Secondary batteries

Abstract

To disclose a new combination of a positive electrode active material capable of charging / discharging as a secondary battery and an aqueous electrolyte.SOLUTION: A secondary battery of the present disclosure includes a positive electrode active material and an aqueous electrolyte. The positive electrode active material includes a layered oxide represented by AxKyNi1-zMzO2±δ-nH2O where the A represents at least one element selected from the group consisting of Li, Na, Rb, Cs, Mg, Ca, Sr, Ba, Sc and Y, the M represents at least one element selected from the group consisting of transition metal elements, group 2A elements, group 3A elements, group 2B elements and group 3B elements, relationships of 0≤x<0.5, 0<y≤0.5, 0≤z≤0.5, 0<n≤2, and (α*x)+y≤0.5 are satisfied, the α representing a cation valence of the A. The aqueous electrolyte includes water and potassium pyrophosphate dissolved in the water.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present application discloses a secondary battery. [Background technology]

[0002] Patent Document 1 describes a method for producing a secondary battery using Li as a positive electrode active material. 0.1 K 0.2 NiO 1.9 Patent Document 2 discloses a nickel-containing oxide represented by the formula A. 0.7H2O as a positive electrode active material for a secondary battery. x Ni 1-z M z Patent Document 3 discloses a nickel-containing layered oxide represented by A.O2 as a positive electrode active material for a secondary battery. x Ni 1-z M z A nickel-containing layered oxide represented by O2·nH2O is disclosed. Patent Documents 1 to 3 also disclose, as electrolytes to be combined with the above-mentioned positive electrode active material, a nonaqueous electrolyte containing lithium ions (LiPF6 dissolved in a carbonate-based solvent) and an aqueous solution containing sodium ions (aqueous sodium chloride solution). Meanwhile, Patent Document 4 discloses, as an electrolyte for use in an aqueous potassium-ion battery, an aqueous electrolyte containing water and potassium pyrophosphate dissolved in water. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-332259 [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-151549 [Patent Document 3] Japanese Patent Application Laid-Open No. 2005-322551 [Patent Document 4] Japanese Patent Application Publication No. 2019-220294 Summary of the Invention [Problem to be solved by the invention]

[0004] When a non-aqueous electrolyte in which a potassium compound is dissolved in a non-aqueous solvent is employed as the electrolyte of a secondary battery, there is a problem that highly reactive metallic potassium is likely to precipitate. In order to avoid this problem, in a secondary battery, instead of the above non-aqueous electrolyte, an aqueous electrolyte in which a potassium compound is dissolved in water may be employed. However, conventionally, there is little known about a positive electrode active material that can be charged and discharged in an electrolyte in which a potassium compound other than a hydroxide is dissolved, whether non-aqueous or aqueous, and there is a problem that the degree of freedom in designing a secondary battery is low. In this regard, a new combination of an electrolyte in which a potassium compound other than a hydroxide is dissolved and a positive electrode active material, which enables charging and discharging of the positive electrode active material, is required.

Means for Solving the Problem

[0005] As one of the means for solving the above problem, the present application provides a secondary battery having a positive electrode active material and an aqueous electrolyte, where the positive electrode active material contains a layered oxide represented by A x K y Ni 1-z M z O 2±δ ·nH2O, where A is at least one element selected from the group consisting of Li, Na, Rb, Cs, Mg, Ca, Sr, Ba, Sc, and Y, where M is at least one element selected from the group consisting of transition metal elements, Group 2A elements, Group 3A elements, Group 2B elements, and Group 3B elements, [[ID=*29]]a relationship of 0 ≦ x < 0.5, 0 < y ≦ 0.5, 0 ≦ z ≦ 0.5, 0 < n ≦ 2, and (α·x) + y ≦ 0.5 is satisfied, where α is the valence of the cation of A, where the aqueous electrolyte contains water and potassium pyrophosphate dissolved in the water, a secondary battery is disclosed. [[ID=*36]] [[ID=*37]]

[0006] [[ID=*38]] Note: There seems to be an issue with the numbering in the original text as the equations in ID=29 are not properly formatted. I've translated it as best as possible while keeping the original structure. If this is a formatting error in the original, it might need to be corrected for a more accurate translation. Also, the ID=36, ID=37, and ID=38 tags seem to be empty in the original, so I've left them as is in the translation.In the secondary battery of the present disclosure, the potassium pyrophosphate may be dissolved in the water at a concentration of 2 mol or more per 1 kg of the water.

[0007] In the secondary battery of the present disclosure, the potassium pyrophosphate may be dissolved in the water at a concentration of 5 mol or more per 1 kg of the water. [Effects of the Invention]

[0008] The secondary battery of the present disclosure uses an aqueous electrolyte solution, which makes it difficult for highly reactive metallic potassium to precipitate. Furthermore, the secondary battery of the present disclosure combines a predetermined layered oxide as a positive electrode active material with an aqueous electrolyte solution containing dissolved potassium pyrophosphate, and this combination enables charging and discharging of the positive electrode active material. [Brief explanation of the drawings]

[0009] [Figure 1] 1 shows a schematic diagram of an example of the configuration of a secondary battery. [Figure 2] 1 shows the charge / discharge curves of Example 1. [Figure 3] 1 shows the charge / discharge curves of Comparative Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0010] 1. Secondary battery The secondary battery of the present disclosure includes a positive electrode active material and an aqueous electrolyte solution. x K y Ni 1-z M z O 2±δIt contains a layered oxide represented by ·nH2O. Here, the said A is at least one element selected from the group consisting of Li, Na, Rb, Cs, Mg, Ca, Sr, Ba, Sc, and Y, the said M is at least one element selected from the group consisting of transition metal elements, Group 2A elements, Group 3A elements, Group 2B elements, and Group 3B elements, and the relationships 0≦x<0.5, 0<y≦0.5, 0≦z≦0.5, 0<n≦2, and (α·x)+y≦0.5 are satisfied. Note that the said α is the valence of the cation of the said A. Also, the said aqueous electrolyte contains water and potassium pyrophosphate dissolved in the water.

[0011] 1.1 Cathode active material The secondary battery of the present disclosure has a layered oxide as a cathode active material. The layered oxide is A x K y Ni 1-z M z O 2±δ represented by ·nH2O. It can be said that in the layered oxide, K and optionally A and H2O exist between the layers mainly composed of Ni, M, and O.

[0012] 1.1.1 Elements A and K In the layered oxide, A is at least one element selected from the group consisting of Li, Na, Rb, Cs, Mg, Ca, Sr, Ba, Sc, and Y. However, the layered oxide does not necessarily contain A. On the other hand, K is essentially contained in the layered oxide. In the layered oxide, A and K can exist between the layers mainly composed of Ni, M, and O and can exist in a cationic state sandwiched between oxygen atoms. The presence of A and K between the layers increases the interlayer distance and makes it easier to accommodate carrier ions. For example, it is considered that the presence of K between the layers makes it easier to accommodate potassium ions as carrier ions.

[0013] The presence of A or K as a cation between layers of a layered oxide stabilizes the water between layers, making it easier to maintain the structure. Here, in a layered oxide, when the valence of the cation A is α, the relationship (α·x) + y ≦ 0.5 must be satisfied, as described below. The smaller (α·x) is, the more likely this relationship is satisfied. For example, when the cation valence of A is 1 or 2, it is easier to reduce the value of α·x. In this regard, A may be at least one element selected from the group consisting of Li, Na, Rb, Cs, Mg, Ca, Sr, and Ba, or at least one element selected from the group consisting of Li, Na, Rb, and Cs.

[0014] 1.1.2 Ni and element M In the layered oxide, Ni can have a valence of 2 to 4. It is believed that in the secondary battery of the present disclosure, the change in the valence of Ni in the layered oxide causes carrier ions to be absorbed or released, thereby functioning as a positive electrode active material. Here, in the layered oxide, a portion of Ni may be substituted with M. By substituting a portion of Ni with M, improvements in reversibility, thermal stability, and storage stability can be expected. However, M does not necessarily need to be present in the layered oxide. M is at least one element selected from the group consisting of transition metal elements, Group 2A elements, Group 3A elements, Group 2B elements, and Group 3B elements. M may have an ionic radius close to the ionic radius of Ni, and may be, for example, at least one element selected from the group consisting of Al, Ti, V, Cr, Mn, Fe, Co, and Ga.

[0015] 1.1.3 H2O Water may exist between the layers of the layered oxide. As described above, the presence of A or K as cations between the layers is considered to enable the water between the layers to exist more stably. Further, in the secondary battery of the present disclosure, an aqueous electrolyte described later is adopted as the electrolyte, and due to this, it is considered that it is difficult for water to escape from between the layers of the layered oxide, and water is likely to be retained between the layers even during and after charge and discharge of the positive electrode active material. The presence of water between the layers is considered to further increase the interlayer distance and further increase the space capable of accommodating carrier ions.

[0016] 1.1.4 Composition Ratio Regarding the composition ratios x, y, and z in the chemical composition of the layered oxide, any relationship that satisfies 0 ≦ x < 0.5, 0 < y ≦ 0.5, 0 ≦ z ≦ 0.5, 0 < n ≦ 2, and (α·x) + y ≦ 0.5 may be used. Here, α is the valence of the cation of A. Also, the composition ratio of oxygen is not limited to 2, and a certain degree of variation is allowed. For example, δ may be 0.5, 0.3, or 0.1.

[0017] Since A is an optional element in the layered oxide, the lower limit of x is 0. The upper limit of x only needs to satisfy the relationship (α·x) + y ≦ 0.5 in relation to y, and it will naturally be less than 0.5. x may be 0.4 or less, 0.3 or less, or 0.2 or less.

[0018] Since K is necessarily included in the layered oxide, the lower limit of y is greater than 0. The upper limit of y only needs to satisfy the relationship (α·x) + y ≦ 0.5 in relation to x, and it will naturally be 0.5 or less. y may be 0.4 or less, 0.3 or less, or 0.2 or less.

[0019] Since M is an optional element in the layered oxide, the lower limit of z is 0. On the other hand, if z is too large, the relative amount of Ni will be too small, and it may be difficult to ensure sufficient capacity. In this regard, the upper limit of z is 0.5 or less, and may be 0.4 or less, 0.3 or less, 0.2 or less, or 0.1 or less.

[0020] If no water is present between the layers of the layered oxide, the spacing between the layers may become narrow, making it difficult to accommodate carrier ions. In this regard, the lower limit of n is greater than 0, and may be 0.1 or more, 0.2 or more, 0.3 or more, 0.4 or more, 0.5 or more, 0.6 or more, or 0.7 or more. On the other hand, if the amount of water between the layers of the layered oxide is too large, many portions will not participate in the battery reaction, and the energy density may decrease. In this regard, the upper limit of n is 2.0 or less, and may be 1.8 or less, 1.6 or less, 1.4 or less, 1.2 or less, or 1.0 or less.

[0021] If the layered oxide contains excessive amounts of A or K, i.e., if (α·x)+y is too large, the valence of Ni in the layered oxide will be small, which may result in insufficient capacity and the layered structure may not be maintained. By keeping (α·x)+y at 0.5 or less, the above problems can be easily avoided. (α·x)+y may also be 0.4 or less, 0.3 or less, or 0.2 or less.

[0022] 1.1.5 Shape The layered oxide may have any shape commonly used as a positive electrode active material for a battery. The layered oxide may be, for example, particulate. In this case, the particle size is not particularly limited, and an appropriate size may be selected depending on the battery design. The layered oxide may have a primary particle size of 1 nm or more, 5 nm or more, 10 nm or more, or 50 nm or more, or 500 μm or less, 100 μm or less, 50 μm or less, 30 μm or less, or 10 μm or less. Furthermore, the layered oxide may be formed by aggregation of primary particles to form secondary particles. In this case, the particle size of the secondary particles is not particularly limited, but may be, for example, 100 nm or more, 500 nm or more, or 1 μm or more, or 1000 μm or less, 500 μm or less, 100 μm or less, 50 μm or less, 30 μm or less, or 20 μm or less.

[0023] 1.1.6 Other positive electrode active materials The secondary battery of the present disclosure may contain other positive electrode active materials together with the layered oxides, depending on the purpose. In the secondary battery of the present disclosure, the layered oxides may account for 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 95% by mass or more of the total positive electrode active material (100% by mass).

[0024] 1.2 Aqueous electrolyte In the secondary battery of the present disclosure, the aqueous electrolyte solution contains water and potassium pyrophosphate dissolved in the water.

[0025] 1.2.1 Solvent An aqueous electrolyte 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 electrolyte (100 mol%), water accounts for 50 mol% or more, 70 mol% or more, 90 mol% or more, or 95 mol% or more. On the other hand, there is no particular upper limit to the proportion of water in the solvent. The solvent may consist of only water.

[0026] The solvent may contain a solvent other than water in addition to water. For example, a solid electrolyte interphase (SEI) may be formed on the surface of the active material by the solvent other than 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 of the total amount of the solvents constituting the electrolyte (100 mol%).

[0027] 1.2.2 Electrolytes In the aqueous electrolyte, potassium pyrophosphate is dissolved as an electrolyte. Here, the "dissolved potassium pyrophosphate" does not have to be completely ionized into potassium ions and pyrophosphate ions in the aqueous electrolyte. That is, in the aqueous electrolyte, the "dissolved potassium pyrophosphate" is K + , P2O7 4-, KP2O7 3- , K2P2O7 2- , K3P2O7 - The aqueous electrolyte may contain ions such as these, or associations of these ions. Furthermore, in the aqueous electrolyte, the "dissolved potassium pyrophosphate" does not have to be derived from a salt of potassium and pyrophosphate (K4P2O7) (obtained by adding K4P2O7 to water). For example, the aqueous electrolyte may contain a potassium ion source (e.g., KOH or CH3COOK) and a pyrophosphate ion source (e.g., H4P2O7) separately added to water and dissolved therein, resulting in the formation of the above ions or associations.

[0028] The concentration of potassium pyrophosphate in the aqueous electrolyte is not particularly limited and may be appropriately selected depending on the desired battery performance. In the aqueous electrolyte, potassium pyrophosphate may be dissolved in water at a concentration of 2 mol or more or 5 mol or more per kg of water. According to the inventor's new findings, the higher the concentration of potassium pyrophosphate in the aqueous electrolyte, the smaller the hysteresis during charge and discharge of the positive electrode active material, making it easier to obtain high performance as a secondary battery. Furthermore, the higher the concentration of potassium pyrophosphate in the aqueous electrolyte, the lower the overvoltage and the easier it is to achieve a good charge and discharge plateau. Furthermore, the higher the concentration of potassium pyrophosphate in the aqueous electrolyte, the more likely it is that pyrophosphate ions and potassium ions will come into close proximity to form an association complex. Therefore, for example, during charging of a secondary battery, pyrophosphate ions are likely to migrate toward the negative electrode, dragged by potassium ions. It is believed that the pyrophosphate ions that reach the negative electrode decompose at the high work function sites on the surface of the negative electrode, forming a coating on the surface of the negative electrode. As a result, direct contact between the aqueous electrolyte and the high work function sites on the surface of the negative electrode is suppressed, and electrolysis of the aqueous electrolyte is likely to be suppressed.

[0029] The concentration of "dissolved potassium pyrophosphate" in the aqueous electrolyte can be determined as follows. For example, elements and ions contained in the aqueous electrolyte are identified by elemental analysis or ion analysis, and the potassium ion concentration, pyrophosphate ion concentration, etc. in the aqueous electrolyte are determined, and the determined ion concentration is converted into the potassium pyrophosphate concentration. Alternatively, the solvent is removed from the aqueous electrolyte, and the solid content is chemically analyzed and converted into the potassium pyrophosphate concentration.

[0030] In an aqueous electrolyte, all of the potassium ions contained in the electrolyte do not have to be calculated as "dissolved potassium pyrophosphate." That is, the aqueous electrolyte may contain more potassium ions than can be calculated as potassium pyrophosphate. For example, when producing an aqueous electrolyte, if a potassium ion source other than the potassium pyrophosphate source (e.g., KOH, CH3COOK, K3PO4, etc.) is added and dissolved in water together with the potassium pyrophosphate source, the aqueous electrolyte will contain more potassium ions than can be calculated as potassium pyrophosphate.

[0031] The aqueous electrolyte may contain cations other than potassium ions. For example, alkali metal ions other than potassium ions, alkaline earth metal ions, transition metal ions, etc. may be contained. The aqueous electrolyte may also contain pyrophosphate ions (as described above, P2O7 4- In addition, KP2O7 3- , K2P2O7 2- , K3P2O7 - The electrolyte may contain anions other than those mentioned above (which may be present in a state bound to a cation, such as anions of other electrolytes, which will be described later).

[0032] The aqueous electrolyte solution of the present disclosure may contain other electrolytes dissolved therein. For example, KPF6, KBF4, K2SO4, KNO3, CH3COOK, (CF3SO2)2NK, KCF3SO3, (FSO2)2NK, K2HPO4, KH2PO4, etc. The other electrolytes may account for 50 mol% or less, 30 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less of the total amount of electrolytes dissolved in the electrolyte solution (100 mol%).

[0033] 1.2.3 Other ingredients In addition to the solvent and electrolyte, the aqueous electrolyte may contain an acid, hydroxide, or the like to adjust the pH of the aqueous electrolyte, and may also contain various additives. The pH of the aqueous electrolyte is not particularly limited. However, if the pH is too high, the oxidation-side potential window of the aqueous electrolyte may be narrowed. In this regard, the pH of the aqueous electrolyte may be 13 or less or 12 or less. The pH may be 3 or more, 4 or more, 5 or more, 6 or more, or 7 or more.

[0034] 1.3 Other configurations The secondary battery of the present disclosure may have the above-described positive electrode active material and aqueous electrolyte solution, and other configurations are not particularly limited. The secondary battery of the present disclosure may have a configuration in which the above-described positive electrode active material is in contact with the aqueous electrolyte solution. FIG. 1 schematically illustrates the configuration of a secondary battery 100 according to one embodiment. As illustrated in FIG. 1, the secondary battery 100 may include a positive electrode 10, an electrolyte layer 20, and an anode 30. The positive electrode 10 may include a positive electrode active material layer 11 and a positive electrode current collector 12, and the anode 30 may include an anode active material layer 31 and an anode current collector 32. In this case, the positive electrode active material layer 11 may contain the above-described positive electrode active material. The positive electrode 10, the electrolyte layer 20, and the anode 30 may all contain the above-described aqueous electrolyte solution.

[0035] 1.3.1 Positive electrode The positive electrode 10 may have a known configuration except that it contains the above-described positive electrode active material and aqueous electrolyte solution. For example, the positive electrode 10 may include a positive electrode active material layer 11 and a positive electrode current collector 12.

[0036] The positive electrode active material layer 11 contains the above-mentioned positive electrode active material, and may further contain, optionally, a conductive additive, a binder, etc. The thickness of the positive electrode active material layer 11 is not particularly limited, but may be, for example, 0.1 μm or more or 1 μm or more, or 1 mm or less or 100 μm or less.

[0037] The type of the positive electrode active material contained in the positive electrode active material layer 11 is as described above. The amount of the positive electrode active material contained in the positive electrode active material layer 11 is not particularly limited. For example, based on the entire positive electrode active material layer 11 (100 mass%), the positive electrode active material may be contained in an amount of 20 mass% or more, 40 mass% or more, 60 mass% or more, or 70 mass% or more, or 99 mass% or less, 97 mass% or less, or 95 mass% or less.

[0038] The conductive additive optionally contained in the positive electrode active material layer 11 may be any of those known as conductive additives used in secondary batteries. Examples include carbon materials. Specific examples include ketjen black (KB), vapor-grown carbon fiber (VGCF), acetylene black (AB), carbon nanotubes (CNT), carbon nanofibers (CNF), carbon black, coke, and graphite. Alternatively, the conductive additive may be a metal material that can withstand the environment in which the battery is used. Only one conductive additive may be used alone, or two or more conductive additives may be used in combination. The conductive additive may take various forms, such as powder or fiber. The amount of the conductive additive contained in the positive electrode active material layer 11 is not particularly limited.

[0039] The binder optionally contained in the positive electrode active material layer 11 may be any of the binders known to be used in secondary batteries. Examples include styrene butadiene rubber (SBR)-based binders, carboxymethyl cellulose (CMC)-based binders, acrylonitrile butadiene rubber (ABR)-based binders, butadiene rubber (BR)-based binders, polyvinylidene fluoride (PVDF)-based binders, and polytetrafluoroethylene (PTFE)-based binders. One type of binder may be used alone, or two or more types may be used in combination. The amount of binder contained in the positive electrode active material layer 11 is not particularly limited.

[0040] The positive electrode current collector 12 may be made of a known metal that can be used as a positive electrode current collector for a secondary battery. Examples of such metals include metal materials containing at least one element selected from the group consisting of Cu, Ni, Al, V, Au, Pt, Mg, Fe, Ti, Pb, Co, Cr, Zn, Ge, In, Sn, and Zr. The shape of the positive electrode current collector 12 is not particularly limited. It may be in various shapes, such as a foil, a mesh, or a porous shape. The above metal may be deposited or plated on the surface of a substrate.

[0041] 1.3.2 Electrolyte layer In the secondary battery 100, for example, an electrolyte layer 20 may be disposed between the positive electrode active material layer 11 and the negative electrode active material layer 31. The electrolyte layer 20 may be composed of a separator and the above-mentioned aqueous electrolyte solution. As the separator, a separator known to be used in secondary batteries (e.g., nickel-metal hydride batteries, zinc-air batteries, etc.) may be adopted. For example, the separator may be a hydrophilic material such as a nonwoven fabric made of cellulose. The thickness of the separator is not particularly limited and may be, for example, 5 μm or more and 1 mm or less.

[0042] 1.3.3 Negative electrode The negative electrode 30 may have a known configuration as a negative electrode for a secondary battery. For example, the negative electrode 30 may include a negative electrode active material layer 31 and a negative electrode current collector 32.

[0043] The negative electrode active material layer 31 contains a negative electrode active material. The negative electrode active material layer 31 may contain a conductive additive or a binder in addition to the negative electrode active material. The thickness of the negative electrode active material layer 31 is not particularly limited, but may be, for example, 0.1 μm or more or 1 μm or more, or 1 mm or less or 100 μm or less.

[0044] The negative electrode active material contained in the negative electrode active material layer 31 may be selected from active materials having a carrier ion charge / discharge potential lower than that of the positive electrode active material, taking into consideration the potential window of the aqueous electrolyte. Examples include potassium-transition metal composite oxides; titanium oxide; metal sulfides such as Mo6S8; elemental sulfur; KTi2(PO4)3; and NASICON-type compounds. A single negative electrode active material may be used alone, or two or more negative electrode active materials may be used in combination. The shape of the negative electrode active material is not particularly limited, and may be, for example, particulate. In this case, the particle size is not particularly limited, and an appropriate size may be selected depending on the battery design. The primary particle size of the negative electrode active material may be 1 nm or more, 5 nm or more, 10 nm or more, 50 nm or more, or 100 nm or more, or 500 μm or less, 100 μm or less, 50 μm or less, 30 μm or less, or 10 μm or less. Furthermore, the negative electrode active material may be formed by aggregation of primary particles to form secondary particles. In this case, the particle diameter of the secondary particles is not particularly limited, and may be, for example, 100 nm or more, 500 nm or more, or 1 μm or more, or 1000 μm or less, 500 μm or less, 100 μm or less, 50 μm or less, 30 μm or less, or 20 μm or less. The amount of the negative electrode active material contained in the negative electrode active material layer 31 is not particularly limited. For example, based on the entire negative electrode active material layer 31 (100 mass%), the negative electrode active material may be contained in an amount of 20 mass% or more, 40 mass% or more, 60 mass% or more, or 70 mass% or more, or 99 mass% or less, 97 mass% or less, or 95 mass% or less.

[0045] The types of conductive additives and binders optionally contained in the negative electrode active material layer 31 are not particularly limited, and can be appropriately selected from, for example, the conductive additives and binders exemplified as being optionally contained in the positive electrode active material layer 21. The amounts of conductive additives and binders contained in the negative electrode active material layer 31 are not particularly limited.

[0046] The negative electrode current collector 32 may be composed of a known metal usable as a negative electrode current collector for a secondary battery. Examples of such metals include metal materials containing at least one element selected from the group consisting of Cu, Ni, Al, V, Au, Pt, Mg, Fe, Ti, Pb, Co, Cr, Zn, Ge, In, Sn, and Zr. In particular, considering stability in an aqueous electrolyte, the negative electrode current collector 32 may contain at least one element selected from the group consisting of Al, Ti, Pb, Zn, Sn, Mg, Zr, and In, or may contain at least one element selected from the group consisting of Ti, Pb, Zn, Sn, Mg, Zr, and In, or may contain Ti. Al, Ti, Pb, Zn, Sn, Mg, Zr, and In all have low work functions, and are therefore unlikely to cause electrolysis of the aqueous electrolyte even when in contact with the aqueous electrolyte. The shape of the negative electrode current collector 32 is not particularly limited. It may be in various forms such as foil, mesh, porous, etc. The surface of the substrate may be plated or vapor-deposited with the above metals.

[0047] The surface of the negative electrode current collector 32 may be coated with a carbon material. That is, the negative electrode 30 may further include the negative electrode current collector 32 and a coating layer provided on the surface of the negative electrode current collector 32 on which the aqueous electrolyte solution is disposed (between the negative electrode current collector 32 and the negative electrode active material layer 31), and the coating layer may contain a carbon material. Examples of carbon materials include ketjen black (KB), vapor-grown carbon fiber (VGCF), acetylene black (AB), carbon nanotubes (CNT), carbon nanofibers (CNF), carbon black, coke, and graphite. The thickness of the coating layer is not particularly limited. The coating layer may be provided on the entire surface of the negative electrode current collector 32 or on a portion of the surface. The coating layer may contain a binder for binding the carbon materials together and between the carbon material and the negative electrode current collector 32. When a coating layer containing a carbon material is provided on the surface of the negative electrode current collector 32, the withstand voltage on the reduction side of the aqueous electrolyte solution is likely to be improved. Since the edge portions of the carbon material have high reactivity, it is believed that the adsorption and decomposition of pyrophosphate ions contained in the aqueous electrolyte solution occurs easily, and that a coating is easily deposited. Therefore, when the aqueous electrolyte solution is used in the secondary battery 100, the edge portions of the carbon material are inactivated, which suppresses electrolysis of the aqueous electrolyte solution at the edge portions, and as a result, it is believed that the reduction-side potential window of the aqueous electrolyte solution is expanded.

[0048] In addition to the above configuration, the secondary battery 100 may also include obvious battery components such as terminals, a battery case, etc. The secondary battery 100 having the above configuration can be manufactured, for example, by forming a positive electrode active material layer 11 on the surface of a positive electrode current collector 12 by a dry or wet method to obtain a positive electrode 10, forming a negative electrode active material layer 31 on the surface of a negative electrode current collector 32 by a dry or wet method to obtain a negative electrode 30, and disposing a separator between the positive electrode 10 and the negative electrode 30 and impregnating them with an aqueous electrolyte solution.

[0049] 2. Combination of positive electrode active material and aqueous electrolyte The technology of the present disclosure is a combination of a positive electrode active material and an aqueous electrolyte, and also has an aspect as a novel combination enabling charge and discharge of the positive electrode active material. That is, the combination of the present disclosure has a positive electrode active material and an aqueous electrolyte, and the positive electrode active material contains a layered oxide represented by A x K y Ni 1-z M z O 2±δ ·nH2O, where A is at least one element selected from the group consisting of Li, Na, Rb, Cs, Mg, Ca, Sr, Ba, Sc, and Y, M is at least one element selected from the group consisting of transition metal elements, Group 2A elements, Group 3A elements, Group 2B elements, and Group 3B elements, the relationships 0 ≦ x < 0.5, 0 < y ≦ 0.5, 0 ≦ z ≦ 0.5, 0 < n ≦ 2, and (α·x) + y ≦ 0.5 are satisfied, α is the valence of the cation of A, and the aqueous electrolyte contains water and potassium pyrophosphate dissolved in the water.

[0050] 3. Supplementary Conventionally, there are few known positive electrode active materials that can be charged and discharged in an electrolyte in which a potassium compound other than hydroxide is dissolved, regardless of whether it is non-aqueous or aqueous. The optimal combination enabling charge and discharge among the combinations of the electrolyte and the positive electrode active material is limited to only a small part of the infinite combinations of the electrolyte and the positive electrode active material. If the crystal structure of the active material, the components of the electrolyte, the potential window, etc. change even slightly, charge and discharge become impossible. That is, the optimal combination cannot be found without actually combining and evaluating the electrolyte and the positive electrode active material. The technology of the present disclosure has found a novel combination enabling charge and discharge through numerous trials and errors among the infinite combinations of the electrolyte and the positive electrode active material, and it is not easily conceivable from the prior art.

[0051] In the secondary battery of the present disclosure, the carrier ions absorbed or released in the positive electrode active material during charging and discharging of the battery are presumed to be potassium ions. That is, the secondary battery of the present disclosure has the potential to function as a potassium ion battery. Potassium is inexpensive and abundant, making it an attractive battery material. Furthermore, when potassium ions are used as carrier ions, the properties of the ions make it possible to expect a high output secondary battery. [Example]

[0052] 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.

[0053] 1. Preparation of positive electrode active material The predetermined layered oxides and predetermined nickel composite hydroxides were prepared according to the procedures described below.

[0054] 1.1 Layered oxides A mixture was obtained by uniformly mixing lithium nitrate and nickel hydroxide in a molar ratio of Li:Ni = 2:1. The mixture was subjected to a solid-state reaction in air at 700°C for 10 hours to obtain powder. The powder was washed with water to remove excess alkali and dried to obtain LiNiO2. Next, LiNiO2 was dissolved in H + The mixture was mixed with 1.2N sulfuric acid solution at a ratio of Li / Ni=4, stirred and reacted for 3 hours, filtered, washed with water, and dried to obtain Li. 0.1 NiO2 was obtained. 0.1 NiO2, K + The mixture was mixed with a 0.3N potassium hydroxide solution at a ratio of Li / Ni=0.5, and the mixture was stirred and reacted for 3 hours. The product was filtered, washed with water, and dried to obtain Li as the positive electrode active material. 0.1 K 0.2 A layered oxide represented by NiO2·0.7H2O was obtained. Analysis of this positive electrode active material by X-ray diffraction revealed that it could be indexed as a hexagonal crystal (space group R-3m), with lattice constants a = 2.83 (Å), c = 21.09 (Å), and an interlayer distance of 7.03 (Å).

[0055] 1.2 Nickel composite hydroxide A first solution was prepared by dissolving nickel sulfate, aluminum sulfate, and ytterbium sulfate in water in a molar ratio of 0.8:0.15:0.05. A second solution was prepared by mixing sodium hydroxide and water. The concentration of the second solution was 32% by mass. 1000 g of water was placed in a glass beaker as a reaction vessel, and the second solution was appropriately added to maintain the pH at 10.0 using a pH sensor and a pH controller. The temperature was maintained at 40°C using a heater and a controller with a temperature sensor. The first and second solutions were added to the reaction vessel and stirred to maintain the above temperature and pH, thereby forming a precipitate. The mixed solution containing the precipitate was cooled to room temperature and then filtered, thereby obtaining a precipitate. The precipitate was washed with pure water and dried, thereby obtaining a nickel composite hydroxide as a positive electrode active material. In addition, when measured by inductively coupled plasma (ICP) emission spectroscopy, the molar ratio of nickel, aluminum, and ytterbium in this composite hydroxide was 0.8:0.15:0.05. Furthermore, as a result of analysis by X-ray diffraction, the nickel composite hydroxide 0.8 Al 0.15 Yb 0.05 (OH) 2.2 It had a crystal structure represented by the following formula:

[0056] 2. Preparation of the Positive Electrode The layered oxide or nickel composite hydroxide described above was used as the positive electrode active material, acetylene black was used as the conductive additive, and PVDF and CMC were used as the binders in a mass ratio of 85:10:4.5:0.5 to prepare an ink. The ink was uniformly applied to the surface of Ni foil using a doctor blade and then dried to obtain a positive electrode for evaluation. That is, the positive electrode was a positive electrode mixture layer containing the positive electrode active material and the like formed on the surface of Ni foil as the positive electrode current collector.

[0057] 3. Preparation of evaluation cells An evaluation cell having the following configuration was fabricated. Cell: VM5 (manufactured by EC Frontier) Working electrode: the above positive electrode, opening area 1 cm 2 Counter electrode: Pt mesh Reference electrode:Ag / AgCl Electrolyte: Aqueous electrolyte shown in Table 1 below

[0058] 4. Cell Evaluation The evaluation cell was charged and discharged under the following conditions, and the ratio of the discharge capacity to the charge capacity (Coulomb efficiency) was measured. Charge / discharge current value: 0.1mA / cm 2 Cutoff voltage: 0.25-0.75V vs. Ag / AgCl Measurement temperature: 25℃

[0059] 5. Evaluation Results The evaluation results are shown in the following Table 1. FIG. 2 shows the charge / discharge curve for Example 1, and FIG.

[0060] [Table 1]

[0061] As is clear from the results shown in Table 1 and Figures 2 and 3, when a specific layered oxide as a positive electrode active material was combined with an aqueous electrolyte solution containing dissolved K4P2O7 (Examples 1 and 2), the positive electrode active material was able to be charged and discharged. On the other hand, when the electrolyte of the aqueous electrolyte solution was changed to K2SO3 (Comparative Examples 1 and 2) or when the type of positive electrode active material was changed (Comparative Example 3), it was also found that charging and discharging of the positive electrode active material became difficult. Note that in the comparative example, the charging potential of the positive electrode active material may have been more noble than the decomposition potential of the electrolyte, and the oxidative decomposition of the solution may have proceeded preferentially over the charging reaction, resulting in no discharge capacity being obtained.

[0062] As described above, it has been found that when a layered oxide as a positive electrode active material is combined with an aqueous electrolyte in which K4P2O7 is dissolved, charge and discharge of the positive electrode active material becomes possible specifically. In the above example, A x K y Ni 1-z M z O 2±δ In the layered oxide represented by ·nH2O, examples were given of those containing Li as A, not containing M, and having x, y, and n as predetermined values. However, the positive electrode active material employed in the technology of the present disclosure is not limited to this. The presence or absence of A, the type of A, and the composition ratio of K are not particularly limited. Also, a part of Ni may be substituted with M for the purpose of improving reversibility, thermal stability, and storage stability. Furthermore, the amount of moisture can also be adjusted as appropriate. As far as the present inventor has confirmed, when the layered oxide has the following chemical composition, charge and discharge are possible in combination with the above aqueous electrolyte.

[0063] Chemical composition: A x K y Ni 1-z M z O 2±δ ·nH2O Here, A is at least one element selected from the group consisting of Li, Na, Rb, Cs, Mg, Ca, Sr, Ba, Sc, and Y, M is at least one element selected from the group consisting of transition metal elements, Group 2A elements, Group 3A elements, Group 2B elements, and Group 3B elements, the relationships 0≦x<0.5, 0<y≦0.5, 0≦z≦0.5, 0<n≦2, and (α·x)+y≦0.5 are satisfied, and α is the valence of the cation of A.

Explanation of symbols

[0064] 10 Positive electrode 11 Positive electrode active material layer 12 Positive electrode current collector 20 Electrolyte layer 30 Negative electrode 31 Negative electrode active material layer 32 Negative electrode current collector 100 Secondary battery

Claims

1. A secondary battery having a positive electrode active material and an aqueous electrolyte solution, The positive electrode active material is ｘ K ｙ Ni １－ｚ M ｚ O ２±δ ・nH ２ O, A is at least one element selected from the group consisting of Li, Na, Rb, Cs, Mg, Ca, Sr, Ba, Sc, and Y, M is at least one element selected from the group consisting of transition metal elements, Group 2A elements, Group 3A elements, Group 2B elements, and Group 3B elements; the relationships 0≦x<0.5, 0<y≦0.5, 0≦z≦0.5, 0<n≦2, and (α x)+y≦0.5 are satisfied, and α is the valence of the cation A; The aqueous electrolyte solution contains water and potassium pyrophosphate dissolved in the water. Secondary battery.

2. The potassium pyrophosphate is dissolved in the water at a concentration of 2 mol or more per 1 kg of the water. The secondary battery according to claim 1 .

3. The potassium pyrophosphate is dissolved in the water at a concentration of 5 mol or more per kg of the water. The secondary battery according to claim 1 .

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