Aqueous battery

Abstract

To disclose a novel active material operable in an aqueous battery.SOLUTION: An aqueous battery of the present disclosure includes a positive electrode, an aqueous electrolyte, and a negative electrode. The positive electrode contains a positive electrode active material, and the negative electrode contains a negative electrode active material. One or both of the positive electrode active material and the negative electrode active material contain a composite oxide. The composite oxide contains Na, at least one transition metal element of Fe, Ti, Ni and Mn, and O. The aqueous electrolyte contains water and potassium polyphosphate dissolved in the water.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This application discloses an aqueous battery. [Background technology]

[0002] Patent Document 1 discloses an aqueous electrolyte solution for use in aqueous batteries, which contains water and potassium pyrophosphate dissolved at a concentration of 2 mol or more per kg of water. When an aqueous battery is constructed using the aqueous electrolyte solution disclosed in Patent Document 1, the wide reduction-side potential window of the aqueous electrolyte solution makes it easy to suppress decomposition of the aqueous electrolyte solution on the electrode surfaces even when the aqueous battery is charged and discharged. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-220294 Summary of the Invention [Problem to be solved by the invention]

[0004] In aqueous batteries that use aqueous electrolytes containing potassium polyphosphate, there is a need for new materials that can be used as active materials using Na (sodium), a relatively abundant resource. [Means for solving the problem]

[0005] The present application discloses the following aspects as means for solving the above problems. <Aspect 1> An aqueous battery having a positive electrode, an aqueous electrolyte, and a negative electrode, the positive electrode includes a positive electrode active material, the negative electrode includes a negative electrode active material, one or both of the positive electrode active material and the negative electrode active material contains a composite oxide, The composite oxide is Na and at least one transition metal element selected from Fe, Ti, Ni, and Mn; O and Including, The aqueous electrolyte solution is Water and potassium polyphosphate dissolved in the water; Including, water-based battery. <Aspect 2> 2. The aqueous battery of embodiment 1, one or both of the positive electrode active material and the negative electrode active material contains at least one of a first composite oxide, a second composite oxide, and a third composite oxide; The first composite oxide is Na x Fe 1-y M 1 y O2, The second composite oxide is Na x Ti 1-y M 2 y O2, The third composite oxide is Na x Ni 1-y M 3 y O2, where: 0 <x≦1であり、 0≦y≦1, M 1 contains one or both of Ti and Mn, and does not contain Ni, M 2 contains one or both of Fe and Mn, and does not contain Ni; M 3 contains at least one of Fe, Ti and Mn, water-based battery. <Aspect 3> 3. The aqueous battery of embodiment 2, the negative electrode active material contains one or both of the first composite oxide and the second composite oxide; water-based battery. <Aspect 4> 4. The aqueous battery of embodiment 3, M 1 is one or both of Ti and Mn, M 2 is one or both of Fe and Mn; water-based battery. <Aspect 5> The aqueous battery according to any one of aspects 2 to 4, the positive electrode active material contains the third composite oxide, water-based battery. <Aspect 6> 6. The aqueous battery of embodiment 5, M 3 is at least one of Fe, Ti and Mn; water-based battery. <Aspect 7> The aqueous battery of any one of Aspects 1 to 6, the aqueous electrolyte solution contains the potassium polyphosphate dissolved in a concentration of 3 mol or more per kg of water; water-based battery. <Aspect 8> The aqueous battery of any one of Aspects 1 to 7, the aqueous electrolyte solution contains the potassium polyphosphate dissolved at a concentration of 3 mol or more and 6 mol or less per 1 kg of water; water-based battery. <Aspect 9> The aqueous battery of any one of Aspects 1 to 8, the aqueous electrolyte solution contains the potassium polyphosphate dissolved at a concentration of 4 mol or more and 6 mol or less per 1 kg of water; water-based battery. <Aspect 10> The aqueous battery of any one of Aspects 1 to 9, The aqueous electrolyte does not have a freezing point at -40°C or higher. water-based battery. <Aspect 11> The aqueous battery of any one of Aspects 1 to 10, The aqueous electrolyte does not precipitate salt when cooled to 0°C to -40°C. water-based battery. <Aspect 12> The aqueous battery of any one of Aspects 1 to 11, the aqueous electrolyte has a viscosity of 10 mPa·s or more and 600 mPa·s or less at 20°C; water-based battery. <Aspect 13> The aqueous battery of any one of Aspects 1 to 12, The pH of the aqueous electrolyte is 3 or more and 13 or less. water-based battery. <Aspect 14> The aqueous battery of any one of Aspects 1 to 13, One or both of the positive electrode and the negative electrode has a current collector containing Al. water-based battery. <Aspect 15> 15. The aqueous battery of embodiment 14, comprising: It has a bipolar structure, a positive electrode active material layer is formed on one surface of the current collector; a negative electrode active material layer is formed on the other surface of the current collector; water-based battery. [Effects of the Invention]

[0006] The aqueous battery of the present disclosure is capable of being charged and discharged by combining a predetermined active material with a predetermined aqueous electrolyte solution. [Brief explanation of the drawings]

[0007] [Figure 1] 1 shows a schematic diagram of an example of the configuration of an aqueous battery. [Figure 2] 1 shows a schematic diagram of an example of the configuration of an aqueous battery. [Figure 3] 1 shows the charge / discharge curves of Example 1. [Figure 4] 1 shows charge / discharge curves of Example 2. [Figure 5] 1 shows the charge / discharge curves of Example 3. [Figure 6] 1 shows the charge / discharge curves of Example 4. [Figure 7] 1 shows the charge / discharge curves of Example 5. [Figure 8]1 shows the charge / discharge curves of Example 6. [Figure 9] 1 shows the charge / discharge curves of Example 7. [Figure 10] 1 shows the charge / discharge curves of Example 8. [Figure 11] 1 shows the charge / discharge curves of Example 9. [Figure 12] 1 shows the charge / discharge curves of Example 10. [Figure 13] 1 shows the charge / discharge curves of Example 11. [Figure 14] 1 shows the charge / discharge curves of Example 12. [Figure 15] 1 shows the charge / discharge curves of Example 13. [Figure 16] 1 shows the charge / discharge curves of Example 14. [Figure 17] 1 shows the charge / discharge curves of Example 15. [Figure 18] 1 shows the charge / discharge curves of Example 16. [Figure 19] 1 shows the charge / discharge curves of Example 17. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, one embodiment of the aqueous battery of the present disclosure will be described with reference to the drawings, but the technology of the present disclosure is not limited to the following embodiment.

[0009] As shown in FIG. 1, an aqueous battery 100 according to one embodiment includes a positive electrode 10, an aqueous electrolyte solution 20, and an anode 30. The positive electrode 10 includes a positive electrode active material. The anode 30 includes an anode active material. One or both of the positive electrode active material and the anode active material includes a composite oxide. The composite oxide includes Na, at least one transition metal element selected from Fe, Ti, Ni, and Mn, and O. The aqueous electrolyte solution 20 includes water and potassium polyphosphate dissolved in the water.

[0010] 1.Active material The aqueous battery 100 according to one embodiment has a specific composite oxide as an active material. According to new findings by the inventors, the charge / discharge potential (the potential at which carrier ions are absorbed and released) of the composite oxide varies depending on the type and amount of transition metal elements contained in the composite oxide. Specifically, the charge / discharge potential tends to decrease as the proportion of Fe and Ti in the transition metal elements contained in the composite oxide increases. Furthermore, the charge / discharge potential tends to increase as the proportion of Ni in the transition metal elements contained in the composite oxide increases. In other words, the composite oxide can function as both a positive electrode active material and a negative electrode active material depending on its composition. The positive electrode active material and the negative electrode active material can be appropriately selected, taking into consideration the potential window of the aqueous electrolyte solution 20, etc.

[0011] 1.1 Composition The aqueous battery 100 according to one embodiment may have one or both of the following configurations (1) and (2). (1) The positive electrode 10 has, as a positive electrode active material, a composite oxide containing Na, one or both of the transition metal elements Fe and Ti, and O. The composite oxide serving as the positive electrode active material may also contain Mn. Furthermore, the composite oxide serving as the positive electrode active material does not necessarily have to contain Ni. (2) The negative electrode 30 includes, as a negative electrode active material, a composite oxide containing Na, Ni, and O. The composite oxide serving as the negative electrode active material may contain at least one of Fe, Ti, and Mn.

[0012] In the aqueous battery 100 according to one embodiment, one or both of the positive electrode active material and the negative electrode active material may contain at least one of a first composite oxide, a second composite oxide, and a third composite oxide. x Fe 1-y M 1 y The second composite oxide has a composition represented by NaO2. x Ti 1-y M 2 y The third composite oxide has a composition represented by NaO2. x Ni1-y M 3 y O2, where 0 <x≦1であり、0≦y≦1であり、M 1 contains one or both of Ti and Mn, and does not contain Ni, and M 2 contains one or both of Fe and Mn, and does not contain Ni, and M 3 contains at least one of Fe, Ti and Mn.

[0013] In the above composition, x is greater than 0 and less than or equal to 1. x may be greater than or equal to 0.1, greater than or equal to 0.2, greater than or equal to 0.3, greater than or equal to 0.4, greater than or equal to 0.5, greater than or equal to 0.6, or greater than or equal to 0.7. In one embodiment, x may be greater than or equal to 0.5 and less than or equal to 1.0, or greater than or equal to 0.7 and less than or equal to 1.0. In the above composition, y is greater than or equal to 0 and less than or equal to 1. y may be greater than 0, greater than or equal to 0.1, greater than or equal to 0.2, greater than or equal to 0.3, or greater than or equal to 0.4, or may be less than 1, less than or equal to 0.9, less than or equal to 0.8, less than or equal to 0.7, less than or equal to 0.6, less than or equal to 0.5, or greater than or equal to 0.4. In one embodiment, y may be greater than or equal to 0.5 and less than or equal to 0.4.

[0014] As described above, the charge / discharge potential tends to decrease as the proportion of Fe and Ti in the transition metal elements contained in the complex oxide increases, and in this case, the complex oxide is suitable as a negative electrode active material. In this regard, in the aqueous battery 100 according to one embodiment, the negative electrode active material may contain one or both of the first complex oxide and the second complex oxide. For example, the first complex oxide and the second complex oxide may not contain elements other than Fe, Ti, and Mn as transition metal elements. Specifically, M 1 may be one or both of Ti and Mn. 2 may be one or both of Fe and Mn. In this case, the composite oxide can be made of relatively inexpensive elements.

[0015] As described above, the charge / discharge potential tends to increase as the proportion of Ni in the transition metal elements contained in the complex oxide increases, and in this case, the complex oxide is suitable as a positive electrode active material. In this regard, in the aqueous battery 100 according to one embodiment, the positive electrode active material may contain the third complex oxide. For example, the third complex oxide may not contain elements other than Ni, Fe, Ti, and Mn as transition metal elements. Specifically, M 3 may be at least one of Fe, Ti, and Mn. In this case, the composite oxide can be made of relatively inexpensive elements.

[0016] 1.2 Crystal structure The composite oxide may have, for example, a layered structure (e.g., at least one selected from an O3 type structure, an O2 type structure, and a P2 type structure), a spinel type structure, or a tunnel structure such as hollandite, romanekite, ramsdellite, nüstite, or pyrolusite. The composite oxide may have multiple types of crystal phases.

[0017] 1.3 Shape The positive electrode active material and the negative electrode active material may have any shape as long as they function as active materials in an aqueous battery. The positive electrode active material and the negative electrode active material may be, for example, particulate. The positive electrode active material and the negative electrode active material may be solid particles, hollow particles, particles having voids, or porous particles. The positive electrode active material and the negative electrode active material may be primary particles or secondary particles formed by aggregation of multiple primary particles. The average particle diameter D50 of the positive electrode active material and the negative electrode active material may be, for example, 1 nm or more, 5 nm or more, or 10 nm or more, or 500 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less. The average particle diameter D50 referred to in this application is the particle diameter (median diameter) at 50% of the cumulative value in a volume-based particle size distribution determined by a laser diffraction / scattering method.

[0018] 1.4 Other matters As described above, in the aqueous battery 100 according to one embodiment, one or both of the positive electrode active material and the negative electrode active material may contain the above-described composite oxide. For example, in one embodiment, the positive electrode active material contains the above-described composite oxide, and the negative electrode active material contains a composite oxide other than the composite oxide employed as the positive electrode active material. Alternatively, in one embodiment, the positive electrode active material contains the above-described composite oxide, and the negative electrode active material does not contain the above-described composite oxide. Alternatively, in one embodiment, the positive electrode active material does not contain the above-described composite oxide, and the negative electrode active material contains the above-described composite oxide. Only one type of positive electrode active material may be used alone, or two or more types may be used in combination. Only one type of negative electrode active material may be used alone, or two or more types may be used in combination.

[0019] As the active material other than the composite oxide, any known active material for aqueous batteries may be used. The positive electrode active material (other positive electrode active material) other than the composite oxide may be appropriately selected taking into consideration the potential window of the aqueous electrolyte 20, etc. The other positive electrode active material may be, for example, a compound known as a positive electrode active material for aqueous proton batteries (e.g., a transition metal oxide) or a compound known as a positive electrode active material for aqueous potassium ion batteries (e.g., an organic active material such as Prussian blue). The negative electrode active material (other negative electrode active material) other than the composite oxide has a lower charge / discharge potential than the positive electrode active material and may be appropriately selected taking into consideration the potential window of the aqueous electrolyte 20, etc. The other negative electrode active material may be, for example, a potassium-transition metal composite oxide; titanium oxide; metal sulfide such as Mo6S8; elemental sulfur; KTi2(PO4)3; NASICON-type compound; etc. The other positive electrode active materials and other negative electrode active materials may be those which deintercalate and insert carrier ions by intercalation, or may be those which deintercalate and insert carrier ions by a conversion reaction, an alloying reaction, or the like.

[0020] 2.Aqueous electrolyte In the aqueous battery 100, the aqueous electrolyte solution 20 contains water and potassium polyphosphate dissolved in the water. The composite oxides used as the positive electrode active material and / or negative electrode active material can insert and extract carrier ions in the aqueous electrolyte solution at a predetermined potential. The carrier ions correspond to various ions contained in the aqueous electrolyte solution 20. In one embodiment, the carrier ions may be protons. That is, the aqueous battery 100 may be an aqueous proton battery. Alternatively, in one embodiment, the carrier ions may be potassium ions. That is, the aqueous battery 100 may be an aqueous potassium ion battery. Alternatively, in one embodiment, the carrier ions may be hydroxide ions or polyphosphate ions. That is, the aqueous battery 100 may be an aqueous anion battery.

[0021] As described above, the aqueous electrolyte solution 20 contains water and potassium polyphosphate dissolved in the water. The aqueous electrolyte solution 20 according to one embodiment may contain water, potassium ions, and polyphosphate ions. The aqueous electrolyte solution 20 may also contain other components in addition to water and potassium polyphosphate. For example, the aqueous electrolyte solution 20 may contain water, potassium ions, and polyphosphate ions dissolved in the water. 3-x H x The aqueous electrolyte solution 20 may contain PO4 (1≦x) or polyphosphoric acid. The aqueous electrolyte solution 20 may be held between the positive electrode 10 and the negative electrode 30 by a separator 40, and may be in contact with the positive electrode 10 and the negative electrode 30.

[0022] 2.1 Solvent The aqueous electrolyte solution 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 solution (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, there is no particular upper limit on the proportion of water in the solvent. The solvent may consist of only water (100 mol% water).

[0023] The solvent may contain a solvent other than water in addition to water, for example, from the viewpoint of forming a solid electrolyte interphase (SEI) on the surface of the active material. 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 of the total amount of solvents constituting the electrolyte solution (100 mol%).

[0024] 2.2 Electrolytes An electrolyte is dissolved in the aqueous electrolyte solution 20, and the electrolyte can be dissociated into cations and anions in the aqueous electrolyte solution 20. In the aqueous electrolyte solution 20, the cations and anions may be close to each other to form an association complex.

[0025] 2.2.1 Potassium polyphosphate The aqueous electrolyte solution 20 contains potassium polyphosphate dissolved in the water. "Potassium polyphosphate" refers to a salt in which at least a portion of the hydrogen atoms in polyphosphate are 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 P2O7), potassium tripolyphosphate (K 5-x H x P3O 10 Among them, potassium polyphosphates include potassium pyrophosphate (K 4-x H xWhen a cation source (e.g., a potassium compound) and an anion source (e.g., polyphosphoric acid) are used, even higher performance is likely to be ensured. In the aqueous electrolyte solution 20, the "potassium polyphosphate dissolved in water" may exist as potassium ions, polyphosphate ions, and associations of these ions, or associations with ions derived from potassium hydrogen phosphate, phosphoric acid, and polyphosphoric acid, as described below. In the aqueous electrolyte solution 20, the "concentration of potassium polyphosphate dissolved in water" can be determined by converting the ions and associations contained in the aqueous electrolyte solution 20 into potassium polyphosphate. In the present application, the "potassium polyphosphate dissolved in water" may also refer to the aqueous electrolyte solution 20 in which the ions and associations thereof are formed as a result of separately adding a cation source (e.g., a potassium compound) and an anion source (e.g., polyphosphoric acid) to the aqueous electrolyte solution 20.

[0026] The concentration of potassium polyphosphate in the aqueous electrolyte solution 20 is not particularly limited. According to the inventor's new findings, when the aqueous electrolyte solution 20 contains potassium polyphosphate dissolved at a concentration of 3 mol or more per 1 kg of water, particularly when the aqueous electrolyte solution 20 contains potassium polyphosphate dissolved at a concentration of 3 mol or more and 6 mol or less per 1 kg of water, and even more particularly when the aqueous electrolyte solution 20 contains potassium polyphosphate dissolved at a concentration of 4 mol or more and 6 mol or less per 1 kg of water, it is expected to be effective in improving other properties of the electrolyte solution, such as electrochemical stability. Furthermore, when the concentration of potassium polyphosphate in the aqueous electrolyte solution 20 is such a concentration, it is easy to obtain an aqueous electrolyte solution 20 that does not have a freezing point at or above −60° C.

[0027] 2.2.2 Cations The aqueous electrolyte solution 20 may contain protons or potassium ions as cations. In the aqueous electrolyte solution 20, some of the potassium ions contained in the aqueous electrolyte solution 20 may be converted into "dissolved potassium polyphosphate." However, the aqueous electrolyte solution 20 may contain more potassium ions than can be converted into potassium polyphosphate. For example, when producing the aqueous electrolyte solution 20, potassium polyphosphate and other potassium ion sources (e.g., KOH, CH3COOK, K3PO4, KH2PO4, K2HPO4, K5PO4) may be added to water. 10 , K6P4O 13 , K7P5O 16 When a potassium ion-containing solution (e.g., (KPO3)n) is added and dissolved in the aqueous electrolyte solution 20, the aqueous electrolyte solution 20 contains more potassium ions than can be converted into potassium polyphosphate. The aqueous electrolyte solution 20 may contain other cations as long as the above-mentioned problems can be solved. For example, the aqueous electrolyte solution 20 may contain alkali metal ions other than potassium ions, alkaline earth metal ions, transition metal ions, etc.

[0028] 2.2.3 Anions The aqueous electrolyte solution 20 may contain hydroxide ions or polyphosphate ions (which may exist in a state bound to a cation, as described above) as anions. The aqueous electrolyte solution 20 may also contain other anions to the extent that the above-mentioned problems can be solved. For example, the aqueous electrolyte solution 20 may contain anions derived from other electrolytes, as described below.

[0029] 2.2.4 Other components that may be contained in aqueous electrolytes The aqueous electrolyte solution 20 may contain other electrolytes. For example, the aqueous electrolyte solution 20 may contain at least one of potassium hydrogen phosphate, phosphoric acid, and polyphosphoric acid dissolved in the water. The "potassium hydrogen phosphate" may be one or both of potassium monohydrogen phosphate (K2HPO4) and potassium dihydrogen phosphate (KH2PO4). In the aqueous electrolyte solution 20, "potassium hydrogen phosphate dissolved in water" and "phosphoric acid dissolved in water" are not limited to K + , H + , PO4 3-, KPO4 2- , HPO4 2- , K2PO4 - , H2PO4 - , KHPO4 - The polyphosphate may exist as an ion such as the above, an association of these ions, or an association of the above-mentioned ions derived from potassium polyphosphate. + , polyphosphate anions, or may exist as an associated compound with ions derived from the potassium polyphosphate. In the aqueous electrolyte solution 20, the ions and associated compounds contained in the aqueous electrolyte solution 20 can be converted into potassium hydrogen phosphate, phosphoric acid, or polyphosphoric acid, thereby specifying the "concentration of potassium hydrogen phosphate dissolved in water," the "concentration of phosphoric acid dissolved in water," and the "concentration of polyphosphoric acid dissolved in water." In the present application, "potassium hydrogen phosphate dissolved in water" refers to the aqueous electrolyte solution 20 in which a cation source (e.g., a potassium compound) and an anion source (e.g., phosphoric acid) are separately added, resulting in the aqueous electrolyte solution 20 containing potassium hydrogen phosphate. + , H + , PO4 3- , KPO4 2- , HPO4 2- , K2PO4 - , H2PO4 - , KHPO4 - The ions may be ions such as those mentioned above, or associations of these ions.

[0030] The aqueous electrolyte 20 may contain electrolytes other than the phosphate compound, such as KPF, KBF, KSO, KNO, CHCOOK, (CFSO), NK, KCFSO, (FSO)NK, KHPO, KHPO, KPO, and KP0. 10 , K6P4O 13 , K7P5O 16 , (KPO3)n, and the like.

[0031] The electrolyte other than potassium polyphosphate may account for 50 mol % or less, 30 mol % or less, or 10 mol % or less, based on the total amount (100 mol %) of electrolytes dissolved in the electrolytic solution.

[0032] The aqueous electrolyte solution 20 may contain various additives in addition to the above electrolytes.

[0033] 2.3 Other properties As long as the aqueous electrolyte solution 20 contains the above-mentioned solvent and electrolyte, there are no particular limitations on other properties of the aqueous electrolyte solution 20. An example of other properties of the aqueous electrolyte solution 20 will be described below.

[0034] 2.3.1 Freezing point The aqueous electrolyte 20 may not have a freezing point above −40°C. The presence or absence of a “freezing point” of the aqueous electrolyte 20 is confirmed by differential scanning calorimetry (DSC). The DSC sweep rate is 5°C / min for both temperature increases and decreases, and the sweep range is from room temperature to −120°C and then to 40°C. The DSC atmosphere 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 evaluation, the atmosphere inside the container is air sealed under atmospheric pressure. When the aqueous electrolyte is measured under the above conditions and no crystallization peak temperature (freezing point temperature) is confirmed above −40°C, the aqueous electrolyte is deemed to “not have a freezing point above −40°C.” The aqueous electrolyte solution 20 may have no freezing point at temperatures above −60°C, −80°C, −100°C, or −120°C. In the aqueous battery 100 of the present disclosure, to achieve the condition that the aqueous electrolyte solution 20 has no freezing point at temperatures above −40°C, it is effective to increase the concentration of potassium polyphosphate in the aqueous electrolyte solution 20. When the aqueous electrolyte solution 20 does not have a freezing point at −40°C, dissolution of the current collector into the aqueous electrolyte solution 20 is easily suppressed. Furthermore, when the aqueous electrolyte solution 20 does not have a freezing point at −40°C, the aqueous battery 100 can be used even at extremely low temperatures. That is, the aqueous battery 100 can operate properly even in cold climates.

[0035] 2.3.2 Presence or absence of salt precipitation The aqueous electrolyte solution 20 may be one that does not precipitate salt when cooled from 0°C to -40°C. If the aqueous electrolyte solution 20 does not precipitate salt due to temperature changes, 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 solution 20 contains water and potassium polyphosphate dissolved in the water. According to the inventor's findings, the saturated solubility of potassium polyphosphate in water has little temperature dependency and changes little at low temperatures below 0°C. In this regard, even when the aqueous electrolyte solution 20 is cooled from 0°C to -40°C, salt precipitation in the aqueous electrolyte solution 20 is unlikely to occur.

[0036] 2.3.3 Viscosity 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 potassium polyphosphate is dissolved at a high concentration in the aqueous electrolyte solution 20, the aqueous electrolyte solution 20 may have a viscosity above a certain level. From the above perspective, the aqueous electrolyte solution 20 may have a viscosity of 10 mPa·s or more and 600 mPa·s or less at 20°C. The viscosity may be 500 mPa·s or less, 400 mPa·s or less, 350 mPa·s or less, or 300 mPa·s or less.

[0037] 2.3.4 pH The pH of the aqueous electrolyte solution 20 is not particularly limited. However, if the pH is too high, the oxidation-side potential window of the aqueous electrolyte solution may be narrowed. In this regard, the pH of the aqueous electrolyte solution 20 may be 3 or more and 13 or less. Alternatively, the pH may be 3 or more, 4 or more, 5 or more, 6 or more, or 7 or more, or 14 or less, 13.5 or less, 13 or less, 12.5 or less, 12 or less, 11 or less, 10 or less, or 9 or less. In particular, when the pH of the aqueous electrolyte solution 20 is 3 or more and 13 or less, 4 or more and 12.5 or less, 4 or more and 12 or less, 4 or more and 11.5 or less, 4 or more and 11 or less, or 5 or more and 13 or less, and even more preferably 5 or more and 10 or less, more excellent performance is likely to be ensured.

[0038] 3. Other configurations An example of other configurations of the aqueous battery 100 will be described below.

[0039] 3.1 Positive electrode The positive electrode 10 includes the above-described positive electrode active material. As shown in Fig. 1, the positive electrode 10 may include, for example, a positive electrode active material layer 11 and a positive electrode current collector 12. In this case, the positive electrode active material layer 11 includes the positive electrode active material.

[0040] 3.1.1 Cathode active material layer The positive electrode active material layer 11 is impregnated with an aqueous electrolyte solution 20. The positive electrode active material layer 11 may contain, in addition to the positive electrode active material, a conductive additive, a binder, and the like. The positive electrode active material layer 11 may also contain various other additives. The content of each component in the positive electrode active material layer 11 may be appropriately determined depending on the desired battery performance. For example, when the entire positive electrode active material layer 11 (total solid content) is taken as 100 mass%, the content of the positive electrode active material may be 40 mass% or more, 50 mass% or more, 60 mass% or more, or 70 mass% or more, or may be 100 mass% or less, or 90 mass% or less. The shape of the positive electrode active material layer 11 is not particularly limited, and may be, for example, a sheet-like positive electrode active material layer having a substantially flat surface. The thickness of the positive electrode active material layer 11 is not particularly limited, and may be, for example, 0.1 μm or more, 1 μm or more, or 10 μm or more, and may be 2 mm or less, 1 mm or less, or 500 μm or less.

[0041] Examples of conductive additives that can be contained 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 the electrolyte, including nickel, titanium, aluminum, and stainless steel. The conductive additive may be, for example, particulate or fibrous, and its size is not particularly limited. Only one type of conductive additive may be used alone, or two or more types may be used in combination.

[0042] Examples of binders that can be contained 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, carboxymethyl cellulose (CMC)-based binders, etc. Only one type of binder may be used alone, or two or more types may be used in combination.

[0043] 3.1.2 Positive electrode current collector As shown in FIG. 1 , the positive electrode 10 may include a positive electrode current collector 12 in contact with the positive electrode active material layer 11. The positive electrode current collector 12 is in contact with an aqueous electrolyte solution 20. Any positive electrode current collector that can function as a positive electrode current collector for an aqueous battery can be used as the positive electrode current collector 12. The positive electrode current collector 12 may be in the form of a foil, plate, mesh, punched metal, foam, or the like. The positive electrode current collector 12 may be made of a metal foil or metal mesh. Metal foil is particularly easy to handle. The positive electrode current collector 12 may be made of multiple foils. Examples of metal materials that can be used to make 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. In particular, the positive electrode current collector 12 preferably contains Al. When the positive electrode current collector 12 contains Al, the positive electrode current collector 12 may be entirely made of Al, or at least a portion of its surface may be made of Al. For example, the positive electrode current collector 12 may be made of Al foil, or may be a metal foil or a substrate whose surface is coated with Al. The positive electrode current collector 12 may have Al present on at least a portion of its surface that contacts the aqueous electrolyte solution 20, or may have Al present over the entire surface that contacts the aqueous electrolyte solution 20. Furthermore, when the positive electrode current collector 12 is made of multiple sheets of metal foil, some layer may be present between the multiple sheets of metal foil. 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.

[0044] 3.2 Negative electrode The negative electrode 30 includes the above-described negative electrode active material. As shown in Fig. 1, the negative electrode 30 may include, for example, a negative electrode active material layer 31 and a negative electrode current collector 32. In this case, the negative electrode active material layer 31 includes the negative electrode active material.

[0045] 3.2.1 Negative electrode active material layer The negative electrode active material layer 31 is impregnated with the aqueous electrolyte solution 20. The negative electrode active material layer 31 may contain, in addition to the negative electrode active material, a conductive additive, a binder, and the like. The negative electrode active material layer 31 may also contain various other additives. The content of each component in the negative electrode active material layer 31 may be appropriately determined depending on the desired battery performance. For example, when the entire negative electrode active material layer 31 (total solid content) is taken as 100 mass%, the content of the negative electrode active material may be 40 mass% or more, 50 mass% or more, 60 mass% or more, or 70 mass% or more, or may be 100 mass% or less, or 90 mass% or less. The shape of the negative electrode active material layer 31 is not particularly limited, and may be, for example, a substantially flat sheet-like negative electrode active material layer. The thickness of the negative electrode active material layer 31 is not particularly limited, and may be, for example, 0.1 μm or more, 1 μm or more, or 10 μm or more, or 2 mm or less, 1 mm or less, or 500 μm or less.

[0046] Examples of conductive additives that can be contained in the negative electrode active material layer 31 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 the electrolyte, including nickel, titanium, aluminum, and stainless steel. The conductive additive may be, for example, particulate or fibrous, and its size is not particularly limited. Only one type of conductive additive may be used alone, or two or more types may be used in combination.

[0047] Examples of binders that can be contained in the negative electrode active material layer 31 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, carboxymethyl cellulose (CMC)-based binders, etc. Only one type of binder may be used alone, or two or more types may be used in combination.

[0048] 3.2.2 Negative electrode current collector As shown in FIG. 1 , the negative electrode 30 may include a negative electrode current collector 32 in contact with the negative electrode active material layer 31. The negative electrode current collector 32 is in contact with the aqueous electrolyte solution 20. Any material capable of functioning 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, plate, mesh, punched metal, foam, or the like. The negative electrode current collector 32 may be made of a metal foil or metal mesh. Metal foil is particularly advantageous in terms of ease of handling. The negative electrode current collector 32 may be made of multiple foils. Examples of metal materials 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 negative electrode current collector 32 preferably contains at least one selected from the group consisting of Al, Ti, Pb, Zn, Sn, Mg, Zr, and In, and particularly preferably contains Al. Al, Ti, Pb, Zn, Sn, Mg, Zr, and In all have low work functions, and are therefore thought to make electrolysis of the aqueous electrolyte solution 20 unlikely even when the negative electrode current collector 32 comes into contact with the aqueous electrolyte solution 20 at a reduction potential. The negative electrode current collector 32 may be a metal foil or a substrate plated or vapor-deposited with any of the above metals. When the negative electrode current collector 32 contains Al, the negative electrode current collector 32 may be entirely made of Al, or at least a portion of its surface may be made of Al. For example, the negative electrode current collector 32 may be made of Al foil, or may be a metal foil or a substrate whose surface is coated with Al. The negative electrode current collector 32 may have Al present on at least a portion of the surface that comes into contact with the aqueous electrolyte solution 20, or may have Al present over the entire surface that comes into contact with the aqueous electrolyte solution 20. When the negative electrode current collector 32 is made of multiple metal foils, some layer may be present between the multiple 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, or may be 1 mm or less or 100 μm or less.

[0049] 3.3 Separator In the aqueous battery 100, a separator 40 may be present between the positive electrode 10 and the negative electrode 30. The separator 40 may be a separator used in conventional aqueous batteries (nickel-metal hydride batteries, zinc-air batteries, etc.). For example, it may be a separator made of 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.

[0050] 3.4 Terminals etc. In addition to the above components, the aqueous battery 100 may also include terminals, a battery case, etc. Other components will be obvious to those skilled in the art after reading this application, and therefore will not be described here.

[0051] 4. Manufacturing method of aqueous battery The aqueous battery 100 of the present disclosure can be manufactured, for example, as follows.

[0052] 4.1 Manufacturing method of aqueous electrolyte The aqueous electrolyte solution 20 can be produced, for example, by mixing water with potassium polyphosphate. Alternatively, it can be produced by mixing water with a potassium ion source and a polyphosphate ion source. The mixing means is not particularly limited, and any known mixing means can be used. Simply filling a container with water, potassium polyphosphate, and other optional components and leaving them to stand will allow them to mix together, ultimately producing the aqueous electrolyte solution 20.

[0053] 4.2 Fabrication of the positive electrode The positive electrode 10 is manufactured, for example, as follows. The positive electrode active material and other components that constitute the positive electrode active material layer 11 are dispersed in a solvent to obtain a positive electrode mixture paste (slurry). The solvent used in this case is not particularly limited, and water or various organic solvents can be used. 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 forming the positive electrode 10. As the application method, in addition to the doctor blade method, electrostatic application, dip coating, spray coating, and the like can also be used.

[0054] 4.3 Negative electrode fabrication The negative electrode 30 is manufactured, for example, as follows. The negative electrode active material and other components that constitute the negative electrode active material layer 31 are dispersed in a solvent to obtain a negative electrode mixture paste (slurry). The solvent used in this case is not particularly limited, and water or various organic solvents can be used. 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 forming the negative electrode 30. As the application method, in addition to the doctor blade method, electrostatic application, dip coating, spray coating, and other methods can also be used.

[0055] 4.4 Storage in the battery case, etc. The aqueous electrolyte solution 20, positive electrode 10, and negative electrode 30 are housed in a battery case to form an aqueous battery 100. For example, a separator 40 is sandwiched between the positive electrode 10 and 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 components such as terminals are attached to the laminate as needed. The laminate is housed in a battery case, and the battery case is filled with aqueous electrolyte solution 20. The laminate is immersed in the aqueous electrolyte solution 20, and the laminate and the electrolyte are sealed in the battery case to obtain an aqueous battery 100.

[0056] 5. Effects etc. The aqueous battery 100 of the present disclosure is capable of charging and discharging by combining a predetermined active material with a predetermined aqueous electrolyte. The aqueous battery 100 of the present disclosure employs an inorganic composite oxide as the active material. Inorganic active materials have superior volumetric energy density and other properties compared to organic active materials.

[0057] Furthermore, in the aqueous battery 100 of the present disclosure, the use of a specific aqueous electrolyte solution facilitates the suppression of elution of the current collector into the electrolyte solution. For example, in the aqueous battery 100, when one or both of the positive electrode 10 and the negative electrode 30 have a current collector containing Al, the elution of Al into the electrolyte solution can be suppressed. In conventional aqueous batteries, current collectors containing Ti or Ni are used to prevent corrosion of the current collector (see, for example, Patent Document 1). It has been considered difficult to use metals other than these because they elute, for example, at the positive electrode potential. However, because Ti and Ni are expensive, alternative technologies using less expensive metals are needed to widely popularize aqueous batteries. In this regard, in the aqueous battery 100 of the present disclosure, the use of a current collector containing Al reduces the overall cost of the battery, while the use of the above-described aqueous electrolyte solution 20 suppresses the elution of Al from the current collector into the aqueous electrolyte solution.

[0058] The mechanism by which Al dissolution into the electrolyte is suppressed when Al is contained in the positive electrode current collector is as follows. First, during charge and discharge of the battery, the potential of the positive electrode becomes an oxidizing potential. Therefore, the Al contained in the positive electrode current collector is prone to release electrons and dissolve. Specifically, the Al contained in the positive electrode current collector dissolves into the aqueous electrolyte while coordinating with anions and water molecules contained in the aqueous electrolyte. Here, potassium polyphosphate is dissolved in the aqueous electrolyte 20 of the aqueous battery 100 of the present disclosure. In other words, anions derived from potassium polyphosphate, such as polyphosphate ions, may be present in the aqueous electrolyte 20. Therefore, in the aqueous battery 100 of the present disclosure, the Al contained in the positive electrode current collector 12 is prone to coordinate with anions derived from potassium polyphosphate during charge and discharge of the battery. Here, for example, aluminum polyphosphate has extremely low solubility in the aqueous electrolyte 20. Therefore, the Al coordinated with anions derived from potassium polyphosphate quickly precipitates as a solid. In other words, an insoluble or poorly soluble Al compound precipitates near the surface of the positive electrode current collector 12, and the Al compound adheres to the surface of the positive electrode current collector 12, forming a protective film (passive film) on the surface. As a result, in the aqueous battery 100 of the present disclosure, the protective film can suppress the elution of Al from the positive electrode current collector 12 into the aqueous electrolyte solution 20.

[0059] The mechanism by which Al elution into the electrolyte is suppressed when Al is contained in the negative electrode current collector is as follows. First, during charge and discharge of the battery, the potential of the negative electrode becomes a reduction potential. Therefore, electrolysis of the aqueous electrolyte in contact with the negative electrode generates hydroxide ions, which tends to increase the pH of the aqueous electrolyte near the negative electrode. When the pH of the aqueous electrolyte near the negative electrode increases, the solubility of Al in the aqueous electrolyte increases, making it easier for Al contained in the negative electrode current collector to elute into the aqueous electrolyte. In contrast, in the aqueous battery 100 of the present disclosure, potassium polyphosphate is dissolved in the aqueous electrolyte 20 as described above. Therefore, in the aqueous battery 100 of the present disclosure, even if Al contained in the negative electrode current collector 32 elutes into the aqueous electrolyte 20 during charge and discharge of the battery, it quickly coordinates with anions derived from potassium polyphosphate to form an Al compound and precipitate as a solid. In other words, an insoluble or poorly soluble Al compound precipitates near the surface of the negative electrode current collector 32, and the Al compound adheres to the surface of the negative electrode current collector 32, forming a protective film (passive film) on the surface. As a result, in the aqueous battery 100 of the present disclosure, the protective film can suppress the elution of Al from the negative electrode current collector 32 into the aqueous electrolyte solution 20.

[0060] As described above, the aqueous battery 100 may employ the same material (e.g., Al) for the positive electrode current collector 12 and the negative electrode current collector 32. In this regard, the aqueous battery 100 may employ a bipolar current collector that serves as both the positive electrode current collector 12 and the negative electrode current collector 32. That is, the positive electrode 10 and the negative electrode 30 may share a single current collector. FIG. 2 shows an example of a bipolar structure. As shown in FIG. 2, the aqueous battery 100 may have a bipolar structure, in which a positive electrode active material layer 11 is formed on one surface of a current collector 50 (a bipolar current collector that functions as both the positive electrode current collector 12 and the negative electrode current collector 32), and a negative electrode active material layer 31 is formed on the other surface of the current collector 50. In this case, the current collector 50 may not be liquid permeable, i.e., it may be such that the aqueous electrolyte solution 20 does not pass from the positive electrode active material layer 11 through the current collector 50 to the negative electrode active material layer 31, and vice versa.

[0061] 6.Applications of aqueous batteries The aqueous battery of the present disclosure can be suitably used in at least one vehicle selected from, for example, a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), and an electric vehicle (BEV). Specifically, the technology of the present disclosure also has an aspect of a vehicle having an aqueous battery, the aqueous battery having a positive electrode, an aqueous electrolyte, and a negative electrode, the positive electrode including a positive electrode active material, the negative electrode including a negative electrode active material, one or both of the positive electrode active material and the negative electrode active material including a composite oxide, the composite oxide including Na, at least one transition metal element selected from Fe, Ti, Ni, and Mn, and O, and the aqueous electrolyte including water and potassium polyphosphate dissolved in the water. Details of the positive electrode, aqueous electrolyte, and negative electrode, as well as details of the battery configuration, are as described above. [Example]

[0062] The technology of the present disclosure will be described in more detail below with reference to examples, but the technology of the present disclosure is not limited to the following examples.

[0063] 1. Preparation of Aqueous Electrolyte Potassium pyrophosphate (K4P2O7) was dissolved in pure water at a concentration of 5 mol / kg to obtain an aqueous electrolyte solution.

[0064] 2. Preparation of active material The active materials according to Examples 1 to 8 were obtained by mixing a sodium source (sodium carbonate) and a transition metal source (a transition metal salt or a composite salt containing multiple types of transition metals) in a predetermined ratio and firing the mixture.

[0065] 2.1 Example 1: NaFeO Na2CO3 and Fe3O4 were mixed in a molar ratio of Na:Fe=1:1, and the mixture was baked at 700°C for 12 hours in an air stream to obtain the active material of Example 1.

[0066] 2.2 Example 2: NaFe 0.5 Ti 0.5 O2 Na2CO3, γFe2O3, and Ti2O3 were mixed in a molar ratio of Na:Fe:Ti=1:0.5:0.5, and fired at 700°C for 12 hours under an argon flow, to obtain an active material according to Example 2.

[0067] 2.3 Example 3: NaFe 0.5 Mn 0.5 O2 Na2CO3, γFe2O3, and Mn2O3 were mixed in a molar ratio of Na:Fe:Mn=1:0.5:0.5, and fired at 800°C for 12 hours in an air stream to obtain an active material according to Example 3.

[0068] 2.4 Example 4: Na 0.7 Fe 0.5 Mn 0.5 O2 Na2CO3, γFe2O3, and Mn2O3 were mixed in a molar ratio of Na:Fe:Mn=0.7:0.5:0.5, and fired at 800°C for 12 hours in an air stream to obtain an active material according to Example 4.

[0069] 2.5 Example 5: NaFe 0.75 Ni 0.25 O2 Na2CO3, Fe3O4, and Ni2O3 were mixed in a molar ratio of Na:Fe:Ni=1:0.75:0.25, and the mixture was fired in an air stream at 700°C for 12 hours to obtain an active material according to Example 5.

[0070] 2.6 Example 6: Na 0.7 Mn 0.6 Ni 0.4 O2, Example 7: Na 0.5 Mn 0.75 Ni 0.25 O2 MnSO4·5H2O and NiSO4·6H2O were weighed to the desired composition ratio and dissolved in distilled water to a concentration of 1.2 mol / L to obtain the first solution. In a separate container, Na2CO3 was dissolved in distilled water to a concentration of 1.2 mol / L to obtain the second solution. 1000 mL of pure water was placed in a reaction vessel, and 500 mL of the first solution and 500 mL of the second solution were added dropwise at a rate of approximately 4 mL / min. After the addition, the mixture was stirred at 150 rpm at room temperature for 1 hour to obtain the product. The product was washed with pure water and subjected to solid-liquid separation in a centrifuge to recover the precipitate. The resulting precipitate was dried overnight at 120°C, crushed in a mortar, and then separated into coarse and fine particles by air classification. The fine particles were removed to obtain precursor particles, which are Mn and Ni composite salts. The obtained composite salt and Na2CO3 were weighed to obtain the desired composition ratio, mixed, and then baked at 800°C for 24 hours in an air stream to obtain the active materials according to Examples 6 and 7.

[0071] 2.7 Example 8: NaNi 0.5 Ti 0.5 O2 Na2CO3, Ni2O3 and TiO2 were mixed in a molar ratio of Na:Ni:Ti=1:0.5:0.5 and fired at 930°C for 25 hours in an air stream to obtain an active material according to Example 8.

[0072] 2.8 Example 9: NaFe 0.5 Ni 0.5 O2 Na2CO3, γFe2O3, and Ni2O3 were mixed in a molar ratio of Na:Fe:Ni=1:0.5:0.5, and fired at 800°C for 12 hours in an air stream to obtain an active material according to Example 9.

[0073] 2.9 Example 10: Na 0.7 Fe 0.5 Ni 0.5 O2 Na2CO3, γFe2O3, and Ni2O3 were mixed in a molar ratio of Na:Fe:Ni=0.7:0.5:0.5, and fired at 800°C for 12 hours in an air stream to obtain an active material according to Example 10.

[0074] 2.10 Example 11: NaTi 0.5 Mn 0.5 O2 Na2CO3, TiO2, and Mn2O3 were mixed in a molar ratio of Na:Ti:Mn=1:0.5:0.5, and fired at 850°C for 12 hours in an air stream to obtain an active material according to Example 11.

[0075] 2.11 Example 12: Na 0.7 Ti 0.5 Mn 0.5 O2 Na2CO3, TiO2, and Mn2O3 were mixed in a molar ratio of Na:Ti:Mn=0.7:0.5:0.5, and fired at 850°C for 12 hours in an air stream to obtain an active material according to Example 12.

[0076] 2.12 Example 13: NaFe 0.75 Ti 0.25 O2 Na2CO3, γFe2O3, and TiO2 were mixed in a molar ratio of Na:Ti:Mn=1:0.75:0.25, and fired at 850°C for 12 hours under argon flow, to obtain an active material according to Example 13.

[0077] 2.13 Example 14: NaNi 0.5 Mn 0.5 O2 Na2CO3, Ni2O3, and Mn2O3 were mixed in a molar ratio of Na:Ni:Mn=1:0.5:0.5, and fired in an oxygen flow at 750°C for 12 hours to obtain an active material according to Example 14.

[0078] 2.14 Example 15: NaFe 0.25 Ni 0.75 O2 Na2CO3, γFe2O3, and Ni2O3 were mixed in a molar ratio of Na:Fe:Ni=1:0.25:0.75, and fired in an oxygen flow at 750°C for 12 hours to obtain an active material according to Example 15.

[0079] 2.15 Example 16: NaTi 0.25 Ni 0.75 O2 Na2CO3, TiO2, and Ni2O3 were mixed in a molar ratio of Na:Ti:Ni=1:0.25:0.75, and fired at 750°C for 12 hours in an oxygen stream to obtain an active material according to Example 16.

[0080] 2.16 Example 17: NaMn 0.25 Ni 0.75 O2 Na2CO3, Mn2O3, and Ni2O3 were mixed in a molar ratio of Na:Mn:Ni=1:0.25:0.75, and fired in an oxygen flow at 750°C for 12 hours to obtain an active material according to Example 17.

[0081] The composition of each active material was as shown in Table 1 below. The crystalline structure of each active material was confirmed by X-ray diffraction pattern. Each active material had the crystalline structure shown in Table 1 below. [Table 1]

[0082] 3. Electrode Fabrication The active material, acetylene black (AB) as a conductive additive, styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) as binders, and water were mixed, followed by degassing at 2000 rpm for 1 minute using a stirring degassing device (Awatori Rentaro). The mass ratio of the active material, AB, SBR, and CMC contained in the ink was 75:20:4:1. The ink was dispensed onto a metal foil fixed on a glass plate and coated with a doctor blade at a gap of 150 μm to form a coating on the surface of the metal foil. The metal foil with the coating was left to stand under reduced pressure, then naturally dried, and further dried overnight at 60°C in a vacuum dryer to obtain an electrode with an active material layer on the surface of the metal foil. The resulting electrode was punched out to a diameter of 16 mm and pressed at a linear pressure of 1 ton to densify it, yielding an electrode for evaluation.

[0083] 4. Electrochemical Measurements 4.1 Preparation of evaluation cell An electrochemical cell (VM4, manufactured by Interchemi) was fabricated using the above electrode as a working electrode, Ni foil as a counter electrode, Ag / AgCl as a reference electrode, and the above aqueous electrolyte as an electrolyte.

[0084] 4.2 Measurement conditions A charge / discharge test was carried out on each electrochemical cell using an electrochemical measurement system to confirm whether or not charge / discharge was possible and the reaction potential. Electrochemical measurement system: VMP3 (Biologic) Evaluated current value: ±0.1mA / cm 2 Cutting conditions: -1.0 to 1.15V vs. SHE

[0085] 5.Results 5.1 Example 1: NaFeO Figure 3 shows the charge / discharge curves when the active material of Example 1 was used. As shown in Figure 3, the active material of Example 1 was able to be charged and discharged in the above aqueous electrolyte. In addition, the reaction potential of the active material of Example 1 was mainly negative, and it was found that it can function suitably as, for example, a negative electrode active material.

[0086] 5.2 Example 2: NaFe 0.5 Ti 0.5 O2 Figure 4 shows the charge / discharge curves when the active material of Example 2 was used. As shown in Figure 4, the active material of Example 2 was able to be charged and discharged in the above aqueous electrolyte. In addition, the reaction potential of the active material of Example 2 was mainly negative, and it was found that it can function suitably as, for example, a negative electrode active material.

[0087] 5.3 Example 3: NaFe 0.5 Mn 0.5 O2 Figure 5 shows the charge / discharge curves when the active material of Example 3 was used. As shown in Figure 5, the active material of Example 3 was able to be charged and discharged in the above aqueous electrolyte solution. In addition, the reaction potential of the active material of Example 3 was mainly negative, and it was found that it can function suitably as, for example, a negative electrode active material.

[0088] 5.4 Example 4: Na 0.7 Fe 0.5 Mn 0.5 O2 Figure 6 shows the charge / discharge curves when the active material of Example 4 was used. As shown in Figure 6, the active material of Example 4 was able to be charged and discharged in the above aqueous electrolyte. In addition, the reaction potential of the active material of Example 4 was mainly negative, and it was found that it can function suitably as, for example, a negative electrode active material.

[0089] 5.5 Example 5: NaFe 0.75 Ni 0.25 O2 Figure 7 shows the charge / discharge curves when the active material of Example 5 was used. As shown in Figure 7, the active material of Example 5 was able to be charged and discharged in the above aqueous electrolyte. In addition, the reaction potential of the active material of Example 5 was mainly positive, and it was found that it can function suitably as, for example, a positive electrode active material.

[0090] 5.6 Example 6: Na 0.7 Mn 0.6 Ni 0.4 O2 Figure 8 shows the charge / discharge curves when the active material of Example 6 was used. As shown in Figure 8, the active material of Example 6 was able to be charged and discharged in the above aqueous electrolyte. In addition, the reaction potential of the active material of Example 6 was mainly positive, and it was found that it can function suitably as, for example, a positive electrode active material.

[0091] 5.7 Example 7: Na 0.5 Mn 0.75 Ni 0.25 O2 Fig. 9 shows charge / discharge curves when the active material of Example 7 was used. As shown in Fig. 9, the active material of Example 7 was able to be charged / discharged in the above aqueous electrolyte solution. Furthermore, it was found that the active material of Example 7 has reaction potentials on both the positive and negative sides and can function as, for example, both a positive electrode active material and a negative electrode active material.

[0092] 5.8 Example 8: NaNi 0.5 Ti 0.5 O2 Figure 10 shows the charge / discharge curves when the active material of Example 8 was used. As shown in Figure 10, the active material of Example 8 was able to be charged and discharged in the above aqueous electrolyte. In addition, the reaction potential of the active material of Example 8 was mainly positive, and it was found that it can function suitably as, for example, a positive electrode active material.

[0093] 5.9 Example 9: NaFe 0.5 Ni 0.5 O2 Fig. 11 shows the charge / discharge curves when the active material of Example 9 was used. As shown in Fig. 11, the active material of Example 9 was able to be charged / discharged in the above aqueous electrolyte solution. Furthermore, it was found that the active material of Example 9 has reaction potentials on both the positive and negative sides and can function as, for example, both a positive electrode active material and a negative electrode active material.

[0094] 5.10 Example 10: Na 0.7 Fe 0.5 Ni 0.5 O2 Fig. 12 shows charge / discharge curves when the active material of Example 10 was used. As shown in Fig. 12, the active material of Example 10 was able to be charged / discharged in the above aqueous electrolyte solution. Furthermore, it was found that the active material of Example 10 has reaction potentials on both the positive and negative sides and can function as, for example, both a positive electrode active material and a negative electrode active material.

[0095] 5.11 Example 11: NaTi 0.5 Mn 0.5 O2 Fig. 13 shows charge / discharge curves when the active material of Example 11 was used. As shown in Fig. 13, the active material of Example 11 was able to be charged / discharged in the above aqueous electrolyte solution. Furthermore, it was found that the active material of Example 11 has reaction potentials on both the positive and negative sides, and can function as, for example, both a positive electrode active material and a negative electrode active material.

[0096] 5.12 Example 12: Na 0.7 Ti 0.5 Mn 0.5 O2 Fig. 14 shows charge / discharge curves when the active material of Example 12 was used. As shown in Fig. 14, the active material of Example 12 was able to be charged / discharged in the above aqueous electrolyte solution. Furthermore, it was found that the active material of Example 12 has reaction potentials on both the positive and negative sides, and can function as, for example, both a positive electrode active material and a negative electrode active material.

[0097] 5.13 Example 13: NaFe 0.75 Ti 0.25 O2 Figure 15 shows the charge / discharge curves when the active material of Example 13 was used. As shown in Figure 15, the active material of Example 13 was able to be charged / discharged in the above aqueous electrolyte. In addition, the reaction potential of the active material of Example 13 was mainly negative, and it was found that it can function suitably as, for example, a negative electrode active material.

[0098] 5.14 Example 14: NaNi 0.5 Mn 0.5 O2 Figure 16 shows the charge / discharge curves when the active material of Example 14 was used. As shown in Figure 16, the active material of Example 14 was able to be charged / discharged in the above aqueous electrolyte. In addition, the reaction potential of the active material of Example 14 was mainly positive, and it was found that it can function suitably as, for example, a positive electrode active material.

[0099] 5.15 Example 15: NaFe 0.25 Ni 0.75 O2 Fig. 17 shows charge / discharge curves when the active material of Example 15 was used. As shown in Fig. 17, the active material of Example 15 was able to be charged / discharged in the above aqueous electrolyte solution. Furthermore, it was found that the active material of Example 15 has reaction potentials on both the positive and negative sides and can function as, for example, both a positive electrode active material and a negative electrode active material.

[0100] 5.16 Example 16: NaTi 0.25 Ni 0.75 O2 Fig. 18 shows charge / discharge curves when the active material of Example 16 was used. As shown in Fig. 18, the active material of Example 16 was able to be charged / discharged in the above aqueous electrolyte solution. Furthermore, it was found that the active material of Example 16 has reaction potentials on both the positive and negative sides and can function as, for example, both a positive electrode active material and a negative electrode active material.

[0101] 2.16 Example 17: NaMn 0.25 Ni 0.75 O2 Figure 19 shows the charge / discharge curves when the active material of Example 17 was used. As shown in Figure 19, the active material of Example 17 was able to be charged and discharged in the above aqueous electrolyte. In addition, the reaction potential of the active material of Example 17 was mainly positive, and it was found that it can function suitably as, for example, a positive electrode active material.

[0102] 6. Supplementary Information In the above examples, potassium pyrophosphate was dissolved in the aqueous electrolyte as potassium polyphosphate. However, the potassium polyphosphate dissolved in the aqueous electrolyte is not limited to potassium pyrophosphate. The inventors have confirmed that the same effects as those described above can be achieved even when a potassium polyphosphate other than potassium pyrophosphate (e.g., potassium tripolyphosphate) is dissolved in the aqueous electrolyte instead of or together with potassium pyrophosphate.

[0103] In the above examples, the aqueous electrolyte solution does not contain any electrolyte other than potassium polyphosphate, but the aqueous electrolyte solution may contain an electrolyte other than potassium polyphosphate.

[0104] 7. Summary From the above results, it can be said that aqueous batteries comprising the following active materials and aqueous electrolyte solution are capable of charging and discharging.

[0105] (1) One or both of the positive electrode active material and the negative electrode active material contains a composite oxide, wherein the composite oxide contains Na, at least one transition metal element selected from Fe, Ti, Ni, and Mn, and O. (2) The aqueous electrolyte contains water and potassium polyphosphate dissolved in the water. [Explanation of symbols]

[0106] 10 positive electrode 11 Cathode 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 Water-based batteries

Claims

1. An aqueous battery having a positive electrode, an aqueous electrolyte, and a negative electrode, the positive electrode includes a positive electrode active material, the negative electrode includes a negative electrode active material, one or both of the positive electrode active material and the negative electrode active material contains a composite oxide, The composite oxide is Na and at least one transition metal element selected from Fe, Ti, Ni, and Mn; O and Including, The aqueous electrolyte solution is Water and potassium polyphosphate dissolved in the water; Including, water-based battery.

2. 10. The aqueous battery of claim 1, one or both of the positive electrode active material and the negative electrode active material contains at least one of a first composite oxide, a second composite oxide, and a third composite oxide; The first composite oxide is Na ｘ Fe １－ｙ M １ ｙ O ２ It has a composition represented by The second composite oxide is Na ｘ Ti １－ｙ M ２ ｙ O ２ It has a composition represented by The third composite oxide is Na ｘ Ni １－ｙ M ３ ｙ O ２ It has a composition represented by where: 0<x≦1, 0≦y≦1, M １ contains one or both of Ti and Mn, and does not contain Ni; M ２ contains one or both of Fe and Mn, and does not contain Ni; M ３ contains at least one of Fe, Ti and Mn; water-based battery.

3. 3. The aqueous battery of claim 2, the negative electrode active material contains one or both of the first composite oxide and the second composite oxide; water-based battery.

4. 4. The aqueous battery of claim 3, M １ is one or both of Ti and Mn, M ２ is one or both of Fe and Mn; water-based battery.

5. 3. The aqueous battery of claim 2, the positive electrode active material contains the third composite oxide, water-based battery.

6. 6. The aqueous battery of claim 5, M ３ is at least one of Fe, Ti and Mn; water-based battery.

7. The aqueous battery according to any one of claims 1 to 6, the aqueous electrolyte solution contains the potassium polyphosphate dissolved in a concentration of 3 mol or more per 1 kg of water; water-based battery.

8. The aqueous battery according to any one of claims 1 to 6, the aqueous electrolyte solution contains the potassium polyphosphate dissolved in a concentration of 3 mol or more and 6 mol or less per 1 kg of water; water-based battery.

9. The aqueous battery according to any one of claims 1 to 6, the aqueous electrolyte solution contains the potassium polyphosphate dissolved at a concentration of 4 mol or more and 6 mol or less per 1 kg of water; water-based battery.

10. The aqueous battery according to any one of claims 1 to 6, The aqueous electrolyte does not have a freezing point at −40° C. or higher. water-based battery.

11. The aqueous battery according to any one of claims 1 to 6, The aqueous electrolyte does not precipitate salt when cooled from 0°C to -40°C. water-based battery.

12. The aqueous battery according to any one of claims 1 to 6, The aqueous electrolyte has a viscosity of 10 mPa·s or more and 600 mPa·s or less at 20°C. water-based battery.

13. The aqueous battery according to any one of claims 1 to 6, The pH of the aqueous electrolyte is 3 or more and 13 or less. water-based battery.

14. The aqueous battery according to any one of claims 1 to 6, One or both of the positive electrode and the negative electrode has a current collector containing Al. water-based battery.

15. 15. The aqueous battery of claim 14, It has a bipolar structure, a positive electrode active material layer is formed on one surface of the current collector; a negative electrode active material layer is formed on the other surface of the current collector; water-based battery.

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