JP6234917B2 - Negative electrode material for metal-air secondary battery and metal-air secondary battery provided with the same - Google Patents

Description

  The present invention relates to a negative electrode material for a metal-air secondary battery and a metal-air secondary battery including the same.

  Among secondary batteries in practical use, the one having the highest energy density (the amount of electric power that can be discharged with respect to the mass of the battery) is regarded as a lithium ion battery. As a secondary battery exceeding the energy density of this lithium ion battery, a metal-air secondary battery has attracted attention. In the metal-air secondary battery, the reactant of the positive electrode is oxygen in the air, and the reactant of the negative electrode is a metal. The greatest feature of this metal-air secondary battery is that, since oxygen in the atmosphere is utilized at the positive electrode, the mass of the reactant in the positive electrode can theoretically be zero. The mass of the battery occupies most of the mass of the reactant at the positive and negative electrodes and the mass of the electrolyte that mediates the reaction. For this reason, the metal-air secondary battery capable of reducing the mass of the reactant on one electrode to zero may greatly improve the energy density.

As a metal-air secondary battery, a combination of a conductive material such as carbon powder and an oxygen reduction catalyst is used as a positive electrode (air electrode), and zinc, aluminum, iron, lithium, or the like is configured as a negative electrode (metal electrode). Things are common. Among the negative electrode materials, iron is excellent in terms of cost and the like. For example, a metal-air all solid secondary battery having a negative electrode in which iron oxide nanoparticles as a negative electrode active material are supported on the surface of a KOH-ZrO ₂ based solid electrolyte. It has been developed (see JP 2012-74371 A). By using such a negative electrode, the characteristics of the metal-air all-solid secondary battery are improved as compared with the case of using a negative electrode made only of iron powder. However, the maximum discharge capacity of the secondary battery having the negative electrode is not sufficient, and development of a negative electrode material for a metal-air secondary battery having higher performance is strongly desired.

JP 2012-74371 A

  The present invention has been made based on the above circumstances, and the object thereof is a negative electrode material for a metal-air secondary battery, which can exhibit good charge / discharge performance of the metal-air secondary battery, And a metal-air secondary battery comprising the same as a metal electrode.

  The invention made to solve the above problems is a negative electrode material used for a metal-air secondary battery, comprising iron or iron oxide having a porous surface portion. It is a negative electrode material for secondary batteries.

  Another invention made to solve the above problems is a metal-air secondary battery comprising the metal-air secondary battery negative electrode material as a metal electrode.

  According to the present invention, a metal-air secondary battery can exhibit good charge / discharge performance.

3 is a SEM image of the surface of the negative electrode material of Example 1. 3 is a graph showing changes in discharge capacity of the metal-air secondary battery of Example 1. FIG. 3 is a SEM image of the surface of the negative electrode material of Example 2. 4 is a graph showing changes in discharge capacity of the metal-air secondary battery of Example 2.

[Anode material for metal-air secondary batteries]
A negative electrode material for a metal-air secondary battery according to an embodiment of the present invention includes iron or iron oxide having a porous surface portion.

Iron or iron oxide becomes an active material of a negative electrode in a metal-air secondary battery. Part or all of iron or iron oxide exists as a metal (Fe) or a metal oxide (for example, Fe ₂ O ₃ , Fe ₃ O _4, etc.) depending on the charge-discharge state. Iron is a metal that is abundant in nature and can be expected to improve battery voltage. In addition, the cost is overwhelmingly lower than other metals, which helps the spread of metal-air secondary batteries.

  Here, iron has a broad meaning including not only iron itself but also iron alloys and iron-containing substances. Although the absolute value of the oxidation-reduction potential is relatively small, iron has an advantage that the metal electrode is stabilized even when repeatedly charged and discharged because the ionized product (iron ion) does not move. In addition, iron is inexpensive and has a large reserve, and dendrite formation does not occur even when used as a secondary battery, so that a safe secondary battery can be provided.

  However, when iron or iron oxide is used as a negative electrode material for a metal-air secondary battery, this iron or iron oxide does not elute unlike zinc or magnesium, so that iron or iron oxide is efficiently used for redox reaction. The contributing region is limited to the vicinity of the surface of iron or iron oxide (for example, about 100 nm from the surface). Therefore, in order to increase the rate of contribution to the battery reaction, it is necessary to contact the electrolyte up to the inside of iron or iron oxide, and a porous negative electrode (iron or iron oxide having a porous surface portion) having a large surface area is required. The advantage to use is great.

  The surface portion of iron or iron oxide is porous. The porous shape refers to a shape having a large number of pores with such a size that a liquid electrolyte or a solid electrolyte can penetrate into the pores. When the surface of iron or iron oxide has such a shape, in the metal-air secondary battery, the contact area between the negative electrode material (metal electrode) and the electrolyte is widened, so that the reaction efficiency is increased, and it is caused by the redox reaction It can carry electrons quickly. Therefore, this enhances the electrochemical characteristics and enables efficient charging and discharging.

  Iron or iron oxide (negative electrode material) may be porous at least on the surface, and the inside may be bulk or porous. That is, a through hole may be formed in the current collector. In the case where a part is oxidized, it can serve as a current collector. When all are oxidized, the utilization efficiency of the negative electrode is improved. In this case, a separate current collector is provided. The porous surface portion is preferably present in the form of a film. The lower limit of the average thickness of the porous surface portion is, for example, 0.1 μm, preferably 1 μm, more preferably 3 μm, and even more preferably 5 μm. By setting the average thickness (region) of the porous surface portion to the above lower limit or more, the surface area can be efficiently expanded. The upper limit of the average thickness is not particularly limited, and can be, for example, 20 μm, and preferably 10 μm.

  The upper limit of the average diameter of the pores existing in the porous surface portion is preferably 100 nm, and more preferably 60 nm. On the other hand, as this lower limit, it can be 10 nm, for example, and 20 nm is preferable. Let the average diameter of a hole be the average value of the diameter of the opening part of arbitrary five holes which can be confirmed with a SEM (scanning electron microscope) image. The diameter of the opening is a diameter of a perfect circle having the same area as the opening. Moreover, as an upper limit of the average distance of the adjacent holes in the surface portion, 500 nm is preferable, and 200 nm is more preferable. On the other hand, the lower limit is, for example, 10 nm, and preferably 20 nm. The average distance between adjacent holes is the average value of the distances between the centers of gravity of the opening portions between any five sets of adjacent holes that can be confirmed by SEM images. By setting the average diameter and average distance of the holes in the above ranges, the surface area can be sufficiently expanded and the charge / discharge performance can be enhanced.

  The method for forming the porous surface portion is not particularly limited, but it is preferably formed by iron anodization. By porous anodization of iron, a porous surface portion (porous iron oxide film) having pores having a preferred size can be efficiently formed.

  Anodization of iron can be performed by immersing iron in an electrolyte solution and energizing the iron as an anode (positive electrode). For example, platinum or the like is used as the cathode (negative electrode) at this time. In the anodic oxidation of iron, iron is oxidized to iron ions, and these iron ions are combined with oxygen atoms of water in the electrolyte to form iron oxide, which is deposited on the surface of the iron as the anode. . At this time, the film becomes porous due to the progress of the formation of the iron oxide film and the elution of iron in the film in parallel. The thickness, pore diameter, etc. of the porous portion (film) can be controlled by changing the voltage, temperature, electrolyte concentration, electrolyte type, etc. in the anodizing treatment.

As the electrolyte solution for the anodic oxidation of iron, for example, water, a mixed solution of ethylene glycol and NH ₄ F, or the like can be used. Moreover, as a voltage at the time of anodizing, it is 30 V or more and 100 V or less, for example.

It is preferable to perform a heating (annealing) treatment after forming the porous film by anodic oxidation. By this heat treatment, for example, iron fluoride generated by NH ₄ F in the electrolyte solution can be converted to iron oxide, and iron oxidation can be sufficiently advanced. The heating temperature at this time can be, for example, 300 ° C. or more and 600 ° C. or less. Moreover, as heating time, it can be set as 1 hour or more and 5 hours or less, for example.

  The shape of the negative electrode material (iron or iron oxide) is not particularly limited, and may be a plate shape, a particle shape, a linear shape, or the like. In the case of a linear current collector such as an iron wire, for example, (a) a mesh structure in which iron wires are regularly arranged, (b) a non-woven fabric structure (steel wool-like structure) in which iron wires are randomly arranged, (c) A three-dimensional network structure (sponge structure), (d) a structure in which these structures are combined, or the like can be adopted.

  The shape of the negative electrode material can be a sheet including the current collector. The sheet-shaped current collector includes, in addition to iron foil, a sheet-shaped current collector having a mesh structure or a sponge structure formed using, for example, an iron wire. By making the current collector (negative electrode material) into a sheet shape, a metal-air secondary battery in which the positive electrode, the electrolyte, and the negative electrode are formed in layers can be easily formed. The average thickness of the sheet-like current collector is not particularly limited, but the lower limit is, for example, 0.01 mm, and preferably 0.1 mm. On the other hand, the upper limit is, for example, 10 mm, preferably 3 mm, and more preferably 1 mm. By setting it as such thickness, a more sufficient function as a negative electrode can be exhibited.

  The lower limit of the porosity of the entire current collector (iron or iron oxide) is, for example, 1% by volume, preferably 10% by volume, and more preferably 20% by volume. The upper limit is, for example, 80% by volume. By setting the porosity of the entire current collector to the above lower limit or more, the surface area can be expanded and the charge / discharge efficiency can be increased. In addition, the porosity of the whole electrical power collector can be effectively raised by making a collector into a mesh structure, a nonwoven fabric structure, a three-dimensional network structure, etc. The porosity means the ratio of the total volume of pores to the apparent volume of the entire current collector.

  The negative electrode material for a metal-air secondary battery can further include a solid electrolyte (first solid electrolyte) disposed so as to be in contact with iron or iron oxide. The arrangement of the solid electrolyte is, for example, by applying a solid electrolyte solution to one or both sides of a sheet-like current collector (iron or iron oxide having a porous surface portion), or a current collector in the solid electrolyte solution. It can be performed by dipping. By drying after coating or dipping, a sheet body (negative electrode material) in which the solid electrolyte is impregnated in the pores and the like of the current collector is obtained. It does not specifically limit as a solid electrolyte, It is the same as that of what is illustrated as what is used for the metal-air secondary battery mentioned later. Thus, according to the negative electrode material in which the solid electrolyte is combined, a metal-air secondary battery can be easily manufactured by combining with the positive electrode (air electrode).

  The negative electrode material for a metal-air secondary battery may further include a powder having electron conductivity (electron conductive powder) disposed so as to be in contact with iron or iron oxide. Examples of the electron conductive powder include carbon black and metal powder. The effect is particularly remarkable when all of the negative electrode material is oxidized. For example, when the current collector (iron or iron oxide) has a mesh structure or a sponge structure, the electron conductive powder may be filled or embedded in the space of this structure, or inside the porous surface portion ( It may be filled or embedded in the hole). Moreover, the electron conductive powder may be laminated on the surface of the current collector. Thus, the surface area of the negative electrode material can be further increased by combining the electron conductive powder with iron or iron oxide. In addition, this electronically conductive powder may be mixed with a solid electrolyte, for example, and disposed.

  According to the negative electrode material for a metal-air secondary battery, the current collector (iron or iron oxide) has electronic conductivity and porosity that allows a liquid or solid electrolyte to come into contact with the inside of the hole. The charge / discharge performance of the air secondary battery can be improved. Thereby, the metal-air secondary battery which shows a plateau area | region (potential flat part) on a discharge curve can also be obtained.

[Metal-air secondary battery]
The metal-air secondary battery which concerns on one Embodiment of this invention is equipped with the said negative electrode material for metal-air secondary batteries as a metal electrode. Other configurations of the metal-air secondary battery can be the same as those of a general metal-air secondary battery. That is, the metal-air secondary battery includes an air electrode and an electrolyte interposed between the metal electrode and the air electrode in addition to the metal electrode. A separator may be provided between the positive electrode and the negative electrode.

The air electrode functions as a positive electrode of the metal-air secondary battery. In the air electrode, the reaction of the following formula (1) occurs. Although what is used for a normal metal-air secondary battery can be used for an air electrode, it is preferable that a carbon and an oxygen reduction catalyst are included. A sheet-like thing can be used for an air electrode, and it is preferable to use a carbon layer with a catalyst.
O ₂ + 2H ₂ O + 4e ⁻ → 4OH ⁻ (1)

Examples of the catalyst (oxygen reduction catalyst) include Pt and MnO ₂ . As the form of the carbon layer (carbon sheet) constituting the catalyst-attached carbon layer, for example, a green powder compact or carbon paper can be used.

  The lower limit of the average thickness of the air electrode is preferably 0.05 mm, and more preferably 0.1 mm. By setting the average thickness of the air electrode to the above lower limit or more, a sufficient reaction can be caused. On the other hand, the upper limit is, for example, 0.3 mm, and preferably 0.2 mm. If the air electrode becomes too thick, it tends to be difficult to efficiently form a three-phase interface of electrolyte, catalyst, and air.

  As the electrolyte, those usually used for metal-air secondary batteries, such as a liquid electrolyte and a solid electrolyte (second solid electrolyte), can be used. A plurality of types of electrolytes may be used, and a plurality of electrolytes may be used in multiple layers.

  Examples of the liquid electrolyte include a solution in which a salt is dissolved in a solution and an ionic liquid. Examples of the solution liquid electrolyte include alkaline aqueous solutions such as an aqueous potassium hydroxide solution and an aqueous sodium hydroxide solution.

  When a solid electrolyte is used as the electrolyte, the metal-air secondary battery is usually configured as a metal-air all-solid secondary battery having a layer structure including a metal electrode layer, a solid electrolyte layer, and an air electrode layer. The

  The solid electrolyte refers to an electrolyte having no fluidity, and examples thereof include a gel body in which a salt such as a basic hydroxide is held in a gel. Examples of the salt in the gel solid electrolyte include basic hydroxides such as potassium hydroxide and sodium hydroxide, and examples of the gel include zirconia gel. A binder such as polyvinylidene fluoride (PVdF) may be mixed in the solid electrolyte.

  When the solid electrolyte is layered, it is preferable that the average film thickness be 0.05 mm or more in order to sufficiently exhibit the action of conducting hydroxide ions and to prevent short circuit. However, since the actual resistance (battery internal resistance) increases when the thickness is too large, the average film thickness is preferably set to 0.3 mm or less, for example.

  In addition, as one mode of the metal-air all-solid secondary battery, a negative electrode material in which the above-described solid electrolyte (first solid electrolyte) is impregnated in a hole portion or the like of the current collector can be used. Such a negative electrode material and an air electrode constitute a metal-air all-solid secondary battery. In the case of this metal-air all-solid secondary battery, the solid electrolyte is in wide contact with iron or iron oxide which is the current collector of the negative electrode, and hydroxide ions generated at the air electrode are continuously supplied to the negative electrode side. Make it possible. In addition, when using such a negative electrode material, it is not necessary to arrange | position a 2nd solid electrolyte and a liquid electrolyte further between a negative electrode material (metal electrode) and an air electrode, but you may arrange | position.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and can be implemented with modifications within a range that can be adapted to the gist of the present invention, all of which are included in the technical scope of the present invention. .

[Example 1]
In the following procedure, anodization of iron was performed to produce a negative electrode material for a metal-air secondary battery. Ethylene glycol, water and NH ₄ F were mixed and stirred at room temperature for 1 hour to obtain an electrolytic solution for anodizing treatment. A platinum rod was used as the negative electrode (cathode), and an iron plate (manufactured by Nilaco Corporation, 0.2 mm × 10 mm × 10 mm) was used as the positive electrode (anode). After anodizing at 60 V, heating (annealing) was performed at 450 ° C. for 3 hours. The structure of the surface portion (iron oxide porous film) of the obtained negative electrode material was observed by SEM, and the oxidation-reduction behavior was evaluated by a cyclic voltammetry (CV) method in an aqueous potassium hydroxide solution. A metal-air secondary battery was constructed using the obtained negative electrode material, and a charge / discharge test was performed. The negative electrode of the metal-air secondary battery (charge / discharge test cell) is the negative electrode material obtained by anodic oxidation, the electrolyte is an 8M aqueous potassium hydroxide solution, and the air electrode is an electrolytic dioxide that is a redox catalyst. Water-repellent carbon paper coated with manganese (MnO ₂ ) to form a catalyst layer was used. The charge / discharge test was conducted at a charge rate and a discharge rate of 1 mA / cm ² and 0.2 mA / cm ² , respectively.

  The surface SEM image of the obtained negative electrode material is shown in FIG. From the SEM image of FIG. 1, it was confirmed that a porous iron oxide film was formed. The average diameter of the holes in the iron oxide film was 46 nm, and the film thickness was 2.5 μm.

The charge / discharge results of the metal-air secondary battery are shown in FIG. The maximum discharge capacity of 5.94 mAhg ⁻¹ (Fe) was obtained at the second cycle, and the discharge capacity at the 10th time was about 2.3 mAhg ⁻¹ (Fe). The negative electrode material obtained by the anodizing treatment of iron was improved in efficiency by increasing the surface area, and a large discharge capacity was obtained.

[Example 2]
In the following procedure, anodization of iron was performed to produce a negative electrode material for a metal-air secondary battery.
Ethylene glycol (98.2 ml), water (0.9 ml) and NH ₄ F (1M) were mixed and stirred at room temperature for 1 hour to obtain an electrolytic solution for anodizing treatment. A platinum rod was used as the negative electrode (cathode), and an iron plate (manufactured by Nilaco Corporation, 0.2 mm × 10 mm × 10 mm) was used as the positive electrode (anode). After anodizing at 50 V, heating (annealing) was performed at 450 ° C. for 3 hours. The structure of the surface portion (iron oxide porous film) of the obtained negative electrode material was observed by SEM. A metal-air secondary battery was constructed using the obtained negative electrode material, and a charge / discharge test was performed. The configuration of the electrolyte and air electrode, and the charge rate and discharge rate were the same as in Example 1.

  The surface SEM image of the obtained negative electrode material is shown in FIG. From the SEM image of FIG. 3, it was confirmed that a porous iron oxide film was formed. Moreover, the average diameter of the holes of the iron oxide film was 45 nm and the film thickness was 5.5 μm.

The charge / discharge results of the metal-air secondary battery are shown in FIG. A discharge capacity of 6.7 mAhg ⁻¹ (Fe) was obtained in the 10th cycle. The negative electrode material obtained by the anodizing treatment of iron improves the efficiency by increasing the film thickness of the porous iron oxide film, provides a large discharge capacity, and increases the oxide film thickness to 5.5 μm. It was revealed that the characteristics were improved.

The negative electrode material for a metal-air secondary battery of the present invention can be suitably used as a metal electrode (negative electrode) of a metal-air secondary battery.

Claims (5)

A negative electrode material used for a metal-air secondary battery,
Comprising iron or iron oxide having a porous surface portion ;
The porous surface portion is a film-like region having an average thickness of 0.1 μm or more from the surface, and the average diameter of pores existing in the porous surface portion is 100 nm or less, and is adjacent to the porous surface portion. The average distance between the holes is 500 nm or less,
The porous surface portion, the metal and wherein the anodic oxide coating der Rukoto iron - anode material for air secondary battery. 2. The negative electrode material for a metal-air secondary battery according to claim 1, further comprising a first solid electrolyte disposed in contact with the iron or iron oxide. A metal-air secondary battery comprising the metal-air secondary battery negative electrode material according to claim 1 or 2 as a metal electrode. An air electrode containing carbon and oxygen reduction catalysts;
The metal-air secondary battery according to claim 3 , further comprising: a liquid electrolyte interposed between the air electrode and the metal electrode. An air electrode containing carbon and oxygen reduction catalysts;
The metal-air secondary battery according to claim 3 , further comprising: a second solid electrolyte interposed between the air electrode and the metal electrode.