Positive electrode for air battery, and air battery

Abstract

The present invention addresses the problem of providing: a positive electrode for an air battery whereby it is possible to suppress decreases in discharge capacity; and an air battery. A positive electrode (10) for an air battery according to the present invention comprises a first electrode reaction layer (11), a second electrode reaction layer (12), a gas supply layer (13), and a current collector (14). The first electrode reaction layer (11) contains a metal oxide that exhibits the activity of an oxygen generation reaction during charging. The second electrode reaction layer (12) contains a conductive carbon material that exhibits the activity of an oxygen reduction reaction during discharge.

Description

Positive electrode for air batteries, and air batteries

[0001] This invention relates to a positive electrode for an air battery and to an air battery.

[0002] In recent years, research and development has been conducted on rechargeable batteries that contribute to energy efficiency, in order to ensure that more people have access to affordable, reliable, sustainable, and advanced energy.

[0003] Air batteries, which use oxygen from the air as the positive electrode active material and a metal as the negative electrode active material, have primarily been used as primary batteries. However, due to their extremely high energy density, air batteries are expected to be used not only as primary batteries but also as next-generation secondary batteries, and research and development are progressing from various perspectives.

[0004] For example, Patent Document 1 proposes a zinc-air battery comprising a positive electrode which is an air electrode, a negative electrode which comprises metallic zinc or zinc containing additive elements, and an electrolyte which comprises a salen-based metal complex and is disposed between the positive electrode and the negative electrode. Patent Document 2 also proposes a negative electrode for a metal-air battery comprising a metal substrate which comprises a metal selected from the group consisting of zinc (Zn), lithium (Li), magnesium (Mg), aluminum (Al), and iron (Fe), and a thin film which comprises a titanium oxide nanosheet monolayer film located on the metal substrate.

[0005] Japanese Patent Publication No. 2021-022472 Japanese Patent Publication No. 2024-094991

[0006] The inventions described in Patent Documents 1 and 2 are both inventions for maintaining charge-discharge characteristics by suppressing dendrite deposition. The invention described in Patent Document 1 is an invention that incorporates a specific substance into the electrolyte, and the invention described in Patent Document 2 is an invention that provides a specific thin film on the negative electrode. The present inventors wanted to suppress the decrease in discharge capacity associated with repeated charging and discharging of air batteries and maintain charge-discharge characteristics, based on a completely different perspective from Patent Documents 1 and 2.

[0007] Therefore, the object of the present invention is to provide a positive electrode for an air battery and an air battery that can suppress the decrease in discharge capacity. This will ultimately contribute to energy efficiency.

[0008] To solve the aforementioned problems, the positive electrode for an air battery according to the present invention is a positive electrode for an air battery comprising a first electrode reaction layer, a second electrode reaction layer, a gas supply layer, and a current collector, wherein the first electrode reaction layer contains a metal oxide that exhibits activity in an oxygen evolution reaction during charging, and the second electrode reaction layer contains a conductive carbon material that exhibits activity in an oxygen reduction reaction during discharge.

[0009] The positive electrode for air batteries and the air batteries according to the present invention can suppress the decrease in discharge capacity. This, in turn, contributes to energy efficiency.

[0010] This is a cross-sectional view of the positive electrode for an air battery according to this embodiment. This is a circuit diagram from when a cycle test was performed. This is a table showing the conditions of the charge / discharge device in the cycle test. This is a graph showing the relationship between the number of cycles and the amount of stacked metal oxide in the cycle test.

[0011] The following describes the positive electrode for an air battery according to the present invention, and an embodiment for implementing the air battery (this embodiment).

[0012] [Positive Electrode for Air Battery] The positive electrode for an air battery according to the present embodiment is a positive electrode used for an air battery that is a secondary battery, and is also called an air electrode. As shown in FIG. 1, a positive electrode for an air battery 10 includes a first electrode reaction layer 11, a second electrode reaction layer 12, a gas supply layer 13, and a current collector 14. As a result of intensive studies on the positive electrode 10 for an air battery, the present inventors have found that forming the electrode reaction layer of the positive electrode into at least a two-layer structure including the first electrode reaction layer 11 containing a metal oxide and the second electrode reaction layer 12 containing a conductive carbon material can suppress a decrease in discharge capacity associated with repeated charging and discharging. In addition, the present inventors have found that by adjusting the mass ratio of the metal oxide in the first electrode reaction layer 11 to the conductive carbon material in the second electrode reaction layer 12, not only the effect of suppressing the decrease in discharge capacity is improved, but also swelling of the positive electrode can be suppressed. Hereinafter, each layer constituting the positive electrode for an air battery according to the present embodiment will be described in detail with reference to FIG. 1.

[0013] (First Electrode Reaction Layer) The first electrode reaction layer 11 is, during charging, "4OH － →O ２ +2H ２ O+4e － ", which is a layer where the electrochemical reaction occurs. The first electrode reaction layer 11 contains a metal oxide that exhibits activity for the oxygen evolution reaction (OER, Oxygen Evolution Reaction) during charging. Specifically, the metal oxide that exhibits OER activity during charging is ABO ３ -type oxide (perovskite-type oxide represented by the chemical composition ABO ３ ), where A is one or more selected from the group consisting of La, Pr, Ca, Sr, and Ba, and B is one or more selected from the group consisting of Mn, Ni, Fe, Co, Cu, and Cr. The metal oxide may be a solid solution. The metal oxide exhibiting OER activity is not particularly limited as long as it is a metal oxide having a combination of A and B described above, and examples thereof include LaNiO ３ , LaMnO ３ , LaCoO ３ , LaCrO ３ , LaFeO ３ and the like. Naturally, examples of the metal oxide exhibiting OER activity also include La ０．８Sr ０．２ MnO ３ La ０．６ Sr ０．４ MnO ３ La ０．８ Ca ０．２ MnO ３ Combinations such as those with multiple A's, or LaFe ０．６ Co ０．４ O ３ LaNi ０．６ Mn ０．４ O ３ It may also be a combination where B is multiple, such as the examples above.

[0014] While there are no particular limitations on the method for producing metal oxides exhibiting OER activity, they can be synthesized, for example, by reverse homogeneous precipitation. Reverse homogeneous precipitation involves rapidly changing the pH of an aqueous solution by dropping a small amount of metal ion aqueous solution into a large amount of high-pH aqueous solution, thereby simultaneously precipitating multiple types of metal ions with different solubility products as hydroxides. As a result, reverse homogeneous precipitation allows for the synthesis of metal hydroxide nanoparticles with high homogeneity of the mixture of multiple types of metal ions.

[0015] (Second electrode reaction layer) The second electrode reaction layer 12 is "O ２ +2H ２ O+4e － →4OH － This is a layer where the following electrochemical reaction occurs. The second electrode reaction layer 12 contains a conductive carbon material that exhibits oxygen reduction reaction (ORR) activity during discharge. Examples of conductive carbon materials that exhibit ORR activity during discharge include graphene, Ketjenblack, and acetylene black. Note that while graphene and the like also exhibit OER activity during charging, it is sufficient if they exhibit ORR activity during discharge.

[0016] While there are no particular limitations on the method for producing conductive carbon materials, they can, for example, be produced by a mechanochemical method. A mechanochemical method is a material synthesis method based on chemical changes (mechanochemical reactions) that utilize mechanical energy such as collision energy or shear energy, and one example is the use of a planetary ball mill equipped with large-diameter stainless steel balls as the mechanical energy source.

[0017] (Other materials in the first and second electrode reaction layers) In addition to the metal oxides and conductive carbon materials mentioned above, the first electrode reaction layer 11 and the second electrode reaction layer 12 may also contain conventionally known materials used as reaction layers and catalyst layers for positive electrodes, such as binders, conductive materials, water-repellent materials, and ion conductors.

[0018] (Mass ratio of conductive carbon material to metal oxide) As described above, the inventors have confirmed that the mass ratio of the metal oxide in the first electrode reaction layer 11 and the conductive carbon material in the second electrode reaction layer 12 has a significant effect not only on suppressing the decrease in discharge capacity but also on the swelling of the positive electrode, and the details are as follows. The mass ratio of the conductive carbon material to the metal oxide is preferably 1:3.5 to 1:10.4, and more preferably 1:4 to 1:10.4, 1:5 to 1:10.4, and 1:6 to 1:10.4. When the mass ratio of the two is within a predetermined range, the effect of suppressing the decrease in discharge capacity is strongly exhibited. Furthermore, when the mass ratio of the two is 1:6 to 1:10.4, the effect of suppressing the swelling of the positive electrode is also exhibited.

[0019] Japanese Patent Publication No. 2024-102722 proposes a positive electrode comprising a layer having ORR activity and a support having OER activity. However, this publication does not disclose any technical ideas such as specifying the mass ratio of the metal oxide (a substance exhibiting OER activity) and the conductive carbon material (a substance exhibiting ORR activity).

[0020] (Gas Supply Layer) The gas supply layer 13 is a layer that supplies externally supplied gas (air) to the electrode reaction layers 11 and 12. The gas supply layer 13 also functions to achieve water repellency and suppress evaporation of the electrolyte. The gas supply layer 13 is not particularly limited as long as it is a porous layer capable of diffusing gas, and may be formed of a conventionally known material such as hydrophobic carbon (acetylene black), carbon black, or carbon paper, for example.

[0021] (Current Collector) The current collector 14 is a current collector on the positive electrode side, and is a conductor that collects electricity. The current collector 14 is not particularly limited as long as it is a substance capable of collecting current, and may be formed of a conventionally known material such as nickel, iron, or titanium, for example. Further, the current collector 14 is preferably in a mesh shape to ensure ventilation from the outside to the first electrode reaction layer 11 and the second electrode reaction layer 12.

[0022] (Lamination Configuration of Positive Electrode) The positive electrode 10 in Fig. 1 is laminated in the order of the first electrode reaction layer 11, the second electrode reaction layer 12, the gas supply layer 13, and the current collector 14 from the electrolyte side toward the outside air side (from the right side toward the left side in Fig. 1), but the lamination order is not particularly limited. For example, the order of the first electrode reaction layer 11 and the second electrode reaction layer 12 may be reversed. Further, the positive electrode 10 in Fig. 1 has a configuration in which one layer each of the first electrode reaction layer 11 and the second electrode reaction layer is provided, but may have a plurality of layers. For example, a lamination configuration of the first electrode reaction layer 11, the second electrode reaction layer 12, the first electrode reaction layer 11, the second electrode reaction layer 12, the gas supply layer 13, and the current collector 14 from the electrolyte side toward the outside air side is also acceptable. In this case, the mass ratio of the conductive carbon material to the metal oxide may be calculated based on the total mass of the metal oxide contained in the plurality of first electrode reaction layers 11 and the total mass of the conductive carbon material contained in the plurality of second electrode reaction layers 12.

[0023] [Air Battery] The air battery according to the present embodiment includes a positive electrode, a negative electrode, and an electrolyte provided between the positive electrode and the negative electrode. The positive electrode is as described above, and the negative electrode and the electrolyte are as described below.

[0024] (Negative electrode) The negative electrode may be made of a conventionally known metal material used in air batteries, such as zinc, lithium, aluminum, magnesium, and iron, but zinc is particularly preferred. Alternatively, the negative electrode may be made by plating these metal materials onto the surface of a carbon-based material or the like. When zinc is used as the negative electrode, during discharge, "Zn + 4OH" occurs. － →Zn(OH) ４ ２－ +2e － "Zn(OH)" ４ ２－ → Zn+H ２ O + 2OH － "Zn(OH)" ４ ２－ →Zno ２ ２－ +2H ２ Reactions such as "O" occur. If the negative electrode is equipped with a current collector, it may be made of any conventionally known material used as a current collector, such as gold-plated stainless steel (SUS), nickel, or titanium.

[0025] (Electrolyte) The electrolyte can also be any conventionally known electrolyte used in air batteries, such as liquid electrolytes like aqueous or non-aqueous solutions. Examples of aqueous electrolytes include aqueous potassium hydroxide (KOH) solution, aqueous sodium hydroxide (NaOH) solution, and ammonium chloride (NH₃). ４ Examples include aqueous solutions of Cl.

[0026] (Other Components) In addition to the positive electrode, negative electrode, and electrolyte described above, the air battery according to this embodiment also includes conventionally known components provided in an air battery, such as a separator for suppressing dendrite deposition, a fan for supplying air to the positive electrode, and carbon dioxide (CO2). ２ ) A filter for removal may be provided. Furthermore, the air battery according to this embodiment is not limited to a structure in which each component (positive electrode 10, negative electrode 20, electrolyte 30) is plate-shaped and stacked as shown in Figure 1, but may be a structure composed of, for example, a rod-shaped negative electrode, a cylindrical positive electrode formed to surround the negative electrode at a predetermined interval, and an electrolyte filling the space between the negative electrode and the positive electrode.

[0027] (Positive electrode for air battery and effects of air battery) The positive electrode for an air battery according to this embodiment comprises a first electrode reaction layer, a second electrode reaction layer, a gas supply layer, and a current collector, wherein the first electrode reaction layer contains a metal oxide that exhibits oxygen evolution activity during charging, and the second electrode reaction layer contains a conductive carbon material that exhibits oxygen reduction activity during discharge. According to this embodiment, since the positive electrode for an air battery comprises a first electrode reaction layer containing a metal oxide that exhibits oxygen evolution activity during charging and a second electrode reaction layer containing a conductive carbon material that exhibits oxygen reduction activity during discharge, a decrease in discharge capacity can be suppressed. In the positive electrode for an air battery according to this embodiment, it is preferable that the second electrode reaction layer is formed only between the first electrode reaction layer and the gas supply layer. According to this embodiment, the electrochemical reaction associated with OER activity in the first electrode reaction layer during charging proceeds smoothly. In the positive electrode for an air battery according to this embodiment, it is preferable that the mass ratio of the conductive carbon material to the metal oxide is 1:3.5 to 1:10.4. According to this embodiment, the effect of suppressing the decrease in discharge capacity is more strongly exhibited. In the positive electrode for an air battery according to this embodiment, the mass ratio of the conductive carbon material to the metal oxide is preferably 1:7 to 1:10.4. According to this embodiment, not only the effect of suppressing the decrease in discharge capacity but also the effect of suppressing the swelling of the positive electrode is exhibited. In the positive electrode for an air battery according to this embodiment, the metal oxide is ABO ３It is preferable that the oxide is a composite oxide (A is one or more of La, Pr, Ca, Sr, Ba, and B is one or more of Mn, Ni, Fe, Co, Cu, Cr). According to this embodiment, the effect of suppressing the decrease in discharge capacity is more reliably exhibited. The air battery according to this embodiment is an air battery comprising a positive electrode, a negative electrode, and an electrolyte provided between the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode for air batteries described above, and the first electrode reaction layer is provided in contact with the electrolyte. According to this embodiment, since the positive electrode is the positive electrode for air batteries described above, the decrease in discharge capacity can be suppressed. Furthermore, in this embodiment, since the first electrode reaction layer is provided in contact with the electrolyte, the reactivity between the electrolyte and the first electrode reaction layer is superior compared to a configuration in which an anion exchange membrane is provided between the two.

[0028] Next, the present invention will be described by illustrating examples of embodiments that satisfy the requirements of the present invention and comparative examples that do not.

[0029] [Sample Preparation Procedure] The sample preparation procedure is as follows: (Synthesis of graphene by mechanochemical method) Graphite powder (Fujifilm Wako Pure Chemical Industries, Ltd., 072-03845) was pulverized at 700 rpm for 2 hours under an argon atmosphere using a planetary ball mill (P-7, manufactured by Fritsch Japan Co., Ltd.) to synthesize graphene. The pulverized sample was then collected and stirred in 5% hydrochloric acid for more than 12 hours to remove any contaminating stainless steel, thereby obtaining graphene.

[0030] (Synthesis of metal oxides by reverse homogeneous precipitation method) LaNO ３ NiNO ３ An aqueous solution (adjusted so that the total La ions and Ni ions are 0.1 mol / L) was added dropwise to a 1% TMAH (tetramethylammonium hydroxide) aqueous solution to obtain a hydroxide precipitate of La and Ni. This precipitate was collected by suction filtration, dried at 110°C for more than 12 hours, and then heat-treated in air at 650°C for 5 hours to obtain the metal oxide LaNiO2. ３ I obtained it.

[0031] (Preparation of the positive electrode) First, graphene obtained by mechanochemical method and a PTFE aqueous dispersion (Daikin Industries, Ltd., Polyflon PTFE D-210C) were added to a mixture of distilled water and a dispersant (1-butanol), and the mixture was stirred for 10 minutes. The mixture was then collected by suction filtration and pulverized in a mixer to obtain carbon material powder. Metal oxide powder was prepared by adding PTFE to metal oxides using a similar method. Next, hydrophobic carbon (Denka Li-100) and a PTFE aqueous dispersion were added to a mixture of distilled water and a nonionic surfactant (Triton X-100 (Kishida Chemical Co., Ltd.)), and the mixture was stirred for 10 minutes. The mixture was then collected by suction filtration and pulverized in a mixer to prepare gas supply layer powder. A Φ24 mm Ni mesh (NI-318100, Niraco Co., Ltd.) and gas supply layer powder were stacked in a mold and pressed at 1 MPa. Carbon material powder and metal oxide powder were then layered in that order and pressed at 2.5 GPa. After that, the mold was heated to the melting point of PTFE (365°C), and immediately pressed at 5 GPa for rapid cooling to obtain the positive electrode (Φ24 mm, thickness approximately 1 mm). The method for preparing the positive electrode for each sample was as described above, but only the amount of metal oxide powder layered was changed between samples; all other preparation conditions were kept the same.

[0032] (Cycle Test: Circuit) A cycle test was performed on the positive electrode of each sample obtained by the above manufacturing method using the circuit 100 shown in Figure 2. Specifically, the positive electrode 10 was the positive electrode obtained by the above manufacturing method, and consisted of a first electrode reaction layer 11, a second electrode reaction layer 12, a gas supply layer 13, and a current collector 14 stacked together. The negative electrode 21 was made of a zinc plate, and the current collector 22 on the negative electrode side was made of gold-plated stainless steel (SUS). The electrolyte 30 was an aqueous potassium hydroxide (KOH) solution. The charge / discharge device 40 was a VMP-300 manufactured by Bio-Logic.

[0033] (Cycle Test: Charge and Discharge Conditions) The charge and discharge conditions in the cycle test using the charge and discharge device 40 were as shown in Figure 3. Specifically, as shown in Figure 3, after State 0 → State 1 (discharge) → State 2 (charge), the system returned to State 1 in State 3, and the cycle of discharge and charge, State 1 → 2, was repeated 1000 times. However, when the voltage in State 1 fell below 0.6V, the system moved to State 4 and the cycle test was terminated. The more cycles performed in the cycle test, the more effectively the decrease in discharge capacity was suppressed.

[0034] (Visual Inspection) After the cycle test, the positive electrode of each sample was visually inspected to determine whether or not it had swollen.

[0035] The table below shows the mass ratio and metal oxide content of each sample, as well as the test results from the cycle test.

[0036]

[0037] (Review of Results) Figure 4 shows the relationship between the number of cycles and the amount of multilayer metal oxide in the cycle test. The solid line in Figure 4 was drawn by the inventors to make it easier to understand the change in the number of cycles as the amount of multilayer metal oxide changes.

[0038] From the results in Table 1 and Figure 4, it was confirmed that the decrease in discharge capacity associated with repeated charging and discharging can be suppressed by using a two-layer configuration consisting of a first electrode reaction layer containing a metal oxide and a second electrode reaction layer containing a conductive carbon material. Specifically, for samples 2 to 4, where the mass ratio of conductive carbon material to metal oxide was 1:3.5 to 1:10.4, it was confirmed that the number of cycles increased and the decrease in discharge capacity was suppressed. Furthermore, as shown in Figure 4, it was confirmed that the number of cycles increased sharply as the proportion of metal oxide to conductive carbon material increased (especially in the section from sample 2 to sample 3), and then slightly decreased from sample 3 to sample 4. In addition, for samples 3 to 4, where the mass ratio of conductive carbon material to metal oxide was 1:7 to 1:10.4, it was confirmed that the swelling of the positive electrode could also be suppressed.

[0039] 10 Positive electrode 11 First electrode reaction layer 12 Second electrode reaction layer 13 Gas supply layer 14 Current collector 21 Negative electrode 22 Current collector 30 Electrolyte 40 Charge / discharge device 100 Circuit

Claims

1. A positive electrode for an air battery comprising a first electrode reaction layer, a second electrode reaction layer, a gas supply layer, and a current collector, wherein the first electrode reaction layer contains a metal oxide that exhibits activity in an oxygen evolution reaction during charging, and the second electrode reaction layer contains a conductive carbon material that exhibits activity in an oxygen reduction reaction during discharge.

2. The positive electrode for an air battery according to claim 1, characterized in that the second electrode reaction layer is formed only between the first electrode reaction layer and the gas supply layer.

3. The positive electrode for an air battery according to claim 1 or 2, characterized in that the mass ratio of the conductive carbon material to the metal oxide is 1:3.5 to 1:10.

4.

4. The positive electrode for an air battery according to claim 1 or 2, characterized in that the mass ratio of the conductive carbon material to the metal oxide is 1:7 to 1:10.

4.

5. The metal oxide is ABO ３ The positive electrode for an air battery according to claim 1 or 2, characterized in that it is a system oxide (A is one or more of La, Pr, Ca, Sr, Ba, and B is one or more of Mn, Ni, Fe, Co, Cu, Cr).

6. An air battery comprising a positive electrode, a negative electrode, and an electrolyte provided between the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode for an air battery according to claim 1 or claim 2, and the first electrode reaction layer is provided so as to be in contact with the electrolyte.

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