JP7798618B2 - secondary battery - Google Patents

Description

This application discloses a secondary battery.

Patent Document 1 discloses a nickel _{-containing oxide represented by Li0.1K0.2NiO1.9.0.7H2O as a} positive electrode active material for a secondary battery. Patent Document 2 discloses a nickel-containing layered oxide represented by _{AxNi1 - zMzO2} as a positive electrode active material for a secondary battery. Patent Document ₃ discloses a nickel-containing layered oxide represented by _{AxNi1 -zMzO2.nH2O as} a positive electrode active material for a secondary battery. Furthermore, Patent Documents 1 to 3 disclose, as electrolytes to be combined with the above-mentioned positive electrode active materials, nonaqueous electrolytes containing lithium _{ions ( LiPF6} dissolved in a _carbonate -based solvent) and aqueous solutions containing sodium ions (sodium chloride aqueous solution). Meanwhile, Patent Document 4 discloses an aqueous electrolyte containing water and potassium pyrophosphate dissolved in water as an electrolyte for use in an aqueous potassium ion battery.

Japanese Patent Application Laid-Open No. 2001-332259 Japanese Patent Application Laid-Open No. 2003-151549 Japanese Patent Application Laid-Open No. 2005-322551 Japanese Patent Application Laid-Open No. 2019-220294

When a non-aqueous electrolyte solution in which a potassium compound is dissolved in a non-aqueous solvent is used as the electrolyte for a secondary battery, there is a problem in that highly reactive metallic potassium is likely to precipitate. To avoid this problem, secondary batteries may use an aqueous electrolyte solution in which a potassium compound is dissolved in water instead of the non-aqueous electrolyte solution. However, until now, almost no positive electrode active materials have been known that can be charged and discharged in an electrolyte solution in which a potassium compound other than hydroxide is dissolved, whether non-aqueous or aqueous, resulting in a problem of limited flexibility in the design of secondary batteries. In this regard, there is a need for a new combination of an electrolyte solution in which a potassium compound other than hydroxide is dissolved and a positive electrode active material that allows the positive electrode active material to be charged and discharged.

As one of the means for solving the above problems, the present application provides:
A secondary battery having a positive electrode active material and an aqueous electrolyte solution,
the positive electrode active material contains a layered oxide represented by A _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.
A secondary battery is disclosed.

In the secondary battery disclosed herein, the potassium pyrophosphate may be dissolved in the water at a concentration of 2 mol or more per kg of water.

In the secondary battery disclosed herein, the potassium pyrophosphate may be dissolved in the water at a concentration of 5 mol or more per kg of water.

The secondary battery disclosed herein uses an aqueous electrolyte solution, which makes it difficult for highly reactive metallic potassium to precipitate. Furthermore, the secondary battery disclosed herein combines a specific layered oxide as the positive electrode active material with an aqueous electrolyte solution in which potassium pyrophosphate is dissolved, and this combination enables the positive electrode active material to be charged and discharged.

1 shows a schematic diagram of an example of the configuration of a secondary battery. 1 shows charge/discharge curves of Example 1. 1 shows the charge/discharge curves of Comparative Example 3.

1. Secondary Battery The 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 .} Here, A is at least one element selected from the group consisting of Li, Na, Rb, Cs, Mg, Ca, Sr, Ba, Sc, and Y, and 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. The α is the valence of the cation of A. The aqueous electrolyte includes water and potassium pyrophosphate dissolved in the water.

1.1 Positive Electrode Active Material The secondary battery of the present disclosure has a layered oxide as a positive electrode active material. The layered oxide is expressed as A _x K _y Ni _1-z M _z O _2±δ ·nH ₂ O. The layered oxide can be said to be composed of Ni, M, and O as the main components, with K, optionally A, and H ₂ O present between layers.

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, the layered oxide must contain K. In the layered oxide, A and K can exist between 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 layers increases the interlayer distance, making it easier to accommodate carrier ions. For example, it is thought that the presence of K between layers makes it easier to accommodate potassium ions as carrier ions.

The presence of A or K as a cation between layers of a layered oxide stabilizes the water between the 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.

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, a 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 contained 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.

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 thought to enable the water to exist more stably between the layers. Furthermore, in the secondary battery of the present disclosure, an aqueous electrolyte solution, which will be described later, is used as the electrolyte solution. This makes it difficult for water to escape from between the layers of the layered oxide, and it is thought that water is easily retained between the layers during and after charge/discharge of the positive electrode active material. The presence of water between the layers is thought to further increase the interlayer distance, further increasing the space available for accommodating carrier ions.

1.1.4 Composition Ratio The composition ratios x, y, and z in the chemical composition of the layered oxide may satisfy the relationships 0≦x<0.5, 0<y≦0.5, 0≦z≦0.5, 0<n≦2, and (α x) + y≦0.5. Here, α is the valence of the cation A. Furthermore, the oxygen composition ratio is not limited to 2 and some variation is allowed. For example, δ may be 0.5, 0.3, or 0.1.

Since A is an optional element in layered oxides, the lower limit of x is 0. The upper limit of x is naturally less than 0.5 as long as the relationship between x and y satisfies (α x) + y ≦ 0.5. x may be 0.4 or less, 0.3 or less, or 0.2 or less.

Since K is an essential element in layered oxides, the lower limit of y is greater than 0. The upper limit of y is naturally 0.5 or less, as long as the relationship between y and x satisfies (α x) + y ≦ 0.5. y may also be 0.4 or less, 0.3 or less, or 0.2 or less.

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 amount of Ni will be relatively too small, which may make it difficult to ensure sufficient capacity. In this regard, the upper limit of z is 0.5 or less, and may also be 0.4 or less, 0.3 or less, 0.2 or less, or 0.1 or less.

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 greater, 0.2 or greater, 0.3 or greater, 0.4 or greater, 0.5 or greater, 0.6 or greater, or 0.7 or greater. On the other hand, if the amount of water between the layers of the layered oxide is too great, 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.

If the layered oxide contains excessive amounts of A or K, i.e., if (α x) + y is excessively large, the valence of Ni in the layered oxide will be small, which may result in insufficient capacity and may also make it impossible to maintain the layered structure. By keeping (α x) + y at 0.5 or less, the above problems can be easily avoided. (α x) + y may be 0.4 or less, 0.3 or less, or 0.2 or less.

1.1.5 Shape The shape of the layered oxide may be 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, and may be 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, and may be 1000 μm or less, 500 μm or less, 100 μm or less, 50 μm or less, 30 μm or less, or 20 μm or less.

1.1.6 Other Positive Electrode Active Materials The secondary battery of the present disclosure may contain other positive electrode active materials in addition to the layered oxides described above, 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).

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

1.2.1 Solvent Aqueous electrolytes contain 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 on the proportion of water in the solvent. The solvent may consist of only water.

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, based on the total amount of solvents constituting the electrolyte (100 mol%).

1.2.2 Electrolyte The aqueous electrolyte contains dissolved potassium pyrophosphate as an electrolyte. The "dissolved potassium pyrophosphate" does not have to be completely ionized into potassium ions and pyrophosphate ions in the aqueous electrolyte. That is, the "dissolved potassium pyrophosphate" may exist in the form of ions such as K ⁺ , P ₂ O ₇ ⁴⁻ , KP ₂ O ₇ ³⁻ , K ₂ P ₂ O ₇ ²⁻ , or K ₃ P ₂ O ₇ ⁻ , or as an association of these ions. Furthermore, the "dissolved potassium pyrophosphate" in the aqueous electrolyte does not have to be derived from a salt of potassium and pyrophosphate (K ₄ P ₂ O ₇ ) (obtained by adding K ₄ P ₂ O ₇ to water). For example, _a solution obtained by separately adding and dissolving a potassium ion source (such as KOH or _CH3COOK ) and a pyrophosphate ion source (such _{as H4P2O7} ) in water, resulting in the formation of the above ions and associated complexes in the water, is also included in the above aqueous electrolyte solution.

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 potassium pyrophosphate concentration in the aqueous electrolyte, the smaller the hysteresis during charge and discharge of the positive electrode active material, making it easier to achieve high performance as a secondary battery. Furthermore, the higher the potassium pyrophosphate concentration in the aqueous electrolyte, the lower the overvoltage and the more likely a favorable charge and discharge plateau is to be achieved. Furthermore, the higher the potassium pyrophosphate concentration 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 the secondary battery, pyrophosphate ions are likely to migrate toward the negative electrode, dragged by potassium ions. It is believed that 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, making it easier to suppress electrolysis of the aqueous electrolyte.

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

In an aqueous electrolyte, the entire amount of potassium ions contained in the electrolyte does not have to be calculated as "dissolved potassium pyrophosphate." That is, the aqueous electrolyte may contain more potassium ions than 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.

The aqueous electrolyte may contain cations other than potassium ions. For example, it may contain alkali metal ions other than potassium ions, alkaline earth metal ions, transition metal ions, etc. Furthermore, the aqueous electrolyte may contain anions other than pyrophosphate ions (which may exist in a state bound to a cation, such as P ₂ O ₇ ⁴⁻ , KP ₂ O ₇ ³⁻ , K ₂ P ₂ O ₇ ²⁻ , K ₃ P ₂ O ₇ ⁻ , etc., as described above). For example, it may contain anions derived from other electrolytes, as described below.

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%).

1.2.3 Other Components In addition to the above-mentioned solvents and electrolytes, 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.

1.3 Other Configurations The secondary battery of the present disclosure may include 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 be configured so that 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. Furthermore, 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 include the above-described positive electrode active material. Furthermore, the positive electrode 10, the electrolyte layer 20, and the anode 30 may all include the above-described aqueous electrolyte solution.

1.3.1 Positive Electrode The positive electrode 10 may have a known configuration except for containing 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.

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.

The type of positive electrode active material contained in the positive electrode active material layer 11 is as described above. The amount of 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% by mass), the positive electrode active material may be contained in an amount of 20% by mass or more, 40% by mass or more, 60% by mass or more, or 70% by mass or more, or 99% by mass or less, 97% by mass or less, or 95% by mass or less.

The conductive additive optionally contained in the positive electrode active material layer 11 can be any of the conductive additives known to be 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 capable of withstanding the environment in which the battery is used. A single 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. There is no particular limitation on the amount of conductive additive contained in the positive electrode active material layer 11.

The binder optionally contained in the positive electrode active material layer 11 can 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. A single binder may be used alone, or two or more binders may be used in combination. There are no particular limitations on the amount of binder contained in the positive electrode active material layer 11.

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 metallic 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 can take various forms, such as foil, mesh, or porous. The above metal may also be vapor-deposited or plated onto the surface of a substrate.

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-described aqueous electrolyte solution. As the separator, a separator known for use in secondary batteries (e.g., nickel-metal hydride batteries, zinc-air batteries, etc.) may be used. 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.

1.3.3 Negative Electrode The negative electrode 30 may have a known structure 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.

The negative electrode active material layer 31 contains a negative electrode active material. The negative electrode active material layer 31 may also contain a conductive additive or 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.

The negative electrode active material contained in the negative electrode active material layer 31 may be selected from active materials whose carrier ions have a 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 Mo ₆ S ₈ ; elemental sulfur; KTi ₂ (PO ₄ ) ₃ ; and NASICON-type compounds. Only one type of negative electrode active material may be used alone, or two or more types may be used in combination. The shape of the negative electrode active material 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, but may be, for example, 100 nm or more, 500 nm or more, or 1 μm or more, and may be 1000 μm or less, 500 μm or less, 100 μm or less, 50 μm or less, 30 μm or less, or 20 μm or less. 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.

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.

The negative electrode current collector 32 may be composed of a known metal that can be used as a negative electrode current collector in a secondary battery. Examples of such metals include metallic 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 form of the negative electrode current collector 32 is not particularly limited. It can be in various forms, such as foil, mesh, or porous. It may also be a substrate whose surface is plated or vapor-deposited with the above-mentioned metals.

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 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. Furthermore, the coating layer may be provided on the entire surface of the negative electrode current collector 32, or on only 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 is likely to be improved. Because the edge portions of the carbon material have high reactivity, it is believed that they are prone to adsorption and decomposition of pyrophosphate ions contained in the aqueous electrolyte, and that a coating is likely to accumulate. Therefore, when the above-mentioned aqueous electrolyte is used in secondary battery 100, the edge portions of the carbon material are inactivated, suppressing electrolysis of the aqueous electrolyte at the edge portions, and as a result, it is believed that the reduction-side potential window of the aqueous electrolyte is expanded.

In addition to the above components, the secondary battery 100 may also include other obvious components of a battery, such as terminals and a battery case. A secondary battery 100 having the above components 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.

2. Combination of Positive Electrode Active Material and Aqueous Electrolyte Solution The technology of the present disclosure is a combination of a positive electrode active material and an aqueous electrolyte solution, and also has an aspect of being a novel combination that enables charging and discharging of the positive electrode active material. That is, the combination disclosed herein includes a cathode active material and an aqueous electrolyte solution, wherein the cathode active material includes a layered oxide represented by _{AxKyNi1 -zMzO2 ± δ} · _nH2O , wherein 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, 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, wherein α is the valence of a cation of A, and the aqueous electrolyte solution includes water and potassium pyrophosphate dissolved in the water.

3. Supplementary Information: Conventionally, few positive electrode active materials are known that can be charged and discharged in an electrolyte solution containing a potassium compound other than hydroxide, whether non-aqueous or aqueous. The optimal combinations of electrolytes and positive electrode active materials that allow charging and discharging are limited to only a small subset of the infinite number of combinations of electrolytes and positive electrode active materials. Even slight changes in the crystal structure of the active material, the electrolyte components, or the potential window can render charging and discharging impossible. In other words, the optimal combination cannot be found without actually combining and evaluating the electrolyte and positive electrode active material. The technology disclosed herein has been developed through numerous trial and error processes to find novel combinations that allow charging and discharging from the infinite number of combinations of electrolytes and positive electrode active materials, and these combinations were not easily conceivable based on conventional technology.

In the secondary battery disclosed herein, it is presumed that the carrier ions absorbed or released in the positive electrode active material during charging and discharging of the battery are potassium ions. In other words, the secondary battery disclosed herein has the potential to function as a potassium ion battery. Potassium is inexpensive and abundant, making it an attractive battery material. Furthermore, if potassium ions are used as carrier ions, the properties of the ion make it possible to expect higher output from the secondary battery.

The technology of the present disclosure will be explained in more detail below using examples, but the technology of the present disclosure is not limited to the following examples.

1. Preparation of Positive Electrode Active Material A predetermined layered oxide and a predetermined nickel composite hydroxide were prepared according to the procedure described below.

1.1 Layered Oxide Lithium nitrate and nickel hydroxide were uniformly mixed at a molar ratio of Li:Ni = 2:1 to obtain a mixture. This mixture was subjected to a solid-state reaction in air at 700°C for 10 hours to obtain a powder. The powder was washed with water to remove excess alkali and then dried to obtain _LiNiO2 . Next, _LiNiO2 was mixed with a 1.2N sulfuric acid solution at a H ⁺ / _Ni ratio of 4, stirred and reacted for 3 hours, filtered, washed with water, and dried to obtain _Li0.1NiO2 . Furthermore, _Li0.1NiO2 was mixed with _a 0.3N potassium hydroxide solution at a K ⁺ _{/ Ni} ratio of 0.5, stirred and reacted for 3 hours, and the product was filtered, washed with water, and _dried to obtain a layered oxide represented by _{Li0.1K0.2NiO2.0.7H2O} as a positive electrode active material. When the positive electrode active material was analyzed by X-ray diffraction, it was found that it could be indexed as a hexagonal crystal (space group R-3m), with lattice constants a = 2.83 (Å) and c = 21.09 (Å), and the interlayer distance was 7.03 (Å).

1.2 Nickel Composite Hydroxide Nickel sulfate, aluminum sulfate, and ytterbium sulfate were dissolved in water in a molar ratio of 0.8:0.15:0.05 to prepare a first solution. Meanwhile, sodium hydroxide and water were mixed to prepare a second solution. The concentration of the second solution was 32% by mass. 1000 g of water was placed in a glass beaker serving 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. Measurement by inductively coupled plasma (ICP) emission spectroscopy revealed that the molar ratio of nickel, aluminum, and ytterbium in this composite hydroxide was 0.8: _0.15 :0.05. Analysis by X-ray diffraction revealed that the nickel composite hydroxide had a crystal structure expressed as _{Ni0.8Al0.15Yb0.05} (OH) _{2.2 .}

2. Preparation of Positive Electrode The layered oxide or nickel composite hydroxide described above as the positive electrode active material, acetylene black as the conductive additive, and PVDF and CMC as the binder were mixed 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 dried to obtain a positive electrode for evaluation. That is, the positive electrode was a positive electrode in which a positive electrode mixture layer containing the positive electrode active material and the like was formed on the surface of Ni foil as the positive electrode current collector.

3. Preparation of Evaluation Cell An evaluation cell having the following configuration was prepared.
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

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: 0.1 mA/ ^cm2
Cut voltage: 0.25-0.75 V vs. Ag/AgCl
Measurement temperature: 25℃

5. Evaluation Results The evaluation results are shown in the following Table 1. In addition, Fig. 2 shows the charge/discharge curve for Example 1, and Fig. 3 shows the charge/discharge curve for Comparative Example 3.

As is clear from the results shown in Table 1 and Figures 2 and 3, when a specific layered oxide as _the positive electrode active material is combined with an aqueous electrolyte solution containing dissolved _K4P2O7 (Examples 1 and ₂ ), it is possible to charge and discharge the positive electrode active material. On the other hand, when the electrolyte of the aqueous electrolyte solution is changed _{to K2SO3} (Comparative Examples 1 and 2) or when the type of positive electrode active material is changed (Comparative Example 3), it is also found that charging and discharging the positive electrode active material becomes difficult. Note that in the comparative examples, the charging potential of the positive electrode active material may be more noble than the decomposition potential of the electrolyte, and as a result, oxidative decomposition of the solution may have proceeded preferentially over the charging reaction, resulting in no discharge capacity being obtained.

As described above, it has been found that the combination of a layered oxide as a positive electrode active material with an aqueous electrolyte solution containing dissolved K ₄ P ₂ O ₇ specifically enables charging and discharging of the positive electrode active material. In the above examples, a layered oxide represented by A _x K _y Ni _1-z M _z O _2±δ ·nH ₂ O was exemplified, in which A contains Li, M does not contain M, and x, y, and n are 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. Furthermore, a portion of Ni may be substituted with M to improve reversibility, thermal stability, and storage stability. Furthermore, the water content can also be adjusted as appropriate. As far as the inventors have confirmed, when a layered oxide has the following chemical composition, charging and discharging is possible when combined with the above-mentioned aqueous electrolyte solution.

Chemical composition: A _x K _y Ni _1-z M _z O _2±δ・nH ₂ O
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.

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 (3)

A secondary battery having a positive electrode active material and an aqueous electrolyte solution,
the positive electrode active material contains a layered oxide represented by A _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. 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 . 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 .