WO2014041800A1

WO2014041800A1 - METAL Na CELL

Info

Publication number: WO2014041800A1
Application number: PCT/JP2013/005378
Authority: WO
Inventors: 林　克郎
Original assignee: 国立大学法人東京工業大学
Priority date: 2012-09-11
Filing date: 2013-09-11
Publication date: 2014-03-20
Also published as: JP2015222613A

Abstract

This invention is provided with: a Na⁺ ion conductor (1) having Na⁺ ion conductivity; a negative electrode active material (4) substantively comprising metal Na, disposed on one side of the Na⁺ ion conductor (1); a negative electrode (5) disposed so that a least a part thereof is in contact with the negative electrode active material (4), the negative electrode (5) being capable of conducting electrons generated when metal Na ionizes; an organic electrolyte (2) stored so as to be in contact with the negative electrode (5) and one surface of the Na⁺ ion conductor (1); a positive electrode (8) disposed on the other side of the Na⁺ ion conductor (1); an aqueous electrolyte (3) stored so as to be in contact with the positive electrode (8) and the other surface of the Na⁺ ion conductor (1); and a housing part (13) for housing the aqueous electrolyte (3), the housing part (13) having base resistance.

Description

Metal Na battery

The present invention relates to a metal Na battery.

In recent years, a battery for an electric car which obtains electricity using a chemical reaction has been developed as a substitute for fossil fuel. Conventionally, a lithium ion (hereinafter, also referred to as lithium ion, Li ⁺ ion, or simply referred to as Li ⁺ ) battery has attracted attention as one of such batteries. However, it is difficult to provide an electric vehicle using a lithium ion battery as a storage battery with the same range as a conventional internal combustion engine. This is because commercial Li-ion batteries have only an energy density of about 100 to 200 Wh / kg. In order to obtain the same range as that of an internal combustion locomotive, an Li-ion battery requires an energy density four or more times the current level. Such a significant improvement in energy density is difficult only with the technology in the extension of the conventional Li-ion battery. Therefore, new energy conversion devices have been required.

As one of the means for achieving high energy density, there is known a technology in which a light alkali metal itself having a large change in reaction free energy due to an oxidation reaction is used as the active material of the negative electrode. Among them, so-called Li-air batteries have been mainly studied. For example, there is known a lithium-air battery using a carbon composite electrode positive electrode, a polymer membrane separator, and an organic electrolytic solution that transmits oxygen by using a Li metal foil as a negative electrode (Non-patent Document 1). This Li-air battery has been considered as the basic type of Li-air battery developed later. Under ideal conditions, the Li-air battery is a battery reaction of 4Li + O ₂ → 2Li ₂ O, and the calculated value including O ₂ in the battery reaction weight is 5,200 Wh / kg, calculated without O _2. It has a high theoretical energy density of 11,140 Wh / kg. Therefore, it is supposed that an experimentally higher energy density can be obtained as compared to a Li-ion battery having a theoretical energy density of 250 to 350 Wh / kg.

In addition, as a technique for solving the problem of Non-Patent Document 1 (described later), a Li ⁺ high-speed ion conductor solid electrolyte Rishicon (LISICON) is used as a separator, and an electrode that transmits a water-soluble electrolyte and oxygen is disposed on the positive electrode side. Li-air batteries (referred to herein as "Li-water-air batteries") having a different structure have also been developed. This Li-water-air battery is said to be capable of completely performing a battery reaction because the reaction product at the time of discharge is LiOH which is dissolved in a water-soluble electrolyte. The theoretical energy density of this Li-water-air cell is about 5,700 Wh / kg excluding the weight of oxygen required for the reaction. In addition, a discharge of about 800 Wh / kg has been demonstrated in the experiment (Non-Patent Document 2).

As another means for providing high energy density to the battery, a technology using β-alumina as a solid electrolyte of a sodium-sulfur (Na-S) battery is known. The β-alumina is a crystal having a layered structure in which a structural layer called a spinel block sandwiches a layer composed of Na ⁺ ions (hereinafter also referred to as sodium ions, Na ions or simply Na ⁺ ). β-alumina has two structures in which the layers of hexagonal β-phase and cyclohedral β′-phase overlap slightly differently. For example, the basic composition of the latter is Na _{1 + x} Mg _x Al _11-x O ₁₇ (X = 0.59 to 0.72). The orientation-controlled polycrystal is a solid electrolyte exhibiting high-speed Na ⁺ ion conductivity of 0.1 S · cm ⁻¹ or more at about 350 ° C. because Na ⁺ ions in the layer can diffuse at high speed.

Na-S batteries were initially developed as secondary batteries for electric vehicles, which can achieve high energy density four to five times that of lead acid batteries. However, Na-S batteries are currently commercialized for stationary use because they operate at a high temperature of about 350.degree. In the Na-S battery, metal Na for the negative electrode, an active material of sulfur and sodium polysulfide S / Na ₂ S _x for the positive electrode, and graphite wool for the current collector of the molten molten salt are respectively used. Typical operating temperatures are 300-350 ° C., where the sodium sulfide melts and the conductivity of the electrolyte is sufficiently high. Na-S batteries are characterized by high charge and discharge energy efficiency.

In addition, NASICON is known as a solid electrolyte having high Na ⁺ ion conductivity and ion conductivity close to β or β ′ ′ alumina. Nasacon is AMM ′ P ₃ O ₁₂ (A is an alkali ion It is a series of rhombohedral or monoclinic crystals represented by the general formula of various cations including M, M ', which are divalent to pentavalent cations. Conducting Na ⁺ ions typical chemical composition of NASICON _{_{_{_{to, Na l + x Si x Zr}}}} 2 P 3-x O 12 (0 ≦ x ≦ 3) is expressed by. Additional Rishikon a series of derived materials NASICON that the a and Li However, since Nasicon decomposes when it comes in direct contact with metallic Na at around 300 ° C, which is the operating temperature of Na-S batteries (Non-Patent Document 3), its chemical durability for application to Na-S batteries Inferior to Have gills are in.

As yet another means for providing a battery with high energy density, a technology using β or β ′ ′ alumina electrolyte and metal Na as a negative electrode active material is also known. In this technology, a temperature from room temperature to 100 ° C. or less The primary battery is operated in the region, so that the negative electrode is amalgam of Na-Hg, the separator is a β alumina electrolyte, the positive electrode active material is contained in a water-soluble electrolytic solution or an organic solvent such as propylene carbonate Br ₂ , I ₂ , H ₂ O and O ₂ (Non-Patent Document 4) Since metal Na is solid at room temperature, when only solid metal Na is used as the negative electrode, Na chemical species can not be sufficiently diffused and the discharge characteristics are extremely low. Hg has been used to solve this problem Hg improves diffusion by forming a liquid phase at room temperature with Na.The theoretical energy density of this cell The degree of energy is in the range of 500 to 1,550 Wh / kg, and the actual energy density by the battery test is 1/2 to 1/3 of them, and the energy density is practically higher than that of the Li ion battery.

As mentioned above, several batteries with improved energy density over conventional Li-ion batteries have been considered. However, in any of the techniques, since expensive members are used as main members, there is room for improvement in cost. Further, in the technique of Non-Patent Document 1, the actual reaction product, Li ₂ O _2, does not dissolve in the organic electrolyte and causes clogging in the positive electrode, so that the battery reaction does not proceed completely, and the above theoretical density There was a problem that was not obtained. In addition, even in the technique of Non-Patent Document 2, there is a problem that the solution decomposes when the riscone electrolyte is in direct contact with Li metal or when it is placed in a strongly acidic or strongly basic aqueous solution. Further, the technique of Non-Patent Document 4 has a problem of using harmful Hg. There was also room for improvement in energy density.

The present invention has been made in view of these problems, and an object thereof is to reduce the cost by using only common materials for the main members while reducing the energy density per weight or volume significantly. It is to provide.

In order to solve the above problems, the metal Na battery according to one embodiment of the present invention, Na ⁺ ion conductivity and Na ⁺ ion conductor having a substantially metal disposed on one side of the Na ⁺ ion conductor A negative electrode active material made of Na, and a negative electrode which is disposed so as to at least partially contact the negative electrode active material and can conduct electrons generated when metal Na is ionized, and one surface of the negative electrode and the Na ⁺ ion conductor an organic electrolytic solution stored in contact, Na ⁺ and the positive electrode disposed on the other side of the ion conductor, a cathode and an Na ⁺ ion conductor aqueous electrolyte solution stored in contact with the other surface of the And a water-soluble electrolyte and a container having a basic resistance.

According to this embodiment, the metal Na with light and active metal Na and water, and oxygen which can be taken in from the outside if necessary, is used as the active material, and thus the metal Na with significantly improved energy density per weight or volume A battery can be provided. In addition, since metallic Na is used for the negative electrode, decomposition of the ceramic separator used as the organic electrolytic solution, the current collector, and the Na ⁺ ion conductor is made in comparison with the conventional metal Li-water-air battery or Li ion battery. It is hard to cause. In addition, since the ion conductivity of the Na ⁺ ion conductive solid electrolyte itself is also good, it does not serve as a barrier for improving the power density of the battery.

In addition, since metal Na batteries use only common materials and elements for main components, they can be manufactured inexpensively and do not hinder large-scale use. In particular, Na, which is responsible for charge transfer in the battery, does not have the problems of abundance and uneven distribution of resources as compared with Li used in Li-ion batteries. Moreover, in the negative electrode holding metal Na, it is not necessary to use copper which is often required when using Li, and cheaper aluminum (Al) can be suitably used. The elements and raw materials that make up the β-alumina based and Nasicon ceramics used as the Na ⁺ ion conductor are also inexpensive and touched.

According to the present invention, it is possible to provide a metallic Na battery in which the energy density per weight or volume is significantly improved while suppressing the cost by using only common materials for the main members.

It is the schematic which shows the structure of the metal Na battery concerning embodiment. FIG. 1 (A) is a schematic view showing a sodium-water-air battery of Embodiment 1. FIG. FIG. 1 (B) is a schematic view showing a sodium-water battery of Embodiment 2. It is the schematic which shows the outline | summary of a battery test. The current-voltage characteristics obtained at 50 ° C. are shown for the metal Na battery using the Nashicon separator of Example 1. The discharge characteristics obtained at 50 ° C. are shown for the metal Na battery using the Nashicon separator of Example 1. The current-voltage obtained at 50 ° C. is shown for a metallic Na battery using the Nashicon separator of Example 2. The current-voltage characteristics obtained at 30 ° C., 40 ° C., and 50 ° C. are shown for the metal Na battery using the β-alumina-based separator of Example 3. The current-voltage characteristics measured at 25 ° C. in air for the metal Na battery using the gas-permeable positive electrode of Example 4 are shown. The evaluation of the charge / discharge characteristic with respect to the metal Na battery using the gas-permeable positive electrode of Example 4 is shown.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In all the drawings, similar components are denoted by the same reference numerals, and the description thereof will be omitted as appropriate.

The present inventors are Na ⁺ ion conductors (hereinafter also referred to as separators), which are Na ⁺ ion conductive solid electrolytes, β and β ′ ′ alumina, and Na—Zr—Si—P—O-based Nasacon ceramics. As a result, we developed a battery (metal Na battery) using a negative electrode containing metal Na as a main component, and on top of that, we conducted intensive studies on the battery structure and operating conditions under which the metal Na battery suitably operates. by via reactive species associated with Na, and found to be able to provide a metal Na cell having a common can be configured with resources, and high energy density. in this specification, "Na ⁺ ion conductive "Indicates that Na ⁺ ions can be selectively conducted. Selectively conductive does not necessarily mean that only Na ⁺ ions are conducted and other substances are not conducted at all. That is, the case in which Na ⁺ ions can be conducted at a significantly higher frequency than other substances is also included in the case where they can be conducted selectively.

When the metal Na battery is discharged, Na of the negative electrode active material is ionized to be Na ⁺ ions and dissolved in the organic electrolytic solution to supply electrons to the negative electrode. Na ⁺ ions diffuse through the Na ⁺ ion conductor into the aqueous electrolyte. That is, the partial reaction at the negative electrode is Na → Na ⁺ + e ⁻ .

When oxygen gas is supplied to the positive electrode or a wire mesh is used as the positive electrode, the partial reaction at the positive electrode is 1 / 2H ₂ O + e ⁻ + 1⁄4O ₂ → OH ⁻ . In this case, the total reaction in the metal Na battery is Na + 1/2 H ₂ O + e ⁻ + 1⁄4 O ₂ → NaOH (aq). That is, as the discharge progresses, the concentration of NaOH in the water-soluble electrolyte increases. However, hydrogen gas is not generated from the surface of the positive electrode. This operating condition of the metal Na battery 20 is referred to as "sodium-water-air battery". A sodium-water-air battery will be described as Embodiment 1 with reference to FIG. 1 (A).

On the other hand, when oxygen gas is not supplied to the positive electrode, partial reactions on the positive electrode become H ₂ O + e ⁻ → OH ⁻ + 1⁄2H ₂ by the supply of electrons from the negative electrode side. In this case, the total reaction in the metal Na battery is Na + H ₂ O → NaOH (aq) + 1/2 H ₂ ↑. That is, as the discharge progresses, the concentration of NaOH in the water-soluble electrolyte increases, and hydrogen gas is generated from the surface of the positive electrode. This operating condition of a metal Na battery is referred to as a "sodium-water battery". A sodium-water battery will be described as Embodiment 2 with reference to FIG. 1 (B).

Embodiment 1
FIG. 1 (A) is a schematic view showing the structure of a metal Na battery 20 (sodium-water-air battery) according to the first embodiment. First, an outline of the structure of the metal Na battery 20 is described, and then each configuration is described in detail.

Metal Na battery 20 of this embodiment, the negative active material and Na ⁺ ion conductor 1 having a Na ⁺ ion-conducting, substantially consisting of metallic Na, which is arranged on one side of the Na ⁺ ion conductor 1 4 and at least a portion of the negative electrode active material 4 being in contact with the negative electrode 5 capable of conducting electrons generated when metal Na is ionized and in contact with one surface of the negative electrode 5 and the Na ⁺ ion conductor 1 water to the pooled organic electrolyte 2, stored as a positive electrode 8 disposed on the other side of the Na ⁺ ion conductor 1, in contact with the other surface of the positive electrode 8 and the Na ⁺ ion conductor 1 as And a water-soluble electrolyte 3 and having a base resistance and a positive electrode side housing portion 13.

The housing 7 accommodates the main components of the metal Na battery 20. The housing 7 includes a negative electrode side housing portion 12 and a positive electrode side housing portion 13. The negative electrode side accommodation portion 12 accommodates the negative electrode 5, the negative electrode active material 4 and the organic electrolytic solution 2 which are members on the negative electrode side of the metal Na battery 20. The negative electrode side housing portion 12 has a structure for keeping the inside airtight after housing these. In this state, the negative electrode external terminal 6 extends from the inside to the outside of the negative electrode side housing portion 12.

On the other hand, the positive electrode side accommodation portion 13 accommodates the positive electrode 8 and the water-soluble electrolyte 3 which are members on the positive electrode side of the metal Na battery 20. Further, the positive electrode external terminal 10 extends from the inside of the positive electrode side accommodation portion 13 to the outside. Furthermore, the gas introduction pipe 11 may be inserted into the positive electrode side accommodation portion 13 from the outside as needed.

After the discharge of the metal Na battery 20 starts, the concentration of NaOH (sodium hydroxide) in the aqueous electrolyte solution 3 increases as the reaction proceeds. As a result, the pH of the water-soluble electrolyte 3 may exceed 13. Therefore, at least a portion of the positive electrode side accommodating portion 13 of the housing 7 in contact with the water-soluble electrolytic solution 3 is formed of a basic resistant material, preferably a strongly basic resistant material that can withstand pH 14 or more. As such a positive electrode side housing portion 13, for example, a container such as Pyrex (registered trademark) glass, nylon, polyvinylidene chloride, peak (polyether ether ketone) resin (manufactured by VICTREX), etc. can be suitably used. The negative electrode side accommodation portion 12 and the positive electrode side accommodation portion 13 of the housing 7 may be integrally formed or may be separately formed.

The Na ⁺ ion conductor 1 separates the organic electrolyte solution 2 contained in the negative electrode side housing portion 12 from the water-soluble electrolyte solution 3 contained in the positive electrode side housing portion 13. The Na ⁺ ion conductor 1 is made of a compact Na ⁺ ion conductive solid electrolyte ceramic. Na ⁺ ion conductive solid electrolyte ceramic, Na ⁺ ions selectively conductively solid material, i.e. a dense gas phase and liquid phase is not transmitted directly, a solid material of Na ⁺ ion conductivity. As materials suitable for the Na ⁺ ion conductor 1, the following two materials are mainly considered.

The first material suitable for the Na ⁺ ion conductor 1 is rhombohedral β-alumina and hexagonal β ′ ′-alumina ceramics. These are combined to form a polycrystalline “β-alumina based ceramic”. It is called. Β ′ ′-alumina is preferable to β-alumina because higher Na ⁺ ion conductivity improves cell performance, but there is no significant performance difference even in the presence of two phases, and the β alumina produced actually The ceramics often contain both β-alumina and β ′ ′-alumina as main phases. The metal Na battery 20 can be operated in the range of 0 to 100 ° C. because the water soluble electrolyte 3 is used for the metal Na battery 20, but if the temperature exceeds 80 ° C., the evaporation of the aqueous solution becomes remarkable. desirable. In this case, the ion conductivity of these β-alumina based ceramics is typically in the range of 10 ⁻³ to 0.1 S · cm ⁻¹ . This ionic conductivity is very high as a solid electrolyte, about 1/10 of 1 M HCl aqueous solution. In addition, β alumina-based ceramics have high stability without decomposition even in direct contact with metal Na, as demonstrated for application to Na-S batteries. Therefore, β-alumina-based ceramics play an important role in securing safety while incorporating metallic Na having high activity into a battery structure.

The second suitable material for the Na ⁺ ion conductor 1 is monoclinic or rhombohedral (polycrystalline) Nasicon ceramics. The feature of the crystal structure is that Na ⁺ ions embedded in a network of oxide ions tetrahedrally and hexahedrally coordinated to cations have a structure capable of easily moving. In the case of Nashicon ceramics, an electric conduction of 0-100 ° C, typically 10 ^-4 to 0.1 S · cm ^-1 is exhibited. Since Nasicon ceramics are stable even when immersed in an aqueous solution for a long time (J.J. Auborn & D. W. Johnson., Solid State Ionics, Vol. 5, p. 315, 1981), the water-soluble electrolyte 3 Show excellent characteristics for separating the organic electrolyte 2.

It is desirable that the composition of the ion conductive species in the β-alumina based ceramics and the NASICON ceramics be adjusted so as to be limited to Na ⁺ ions. That is, it is desirable to minimize the amount of chemical species to be monovalent cations other than Na, such as K (potassium). When the amount of monovalent cations other than Na ⁺ ions increases, the ion conductivity decreases and, in addition, the exchange of monovalent cations occurs during the operation of the battery, which causes the volume change in these ceramics, thereby making the ceramics Makes it easy to cause damage. These ceramics are molded into a dense diaphragm. In order to reduce the internal resistance of the battery, the thinner the thickness, the better. However, in the case of self-supporting inside the metal Na battery 20, it is desirable that the thickness of the Na ⁺ ion conductor 1 be 0.1 to 2 mm from the viewpoint of strength. In addition, a structure in which a dense β-alumina-based or NASICON film is formed on a base material made of a material that does not deteriorate with the insulating electrolyte 3 such as porous alumina as the Na ⁺ ion conductor 1. May be used. In this case, the resistance can be further reduced.

The material of the negative electrode 5 is solid (having a melting point of over 100 ° C.) at the operating temperature (0 to 100 ° C.) of the metal Na battery 20 and has a good conductivity to the extent that electrons generated by dissolving metal Na can be conducted. Any material may be used as long as it is a material that does not react significantly with metal Na. As such a material, for example, Al, Cu, Fe, Cr, Ni, Ti, an alloy containing these as main components, and the like can be suitably used. From the viewpoint of cost and weight, metals having Al as a main component are preferable. Forming the negative electrode 5 as a porous metal is more advantageous because the diffusion of sodium is promoted.

The negative electrode active material 4 is accommodated in the negative electrode side accommodation portion 12 so as to be in contact with at least a part of the negative electrode 5. The negative electrode active material 4 contains metal Na as a main component. It is desirable that the negative electrode active material 4 be substantially made of metal Na. “Consisting essentially of metal Na” means that the content of metal Na in the negative electrode active material 4 is 90% by mass or more. The negative electrode active material 4 is electrically connected to the negative electrode 5 and the negative electrode external terminal 6 connected to the negative electrode 5. The metal Na which is the main component of the negative electrode active material 4 may be held by a flat metal plate including the negative electrode 5, a metal wire, a wire mesh or the like. Alternatively, when the negative electrode 5 is formed of a porous metal, Na may be impregnated into the pores to integrate the negative electrode active material 4 and the negative electrode 5.

The negative electrode active material 4 preferably contains substantially no amalgam, which is a liquid alloy of mercury and Na. Specifically, the content of mercury in the negative electrode active material 4 is desirably less than 1.0% by mass, and more desirably less than 0.1% by mass. By reducing the content of mercury in the negative electrode active material 4 to this range, the energy density per weight of the metal Na battery 20 can be improved, the load on the environment can be reduced, and the safety can be improved. Similarly, it is desirable not to contain K substantially. If the metal Na of the negative electrode active material 4 contains a metal that becomes a monovalent cation such as K, the lower the content, the more desirable, and in particular less than 5% by mass. By reducing the content of K in the negative electrode active material 4 to this range, the conductivity and the durability of the ceramic constituting the Na ⁺ ion conductor 1 can be maintained.

The organic electrolytic solution 2 is a substance having Na ⁺ ion conductivity. As the organic electrolytic solution 2, propylene carbonate (PC), ethylene carbonate (EC), or a mixed solution of ethylene carbonate and dimethyl carbonate (EC: DMC) is preferable. In addition, since Na has a smaller absolute value of the standard redox potential than Li, it does not require the stability as in the organic electrolyte of the conventional Li-ion battery or Li-water-air battery. Therefore, it is thought that the electrolyte solution considered unsuitable for Li ion batteries can be utilized. Even if the organic electrolyte solution 2 is omitted and the Na ⁺ ion conductor 1 and the negative electrode active material 4 are brought into direct contact, an electromotive force is generated in principle. However, because the diffusion of sodium is extremely slow, a sufficient current can not be obtained. That is, the organic electrolyte solution 2 plays an important role in order to sufficiently diffuse sodium between the Na ⁺ ion conductor 1 and the negative electrode active material 4. In order to impart Na ⁺ ion conductivity to the organic electrolytic solution 2, sodium salt such as NaPF ₆ , NaClO ₄ , NaBO ₄ or the like, or, if necessary, an electric current at the interface between the negative electrode active material 4 and the organic electrolytic solution Additives are added to improve the properties. Concentration of sodium salts such as NaPF ₆ is more than 0.5M are preferred, more preferably at least 0.8 M, more preferably at least 0.95 M, more 1.0M is more preferable. Also, the addition of fluoroethylene carbonate (FEC) as an additive improves the stability of the metal Na battery 20. It is speculated that the role of the FEC in the metal Na battery 20 is to promote the diffusion of sodium and contribute to the stabilization of the organic electrolyte by forming a stable film on the surface of metal Na.

In the present embodiment, the positive electrode 8 may have any shape of a plate or a wire mesh. As such a material, for example, carbon can be suitably used. On the other hand, in the case of supplying oxygen to the water-soluble electrolyte 3 through the gas introduction pipe 11 and the positive electrode 8, the positive electrode 8 may be a carbon plate, a metal electrode mainly composed of nickel (Ni), platinum (Pt) or palladium It is desirable to use an electrode made of a noble metal such as (Pd) or an electrode in which a noble metal is coated on a stainless steel mesh or the like (for example, carbon plates can be purchased from Tokai Carbon Co., and metal materials can be purchased from Niraco Co., Ltd.). In order to improve the current density, it is particularly preferable to use a carbon plate as the positive electrode 8. Further, in consideration of the price etc., a metal electrode mainly composed of nickel is particularly preferable.

In order to supply oxygen to the water-soluble electrolyte solution 3 through the gas introduction pipe 11 and the positive electrode 8, oxygen or air may be supplied using a compressed gas cylinder. Alternatively, the atmosphere may be supplied by using a pump or the like as needed.

The water-soluble electrolyte 3 is accommodated in the positive electrode side accommodation portion 13. A sodium salt is added to the water-soluble electrolyte solution 3 to impart Na ⁺ ion conductivity. As a sodium salt, NaOH is suitable. The positive electrode 8 is in electrical contact with the positive electrode external terminal 10. FIG. 1A shows the case where a platinum wire mesh (Pt wire mesh) is used as the positive electrode 8. The positive electrode 8 is immersed in the water-soluble electrolyte 3. In the case of using the positive electrode 8 made of a metal mesh, as shown in FIG. 1A, a gas introduction pipe 11 for supplying a gas containing oxygen may be further provided.

A Na salt is added to the water-soluble electrolyte solution 3 in order to impart Na ⁺ ion conductivity. The higher the concentration of Na ⁺ ions in the water-soluble electrolyte 3, which is advantageous from the viewpoint of reducing the internal resistance of the metal Na battery 20, but the electromotive force of the metal Na battery 20 slightly decreases. Therefore, an aqueous solution of NaOH, typically having a concentration of about 0.1 to about 1.0 M, is suitable as the water-soluble electrolyte 3. It is desirable that the water-soluble electrolyte 3 have a pH> 7 (basic). When it becomes acidic, the H ₃ O ⁺ ion concentration in the water-soluble electrolyte 3 increases, and the H ₃ O ⁺ diffuses into the Na ⁺ ion conductor 1 and reduces when reaching the organic electrolyte 2 and the negative electrode active material 4 It may be decomposed to form an ion diffusion barrier layer of oxide accompanied by generation of hydrogen gas. In addition, unlike β-alumina and Nashicon, they do not decompose even with a strong base unlike Rishicon. In addition, since Na ⁺ ions move from the organic electrolyte 2 into the water-soluble electrolyte 3 with the discharge of the metal Na battery 20, the NaOH concentration becomes high. However, there is no particular problem even if the depth of discharge is further increased. This is because an increase in the pH of the water-soluble electrolyte 3 does not adversely affect the Na ⁺ ion conductor 1. Further, in the water-soluble electrolyte solution 3, it is preferable that the concentration of monovalent cations other than Na ⁺ ions such as K ^{+ be} lower. Specifically, it is desirable that the Na ⁺ ion concentration ratio is less than 5%.

As described above, according to the present embodiment, since light and active metal Na and water, and oxygen which can be taken in from the outside as necessary are used as active materials, the energy density per weight or volume is significantly large. An improved metal Na battery 20 can be provided. In addition, since metallic Na is used for the negative electrode, decomposition of the ceramic separator used as the organic electrolytic solution, the current collector, and the Na ⁺ ion conductor 1 as compared with the conventional metal Li-water-air battery or Li ion battery Hard to cause In addition, since the ion conductivity of the Na ⁺ ion conductive solid electrolyte itself is also good, it does not serve as a barrier for improving the power density of the battery.

In addition, since only the common materials and elements are used for the main members, the metal Na battery 20 can be manufactured inexpensively and does not hinder large-scale use. In particular, Na, which is responsible for charge transfer in the battery, does not have the problems of abundance and uneven distribution of resources as compared with Li used in Li-ion batteries. Moreover, in the negative electrode 5 holding metal Na, it is not necessary to use copper which is often required when using Li, and cheaper Al can be suitably used. The elements and raw materials constituting the β-alumina system and the NASICON ceramics used as the Na ⁺ ion conductor 1 are also inexpensive and touched.

In addition, since amalgam, which is a liquid alloy of mercury and sodium, is not substantially used as the negative electrode active material 4, the energy density of the metal Na battery 20 can be improved. In addition, the metal Na battery 20 is excellent not only in manufacture but also in terms of disposal.

In the metal Na battery 20 of the present embodiment, the organic electrolytic solution 2 in which solid metal Na and Na salt are dissolved on the negative electrode 5 side through the Na ⁺ ion conductor 1 is dissolved on the positive electrode 8 side. The use of No. 3 enables stable operation at room temperature to 80 ° C. In particular, when β and β ′ ′ alumina and Nasicon ceramics are used as Na ⁺ ion conductor 1, good conduction of Na ⁺ ions does not become a main resistance component for discharge, and it is a main factor that inhibits the cell reaction. It is possible to prevent the mixing of the organic electrolytic solution 2 and the water-soluble electrolytic solution 3 without becoming a serious factor, and the structure of the metal Na battery 20 is completely completed by the Na ⁺ ion conductor 1 from the structure of the negative electrode 5 side. Thus, the replenishment and replacement of the water-soluble electrolyte 3 are easy.

Second Embodiment
FIG. 1 (B) is a schematic view showing the structure of a metal Na battery 20 (sodium-water battery) according to the second embodiment. Here, only differences from the sodium-water-air battery of Embodiment 1 will be described.

In the present embodiment, the water-soluble electrolyte 3 is accommodated in the positive electrode side accommodating portion 13 by forming the positive electrode 8 on one side of the positive electrode side accommodating portion 13. In this case, the positive electrode 8 has gas permeability that directly takes in oxygen from the outside during discharge and releases the oxygen generated on the electrode during charge to the outside. On the other hand, while the positive electrode 8 is appropriately infiltrated by the water-soluble electrolyte 3, it does not allow the water-soluble electrolyte 3 to pass through. That is, the positive electrode 8 has gas permeability, and the other surface opposite to the one surface facing the Na ⁺ ion conductor 1 is at least partially in contact with air. A porous carbon, a porous metal film, a metal wire mesh, or a porous carbon carrying fine metal particles or a metal oxide catalyst for enhancing the reactivity on at least the surface of the positive electrode 8 in contact with the water-soluble electrolyte 3 It is preferable to use a laminated or composite material.

The gas-permeable positive electrode 8 is water-repellent on the air side and porous and permeable to air, good for oxygen reduction and water oxidation reaction on the aqueous solution side, and excellent in electric conductivity and durability. For example, it is optional. As such a positive electrode 8, a gas diffusion layer, a catalyst layer, and a nickel metal mesh are laminated in this order, and a laminated board obtained by hot pressing them at about 200 ° C., for example, can be suitably used. In this case, the gas diffusion layer can be obtained by mixing highly conductive carbon (Ketjen black) and a binder (PTEF emulsion) and hot pressing the mixture at, for example, about 200 ° C. The catalyst layer can be obtained by mixing activated carbon on which a catalyst consisting of a Mn ₃ O ₄ phase is supported with a binder (PTEF emulsion) and hot-pressing the mixture at, for example, about 200 ° C.

The same effect as that of the first embodiment can be obtained also by the present embodiment.

(Performance of metal Na battery 20)
Table 1 shows theoretical electromotive force (V) at 25 ° C. and theoretical energy density (Wh / kg) at 25 ° C. of sodium-water-air battery (embodiment 1) and sodium-water battery (embodiment 2). In the case where O ₂ is not included and when included. In the table, “s” “l” “aq” “g” respectively represent solid, liquid, aqueous solution and gas.

In the metal Na batteries of

Embodiments

1 and 2, an energy density of about 3 to 15 times that of a typical Li ion battery can be obtained.

Further, the charge / discharge rate (current density) of the metal Na battery 20 is about 0.1 mA / cm ² , more preferably about 0.5 mA / cm ² , and still more preferably about 1 mA / cm ² .

(Example 1: Metallic Na battery using Nashikon separator)
Na ₃ Zr _₂ Si ₂ so as to have the composition of the _{_{_{_{PO 12, Na 3 PO 4,}}}} ZrO 2, SiO 2 raw material is weighed and mixed, and 12 hours calcined at 1100 ° C.. The powder is pulverized by a ball mill, then subjected to uniaxial pressing and cold isostatic pressing and sintered at 1275 ° C. for 15 hours to form a disc having a Nasicon phase purity of 98% or more and a diameter of about 16 mm. A sintered body having a relative density of 98% or more and no gas permeability was obtained. This was polished to a thickness of 1 mm to obtain a Nashicon separator (Na ⁺ ion conductor 1) made of Nashicon ceramics. The temperature characteristics of the sum of the resistance components in the crystal grains and grain boundaries of the above-described Nashicon ceramic were evaluated by the AC impedance method. The activation energy of the electrical conductivity showed a typical value of 0.27 eV. The direct current resistance as the separator was 53 Ω · cm ⁻² at 50 ° C.

FIG. 2 is a schematic view showing an outline of a battery test. The metal Na battery 20 used in the present example was configured as follows. In the present embodiment, the housing 7 is configured by the O-ring 14, the jig 15, the glove box 16, the negative electrode side housing portion 12 including the lid portion 17, and the positive electrode side housing portion 13. First, the obtained Na ⁺ ion conductor 1 was attached to a jig 15 made of peak resin (polyether ether ketone) designed to have an effective area of 1.1 cm ² . The jig 15 has a structure in which the inside of the negative electrode 5 side and the outside of the positive electrode 8 are airtightly separated by a rubber O-ring 14. In a glove box 16 providing a purified atmosphere from which oxygen and water were removed, 20 mg of metal Na pieces were bound to a platinum wire and stored in the jig 15 described above. Furthermore, after pouring the organic electrolyte solution 2 in which 0.5 M NaPF ₆ and 0.05 M fluoroethylene carbonate were dissolved in propylene carbonate so that all metal Na was immersed, O-ring 14 and lid 17 Airtight sealing was performed to form the negative electrode side accommodation portion 12. The platinum wire was drawn out from the negative electrode side accommodation portion 12 so as to maintain the airtightness, and was used as a negative electrode external terminal 6.

As the water-soluble electrolyte 3, 0.1 M NaOH aqueous solution was used. The water-soluble electrolyte solution 3 was accommodated in the positive electrode side accommodation portion 13 made of Pyrex (registered trademark) glass. In the air, the negative electrode side housing portion 12 in which the above-described Na was sealed in the water-soluble electrolyte 3 was placed so that all the Na ⁺ ion conductors 1 were immersed in the water-soluble electrolyte 3. In the water-

soluble electrolyte

3, 4 cm ² of 250 mesh Pt wire netting (a metal material purchased from Nyako Co., Ltd.) was set as a positive electrode 8. The platinum wire welded to the positive electrode 8 was drawn out from the positive electrode side accommodation portion 13 to form a positive electrode external terminal 10. In the vicinity of the

positive electrode

8, 100% O ₂ gas or 5% H ₂ -95% Ar gas at 50 cm ³ / min. Supplied at a flow rate of The battery characteristics were evaluated using the potentio galvanostat 18 (Solartron, 1287 type) through the positive electrode external terminal 10 and the negative electrode external terminal 6.

FIG. 3 shows the current-voltage characteristics obtained at 50 ° C. for the metal Na battery using the Nashicon separator of Example 1. The voltage (V) at current 0 corresponds to the open electromotive force. When 100% oxygen was supplied through the gas inlet tube, the metal Na battery of this example operated as a Na-water-air battery. In this case, the obtained open electromotive force (V) almost matches the theoretical electromotive force. The measured value was slightly smaller than the theoretical electromotive force. It is considered that this is because 0.1 M NaOH was previously dissolved in the aqueous electrolyte. On the other hand, when 5% H ₂ -95% Ar was supplied through the gas introduction pipe, the metal Na battery of this example operated as a Na-water battery. In this case, the obtained open electromotive force (V) showed a value close to the theoretical electromotive force. The measured value was larger than the theoretical electromotive force. It is considered that this is because the dissolved oxygen in the water-soluble electrolyte could not be completely removed. Current - the internal resistance of the battery was estimated from the slope of the voltage characteristic, Na- water - about 200 [Omega ^/ cm 2 in the air battery, the Na- water cell was about 400Ω / cm ^2. This was a value several times larger than the resistance value of the separator of Nashicon ceramics. Therefore, it was judged that Nashicon ceramic itself is not a main component of internal resistance, and works effectively as the Na ⁺ ion conductor without impairing the battery performance itself.

FIG. 4 shows the discharge characteristics obtained at 50 ° C. for the metal Na battery using the Nashicon separator of Example 1. In any of the sodium-water-air battery and the sodium-water battery, the electromotive force (voltage / V) decreases sharply immediately after the integrated current value (discharge amount / mAh) expected from the amount of metal Na charged did. From this, it was shown that the metal Na battery can be completely discharged under any operating condition. Moreover, the metal Na battery of the present Example operated stably for 2 days or more in the state which the positive electrode side accommodating part was immersed in the strongly-basic (pH> 13) water-soluble electrolyte solution.

Example 2 Metallic Na Battery Using Nashicon Separator
The present embodiment is different from the first embodiment in that a sintered body having the same composition as that of the first embodiment is polished to a thickness of 0.5 mm to obtain a separator made of Nashicon ceramics.

The battery test shown in FIG. 2 was performed in the same manner as in Example 1 for the metal Na battery 20 using the obtained Nashicon separator. Point metal Na pieces tied to the platinum wire was 23 mg, that was used NaPF ₆ of 1.0 M, a point used as a positive electrode 8 12cm ² carbon plate (Tokai Carbon Co., Ltd. isotropic graphite) And a battery test was conducted in the same manner as Example 1 except that only 100% O ₂ gas was supplied to the positive electrode 8.

FIG. 5 shows the current-voltage obtained at 50 ° C. for the metal Na battery using the Nashicon separator of Example 2. The open electromotive force (V) was almost in agreement with the theoretical electromotive force. Therefore, it was confirmed that the metal Na battery of this example operated as a Na-water-air battery. The maximum value of the power density (W / kg) is a total of 4.1 mg of the enclosed metal sodium weight of 23 mg and 18 mg of water required for the complete reaction, and 200 W / kg in terms of the effective area of the Nashicon separator. The current density at that time was 6 mA / cm ² . As shown in FIG. 5, in the present example, the unstable behavior seen in Example 1 (FIG. 3) was not confirmed. From this, it is considered that the main reason that the current density was further increased than in Example 1 was that the positive electrode was replaced from a Pt wire mesh to a carbon plate. In addition, it is considered that the increase in the concentration of NaPF ₆ in the organic electrolyte is also a factor.

Table 2 shows the open electromotive force (V), the energy density (Wh / kg) at 50 ° C., and the power density (W / kg) obtained by the metal Na battery using the Nasicon separators of Examples 1 and 2. . The energy and the power density were calculated from the value at the maximum power in the current-voltage characteristics and the mass of the negative electrode and the positive electrode active material excluding oxygen gas involved in the battery reaction.

In the metal Na battery of this example, an energy density of about 3 to 15 times that of a typical Li ion battery was obtained. It was also confirmed that the power density was a value that could sufficiently respond to the target standard value of 100 W / kg.

For example, in order to obtain a cruising distance equivalent to that of a gasoline car having a car weight of about 1000 kg, a conventional Li-ion battery requires a battery of about 1200 kg. Therefore, the conventional Li-ion battery is impractical for practical use. On the other hand, using the Na-water-air battery of this example having an energy density of about 6 times (Table 2), the weight of the battery is about 200 kg. This is a realistic weight comparable to current electric vehicles. In a general-purpose automobile, it is only necessary to constantly obtain a maximum output of about 80 kW for about 1 minute and a steady output of about 20 kW. Therefore, in order to obtain an output of 80 kW in a short time, for example, a Li-ion battery of about 30 kg or a supercapacitor having the same performance as that is used in combination. As a result, the power density required for the Na-water-air battery is about 100 W / kg, which is 1/5 to 1/30 of the Li-ion battery. In addition, the feature that the metal Na battery of the present embodiment has a high energy density, that is, the feature of lightness is not necessarily limited to a mobile object such as an automobile, and is also suitable as a battery for home electric appliances.

(Example 3: β-alumina based metal Na battery using separator)
A raw material was mixed with Na ₂ CO ₃ , g-Al ₂ O ₃ and MgO as a raw material, and calcined at 1600 ° C. for 2 hours. This is subjected to ball milling, uniaxial pressing, and cold isostatic pressing followed by sintering at 1650 ° C. for 24 hours to obtain a diameter of about 6: 4 in weight ratio of β alumina and β ′ ′ alumina phase. A sintered body with 16 mm disk shape and relative density of 98% or more without gas permeability was obtained and polished to a thickness of 1.5 mm to obtain a separator (Na ⁺ ion conductor of β alumina ceramic) The temperature characteristics of the sum of the resistance components within the crystal grains and grain boundaries of the above-mentioned β-alumina-based ceramics were evaluated by the alternating current impedance method. The activation energy of the electrical conductivity was a typical value. The direct current resistance as a Na ⁺ ion conductor was 40 Ω · cm ⁻² at 50 ° C.

Next, in the same manner as in Example 1, a battery test shown in FIG. 2 was performed. A separator of β-alumina based ceramic was used as Na ⁺ ion conductor 1, 10 mg of metal Na pieces bound to a platinum wire, and 0.5 M sodium perchlorate (NaClO instead of NaPF ₆₎ _A battery test was conducted in the same manner as in Example 1 except that ₄ ) was used.

FIG. 6 shows the current-voltage characteristics obtained at 30 ° C., 40 ° C., and 50 ° C. for the metal Na battery using the β-alumina-based separator of Example 3. The open circuit electromotive force (V) almost agrees with the theoretical electromotive force as the Na-water-air battery. From this, it was confirmed that the metal Na battery of this example operated as desired. The internal resistance of the cell estimated from the slope of the current-voltage characteristics at 50 ° C. was about 900 Ω / cm ² . This was a value 10 or more times larger than the resistance value of the β-alumina-based ceramic separator. Therefore, it was determined that the β-alumina ceramic itself was not a main component of the internal resistance. Further, even if the Pt wire mesh was changed to a 1 cm ² Ni plate, no change in current-voltage characteristics was observed.

Example 4 Metallic Na Battery Using Gas Permeable Positive Electrode
Corresponding to FIG. 1 (B), the metal Na battery using the gas-permeable positive electrode was created, and the discharge characteristic was evaluated.

First, a gas-permeable positive electrode was produced by the following procedure. Catalyst consisting mainly of Mn ₃ O ₄ phase by calcining a mixture of activated carbon: manganese nitrate [Mn (NO ₃ ) ₂ · 6H ₂ O] = 3: 1 (weight ratio) at about 350 ° C. in nitrogen atmosphere for 2 h Obtained activated carbon. This was mixed with a binder (PTEF emulsion), and the mixture was hot pressed at about 200 ° C. to obtain a catalyst layer.

Also, a highly conductive carbon (Ketjen black) and a binder (PTEF emulsion) were mixed, and the mixture was hot pressed at about 200 ° C. to obtain a gas diffusion layer.

Furthermore, the gas diffusion layer, the catalyst layer, and the nickel wire netting described above were stacked in this order and hot pressed at about 200 ° C. to obtain a gas-permeable positive electrode.

The metal Na battery 20 shown in FIG. 1 (B) was configured using this gas permeable positive electrode. The effective electrode area of this metal Na battery 20 was 0.79 cm ² . EC which dissolved 10 mg metal Na crimped on a SUS304 stainless steel plate on a negative electrode, 1M sodium perchlorate NaClO ₄ as a sodium salt and 1 vol% fluoroethylene carbonate (FEC) as an additive to a negative electrode electrolyte: An electrolyte of DMC (mixture of ethylene carbonate and dimethyl carbonate in a 1: 1 ratio) was used. A 0.6 mm thick Nashicon ceramic was used as a ceramic separator. The positive electrode electrolyte was a 0.5 M aqueous solution of sodium hydroxide NaOH, and was in contact with one side of the above gas-permeable positive electrode.

FIG. 7 shows the current-voltage characteristics of the metal Na battery using the gas-permeable positive electrode of Example 4 measured at 25 ° C. in air. The maximum output of the metal Na battery of this example was improved to 25 mW / cm ² or more. It is believed that this property surpasses both conventional aqueous and non-aqueous lithium and sodium air batteries. That is, in the present embodiment, a highly conductive sodium ion conductive ceramic material, a non-aqueous electrolytic solution and a water-soluble electrolytic solution are selected as the electrolyte component. This shows that high output can be obtained in the form of an aqueous solution sodium-air battery equipped with a gas-permeable positive electrode.

(Example 5: Evaluation of charge and discharge characteristics)
In the present embodiment, the configuration of the battery and the measurement conditions are the same as in the fourth embodiment. Only the current control of the measurement system for charge and discharge (charge and discharge) differs from that of the fourth embodiment.

FIG. 8 shows the evaluation of charge and discharge characteristics of a metal Na battery using the gas-permeable positive electrode of Example 4.

The charge / discharge rate (current density) of the metal Na battery of this example was about 1 mA / cm ² . This is greater than the charge and discharge rate of the previously reported battery. For example, in Japanese Patent Application No. 2012-537241 is Seramatekku, Inc. patent application ( "solid sodium secondary battery having a sodium-ion conducting ceramic separator"), is carried out charging and discharging speeds of up to 30 .mu.A / cm ² There is. On the other hand, in the metal Na battery of this example, the charge / discharge rate was about 30 times as fast. That is, in the present embodiment, by taking the form of the aqueous solution sodium-air battery, the merit that the material having high conductivity can be selected for the electrolyte component was exhibited.

DESCRIPTION OF SYMBOLS 1 Na ⁺ ion conductor, 2 organic electrolyte solution, 3 water-soluble electrolyte solution, 4 negative electrode active material, 5 negative electrode, 6 negative electrode external terminal, 7 negative electrode, 8 positive electrode, 10 positive electrode external terminal, 11 gas introduction pipe, 12 negative electrode side Housing part, 13 Positive side housing part, 20 metal Na battery

The present invention relates to a metal Na battery.

Claims

Na + ion conductor having Na + ion conductivity,
A negative electrode active material substantially consisting of metallic Na disposed on one side of the Na + ion conductor;
A negative electrode which is disposed so that at least a part is in contact with the negative electrode active material, and which can conduct electrons generated when metal Na is ionized;
An organic electrolytic solution stored so as to be in contact with one surface of the negative electrode active material and the Na + ion conductor;
A positive electrode disposed on the other side of the Na + ion conductor;
A water-soluble electrolyte stored so as to be in contact with the positive electrode and the other surface of the Na + ion conductor;
A metal Na battery containing the water-soluble electrolyte and a container having a basic resistance.
The Na + ion conductor is formed of β ′ ′ alumina, β alumina or Nasicon represented by Nal + x Si x Zr 2 P 3-x O 12 (0 ≦ x ≦ 3). The metal Na battery of claim 1.
The metal Na battery according to claim 1, wherein the negative electrode contains substantially no Hg.
The metal Na battery according to any one of claims 1 to 3, wherein the organic electrolytic solution is propylene carbonate.
The metal Na battery according to any one of claims 1 to 4, wherein the water-soluble electrolyte is an aqueous solution of NaOH.
The positive electrode is gas permeable, and the other surface opposite to the one surface facing the Na + ion conductor is at least partially in contact with air. The metal Na battery of any one of 5.