cause salt precipitation when cooled from 0°C to -60°C. In this way, since the aqueous electrolyte 20 does not cause salt precipitation due to temperature change, stable ion conduction is possible even at low temperatures. For example, the aqueous battery 100 can be used even at extremely low temperatures such as in cold regions. As described above, the aqueous electrolyte 20 contains water and potassium polyphosphate dissolved in the water. Here, according to the knowledge of the present inventor, the saturated solubility of potassium polyphosphate in water has little temperature dependency and hardly changes at low temperatures below 0°C. In this regard, even if potassium polyphosphate is dissolved at a high concentration in the aqueous electrolyte 20 at 0°C and the aqueous electrolyte 20 is cooled from 0°C to -60°C, precipitation of potassium polyphosphate in the aqueous electrolyte 20 does not substantially occur. (4) If the viscosity of the aqueous electrolyte solution 20 is too high, the ionic conductivity of the aqueous electrolyte solution 20 may decrease. On the other hand, if an electrolyte is dissolved in the aqueous electrolyte solution 20 at a high concentration, the aqueous electrolyte solution 20 may have a viscosity of a certain level or higher. The aqueous electrolyte solution 20 may have a viscosity of 10 mPa·s or more and 600 mPa·s or less at 20°C.

[0034] 3.4 Negative electrode active material layer The negative electrode active material layer 31 includes a negative electrode active material. The negative electrode active material layer 31 may include a conductive assistant, a binder, various additives, and the like in addition to the negative electrode active material. The negative electrode active material layer 31 may be impregnated with the aqueous electrolyte solution 20. The content of each component in the negative electrode active material layer 31 may be appropriately determined according to the intended battery performance. For example, the content of the negative electrode active material may be 40% by mass or more, 100% by mass or less, or 90% by mass or less, assuming that the entire negative electrode active material layer 31 (total solid content) is 100% by mass. The shape of the negative electrode active material layer 31 is not particularly limited, and may be, for example, a sheet-like negative electrode active material layer having a substantially flat surface.

[0035] 3.4.1 Negative electrode active material The negative electrode active material may be any of those known as negative electrode active materials for aqueous batteries. Examples of the negative electrode active material include potassium-transition metal composite oxide, titanium oxide, Mo 6 S 8 Metal sulfides such as; Elemental sulfur; KTi 2 (PO 4 ) 3 ;NASICON type compound;WO 3The negative electrode active material may include at least one selected from the above. Alternatively, the negative electrode active material may be a hydrogen storage alloy. Alternatively, the negative electrode active material may be an inorganic compound having a crystal structure belonging to space group I23. The inorganic compound having a crystal structure belonging to space group I23 may include, for example, element A, element M, and O. Here, the element A is at least one of Bi and La, and the element M is at least one of Bi, Mn, Fe, Co, and Ni, and both the element A and the element M may be Bi. The negative electrode active material may be one that deintercalates ions by intercalation, or one that deintercalates ions by conversion reaction, alloying reaction, or the like. Only one type of negative electrode active material may be used alone, or two or more types may be used in combination. The shape of the negative electrode active material may be any shape that can function as a negative electrode active material of a battery. The negative electrode active material may be, for example, particulate. The negative electrode active material may be a primary particle, or a secondary particle in which a plurality of primary particles are aggregated. The average particle diameter D50 of the negative electrode active material may be, for example, 1 nm or more and 1000 μm or less, with a lower limit of 5 nm or more or 10 nm or more, and an upper limit of 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less.

[0036] 3.4.2 Other ingredients The conductive assistant and the binder that can be contained in the negative electrode active material layer 31 may be appropriately selected from those exemplified as those that can be contained in the positive electrode active material layer 11. The conductive assistant and the binder may each be used alone or in combination of two or more kinds.

[0037] 3.5 Negative electrode current collector As shown in FIG. 1, the negative electrode 30 of the aqueous battery 100 may include a negative electrode current collector 32 in contact with the negative electrode active material layer 31. The negative electrode current collector 32 may be in contact with the aqueous electrolyte 20. Any negative electrode current collector that can function as a negative electrode current collector for an aqueous battery can be used as the negative electrode current collector 32. The negative electrode current collector 32 may be in the form of a foil, a plate, a mesh, a punched metal, a foam, or the like. The negative electrode current collector 32 may be made of a metal foil or a metal mesh. In particular, a metal foil is excellent in terms of ease of handling. The negative electrode current collector 32 may be made of a plurality of foils. Examples of the metal material constituting the negative electrode current collector 32 include those 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, the performance of the negative electrode current collector 32 containing at least one selected from the group consisting of Al, Ti, Pb, Zn, Sn, Mg, Zr, and In, particularly the negative electrode current collector 32 containing Al, is high. Al, Ti, Pb, Zn, Sn, Mg, Zr, and In all have low work functions, and it is considered that even if the negative electrode current collector 32 contacts the aqueous electrolyte solution 20 at a reduction potential, electrolysis of the aqueous electrolyte solution 20 is unlikely to occur. The negative electrode current collector 32 may be a metal foil or a substrate plated or vapor-deposited with the above metal. In addition, when the negative electrode current collector 32 is made of a plurality of metal foils, some layer may be present between the plurality of metal foils. The thickness of the negative electrode current collector 32 is not particularly limited. For example, it may be 0.1 μm or more or 1 μm or more, and may be 1 mm or less or 100 μm or less.

[0038] 3.6 Other configurations In addition to the basic configuration described above, the aqueous battery 100 may have other configurations, for example, as described below. As described above, in the aqueous battery 100, the separator 40 may be present between the positive electrode active material layer 11 and the negative electrode active material layer 31. The separator 40 may be a separator used in aqueous electrolyte batteries (nickel-metal hydride batteries, zinc-air batteries, etc.). For example, the separator 40 may be a hydrophilic separator such as a nonwoven fabric made of cellulose. The thickness of the separator 40 is not particularly limited, and may be, for example, 5 μm or more and 1 mm or less. In addition to the configuration described above, the aqueous battery 100 may have terminals, a battery case, etc. The aqueous battery 100 may be a secondary battery.

[0039] 4. Manufacturing method of aqueous batteries The aqueous battery 100 of the present disclosure can be manufactured, for example, as follows. (1) The positive electrode active material constituting the positive electrode active material layer 11 is dispersed in a solvent to obtain a positive electrode mixture paste (slurry). The solvent used in this case may be water or various organic solvents. The positive electrode mixture paste (slurry) is applied to the surface of the positive electrode current collector 12 using a doctor blade or the like, and then dried to form the positive electrode active material layer 11 on the surface of the positive electrode current collector 12, thereby obtaining the positive electrode 10. As the application method, in addition to the doctor blade method, an electrostatic application method, a dip coating method, a spray coating method, or the like may also be adopted. (2) The aqueous electrolyte solution 20 can be produced, for example, by mixing water and an electrolyte. Even if the water and the electrolyte are simply filled into a container and left to stand, they will mix with each other, and the aqueous electrolyte solution 20 will ultimately be obtained. (3) The negative electrode active material constituting the negative electrode active material layer 31 is dispersed in a solvent to obtain a negative electrode mixture paste (slurry). The solvent used in this case may be water or various organic solvents. The negative electrode mixture paste (slurry) is applied to the surface of the negative electrode current collector 32 using a doctor blade or the like, and then dried to form the negative electrode active material layer 31 on the surface of the negative electrode current collector 32, thereby obtaining the negative electrode 30. As the application method, in addition to the doctor blade method, an electrostatic application method, a dip coating method, a spray coating method, or the like may also be used. (4) The positive electrode 10, the aqueous electrolyte 20, and the negative electrode 30 are housed in a battery case to obtain an aqueous battery 100. For example, a separator 40 is sandwiched between the positive electrode 10 and the negative electrode 30 to obtain a laminate having a positive electrode current collector 12, a positive electrode active material layer 11, the separator 40, a negative electrode active material layer 31, and a negative electrode current collector 32 in this order. Other members such as terminals are attached to the laminate as necessary. The laminate is housed in a battery case, and the battery case is filled with the aqueous electrolyte 20. The laminate and the electrolyte are sealed in the battery case so that the laminate is immersed in the aqueous electrolyte 20, to obtain the aqueous battery 100. EXAMPLES

[0040] Hereinafter, the technology of the present disclosure will be described in more detail with reference to examples and comparative examples, but the technology of the present disclosure is not limited to these.

[0041] 1. Preparation of Positive Electrode Active Material 1.1 Example: Preparation of positive electrode active material by soft chemical method 1.1.1 Precursor synthesis As starting materials, sodium carbonate, dimanganese trioxide, and dibismuth trioxide were weighed out to a molar ratio of 1:0.95:0.05, and ground and mixed for 2 hours using a planetary ball mill to obtain a mixture. The obtained mixture was placed in an alumina crucible and placed in a firing furnace. Nitrogen gas was flowed into the furnace to degas oxygen, and then the temperature was raised to 750°C at a heating rate of 150°C / h and held at 750°C for 60 hours to fire the mixture. After that, the mixture was gradually cooled to room temperature, the flow of nitrogen gas was stopped, and the fired precursor was collected from the furnace. When the X-ray diffraction pattern of the obtained precursor was confirmed, the precursor showed a diffraction pattern similar to that of single-phase α-NaMnO2, and it was determined that Bi was incorporated into the crystal structure.

[0042] 1.1.2 Acid treatment of precursor The precursor obtained as described above was placed in a 300 ml beaker, to which 250 ml of sulfuric acid solution (sulfuric acid concentration: 0.3 mol / L) was added, and the mixture was stirred at 20° C. for 24 hours. After stirring, the mixture was suction filtered using a Buchner funnel, and BaCl 2 BaSO in aqueous solution 4 The wet cake was dried in a hot air dryer at 50° C. for 24 hours and then classified using a standard sieve with an opening of 30 μm to obtain a positive electrode active material.

[0043] 1.1.3 Confirmation of crystal structure and composition The X-ray diffraction pattern of the obtained positive electrode active material was confirmed using CuKα as a radiation source. FIG. 2 shows the X-ray diffraction pattern of the positive electrode active material according to the embodiment. As shown in FIG. 2, it was confirmed that the positive electrode active material has a monoclinic layered crystal structure belonging to the space group C2 / m in a single phase. In addition, when the obtained positive electrode active material was subjected to elemental analysis by IPC measurement, it was confirmed that the composition ratio Na / (Mn+Bi) of Na to the total of Mn and Bi in the positive electrode active material was 0.05, the composition ratio Bi / (Mn+Bi) of Bi to the total of Mn and Bi was 0.04, and the positive electrode active material contained 13.7 mass% moisture. In other words, the positive electrode active material according to the embodiment has a monoclinic crystal structure belonging to the space group C2 / m, and has a composition formula: Na 0.05 Mn 0.96 Bi 0.04 O 2 0.83H 2 The composition was represented by O.

[0044] 1.2 Comparative Example: Preparation of Positive Electrode Active Material by Solution Method 1.2.1 Preparation of solutions Ion-exchanged water was added to 19.8 g of manganese(II) chloride tetrahydrate to prepare 200 ml of a 0.5 mol / L manganese chloride aqueous solution. Ion-exchanged water was added to 109 g of a 50 w / w% aqueous sodium hydroxide solution to prepare 250 ml of a 5.5 mol / L aqueous sodium hydroxide solution.

[0045] 1.2.2 Preparation of positive electrode active material by solution reaction The manganese chloride aqueous solution was transferred to a 1 L stirring vessel capable of gas flow, and then placed in a cooling bath to cool the temperature to below 10°C. Ar gas was bubbled in while stirring to expel the air from the vessel. While maintaining the temperature and bubbling the gas, the sodium hydroxide aqueous solution prepared above was added to the manganese chloride aqueous solution over a period of 15 minutes using a roller pump and mixed to produce a cloudy slurry. The gas pipe was then reconnected to an air pump, and air was pumped in at 6 L / min. Bubbling was continued for more than 20 hours, turning the slurry black and promoting the oxidation reaction. After the reaction was completed, filtration was performed using a Buchner funnel, and AgNO 3 The mixture was washed with water until no white precipitate of AgCl was formed in the aqueous solution, to obtain a wet cake. The wet cake was dried overnight in a hot air dryer at 80°C, and then classified using a standard sieve with an opening of 38 μm to obtain a positive electrode active material.

[0046] 1.2.3 Confirmation of crystal structure and composition The X-ray diffraction pattern of the obtained positive electrode active material was confirmed using CuKα as a radiation source. FIG. 2 shows the X-ray diffraction pattern of the positive electrode active material according to the comparative example. As shown in FIG. 2, it was confirmed that the positive electrode active material has a monoclinic layered crystal structure belonging to the space group C2 / m in a single phase. In addition, when the obtained positive electrode active material was subjected to elemental analysis by IPC measurement, it was confirmed that the composition ratio Na / Mn of Na to Mn in the positive electrode active material was 0.29 and that the positive electrode active material contained 10.9 mass % moisture. In other words, the positive electrode active material according to the comparative example has a monoclinic crystal structure belonging to the space group C2 / m and has a composition formula: Na 0.29 MnO 2 0.64H 2 The composition was represented by O.

[0047] 2. Preparation of Positive Electrode The above positive electrode active material, acetylene black as a conductive additive, and PVdF as a binder were mixed with a solvent in a mass ratio of positive electrode active material:conductive additive:binder=75:20:5 to prepare an ink. The ink obtained was dropped onto a Ti foil surface and then coated with a doctor blade to form a uniform film, thereby obtaining a positive electrode having a Ti foil and a positive electrode active material layer provided on the surface of the Ti foil.

[0048] 3. Preparation of Aqueous Electrolyte K per kg of pure water 4 P 2 O 7 The aqueous electrolyte for evaluation had a pH value in the range of 3 to 13, no freezing point at temperatures above -60°C, no salt precipitation when cooled from 0°C to -60°C, and a viscosity at 20°C in the range of 10 mPa s to 350 mPa s.

[0049] 4. Preparation of Evaluation Cell An evaluation cell (VM4, EC Frontier) was produced using the above positive electrode as a working electrode, a Pt mesh as a counter electrode, Ag / AgCl as a reference electrode, and the above aqueous electrolyte.

[0050] 5. Cell Evaluation The evaluation cell was charged and discharged under the following conditions to obtain a discharge curve. Charge / discharge current value: 0.2mA / cm 2 Cutoff voltage: -0.5~0.85V vs. Ag / AgCl Measurement temperature: 25℃

[0051] 6. Evaluation Results FIG. 3 shows the discharge curves of the cell according to the embodiment and the cell according to the comparative example. As shown in FIG. 3, the positive electrode active material according to the embodiment can be charged and discharged in an aqueous electrolyte solution containing potassium polyphosphate, and has a significantly higher discharge capacity than the positive electrode active material according to the comparative example. It is considered that the positive electrode active material according to the embodiment has a monoclinic crystal structure belonging to the space group C2 / m, and that the presence of Bi together with Mn stabilizes the crystal structure and / or changes the electronic state within the crystal. This makes it easier for charge compensation ions (e.g., potassium ions and / or protons) derived from the aqueous electrolyte solution to enter between the layers of the crystal structure, thereby increasing the discharge capacity.

[0052] 7. Supplementary Information FIG. 4 shows the relationship between the concentration of an aqueous potassium pyrophosphate solution and ionic conductivity. As shown in FIG. 4, when the concentration of potassium pyrophosphate in the aqueous potassium pyrophosphate solution is more than 0 mol / kg and not more than 8 mol / kg, the aqueous potassium pyrophosphate solution has excellent ionic conductivity. FIG. 5 shows the relationship between the concentration of the aqueous potassium pyrophosphate solution and pH. As shown in FIG. 5, when the concentration of potassium pyrophosphate in the aqueous potassium pyrophosphate solution is more than 0 mol / kg and not more than 8 mol / kg, the pH of the aqueous potassium pyrophosphate solution is 10.0 to 12.0.

[0053] In the above examples, the performance of the active material contained in the positive electrode was simply evaluated using an evaluation cell having a specific aqueous electrolyte, a specific positive electrode as a working electrode, a Pt mesh as a counter electrode, and Ag / AgCl as a reference electrode. Here, the configuration other than the positive electrode active material is not the essence of the technology of the present disclosure. When actually constructing a water-based battery, it is sufficient to adopt an aqueous electrolyte, a negative electrode, etc. together with a specific positive electrode active material. [Explanation of symbols]

[0054] 10: positive electrode, 11: positive electrode active material layer, 12: positive electrode current collector, 20: aqueous electrolyte, 30: negative electrode, 31: negative electrode active material layer, 32: negative electrode current collector, 40: separator, 100: aqueous battery

Claims

1. A positive electrode active material, It has a monoclinic crystal structure belonging to the space group C2 / m, Composition formula: A ｘ Mn １－ｙ Bi ｙ O ２ ・zH ２ O A: Na and / or K 0<x<1 0<y<1 0<z<2 and Used in an aqueous battery having an aqueous electrolyte solution containing water and potassium polyphosphate dissolved in the water, Cathode active material.

2. An aqueous battery, A positive electrode active material layer, an aqueous electrolyte solution, and a negative electrode active material layer are included, The positive electrode active material layer comprises the positive electrode active material according to claim 1 , The aqueous electrolyte solution contains water and potassium polyphosphate dissolved in the water. water-based battery.

3. 3. The aqueous battery of claim 2, The aqueous electrolyte solution contains water and potassium polyphosphate dissolved in the water at a concentration of 2.0 mol or more per kg of the water. water-based battery.

4. 3. The aqueous battery of claim 2, The aqueous electrolyte solution contains water and potassium pyrophosphate as the potassium polyphosphate dissolved in the water. water-based battery.

5. The aqueous battery according to any one of claims 2 to 4, The pH of the aqueous electrolyte is 3 or more and 13 or less. water-based battery.

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