Negative electrode material for aluminum-air battery

Abstract

To provide a negative electrode material for an aluminum-air battery that exhibits uniform overall dissolution and stabilizes at a low potential in a chloride aqueous solution with a pH of 5-9.SOLUTION: A negative electrode material comprises an aluminum alloy containing 0.3-5 wt.% of Sn, the balance being Al and unavoidable impurities. The aluminum alloy has an Al matrix phase and multiple Sn-enriched regions, the Sn-enriched regions containing 0.5 wt.% or more of Sn, the distance between adjacent Sn-enriched regions being an average of 50 μm or less, the area ratio of the Al matrix phase being 50% or less on a surface or cross section of the aluminum alloy, and an average crystal grain size constituting the Al matrix phase being 5 μm or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a negative electrode material for an aluminum-air battery. [Background technology]

[0002] Aluminum-air batteries utilize the dissolution reaction of aluminum at the negative electrode and the reduction reaction of oxygen at the positive electrode. Aluminum-air batteries have the advantage that they do not need to store positive electrode active material inside the battery because they use oxygen from the air as the positive electrode active material. This allows the majority of the battery to be composed of aluminum or aluminum alloy, which is the negative electrode active material, making it possible to make aluminum-air batteries lightweight and compact.

[0003] Conventionally, basic aqueous solutions such as NaOH and KOH solutions, or chloride solutions with a pH of 5 to 9 such as NaCl and KCl solutions, have been used as electrolytes in aluminum-air batteries. The use of a basic aqueous solution as the electrolyte for an aluminum-air battery increases the battery's discharge voltage. However, there is a problem in that the Al alloy is prone to self-corrosion (self-discharge) in a basic aqueous solution. When self-corrosion of the Al alloy occurs, the negative electrode material is consumed through processes other than the battery reaction, reducing the current obtained per unit weight of the negative electrode material and shortening the battery's lifespan.

[0004] On the other hand, when a chloride aqueous solution with a pH of 5 to 9 is used as the electrolyte for an aluminum-air battery, the self-corrosion of an Al alloy in the chloride aqueous solution is less severe than in a basic aqueous solution. However, in an aqueous solution with a pH of 5 to 9, an oxide film forms on the surface of the Al, which increases the potential of the negative electrode and reduces the discharge voltage of the battery. Therefore, the addition of alloying elements has been investigated to lower the potential of the Al alloy negative electrode. Specifically, alloys consisting of Al, Mg, Sn, and Mn (see, for example, Patent Document 1) and alloys consisting of Al, Si, Fe, Sn, and Ga (see, for example, Patent Document 2) have been disclosed. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 6-179936 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-204712 Summary of the Invention [Problem to be solved by the invention]

[0006] When an aluminum-air battery uses a chloride aqueous solution with a pH of 5 to 9 as its electrolyte, the dissolution reaction of the aluminum alloy in the chloride aqueous solution proceeds locally, concentrating on a small portion of the aluminum alloy surface. Localized dissolution of the aluminum alloy results in uneven current distribution at the negative electrode. This leads to issues such as unstable potential at the negative electrode and therefore unstable discharge voltage of the battery. Therefore, there is a need for an aluminum alloy negative electrode material that exhibits uniform, full-surface dissolution even in a chloride aqueous solution with a pH of 5 to 9.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a negative electrode material for an aluminum-air battery that exhibits a uniform, full-surface dissolution in an aqueous chloride solution having a pH of 5 to 9 and has a stable potential at a low value. [Means for solving the problem]

[0008] The present inventors have conducted various experimental studies to overcome the limitations of the prior art and solve the unresolved problems described above, and have completed the present invention. They have discovered that by adding Sn to an Al alloy to form a Sn-enriched region, controlling the morphology and area ratio of the Al matrix around the Sn-enriched region, and refining the crystal grain size of the Al alloy, the dissolution morphology of the Al alloy in a chloride aqueous solution becomes uniform and the potential is stabilized at a low value.

[0009] That is, the aluminum-air battery negative electrode material according to the present invention is made of an aluminum alloy containing 0.3 to 5 wt% Sn, with the remainder being Al and unavoidable impurities, the aluminum alloy having an Al matrix and a plurality of Sn-enriched regions, each of which contains 0.5 wt% or more of Sn, the distance between adjacent Sn-enriched regions being 50 μm or less on average, the area ratio of the Al matrix on the surface or cross section of the aluminum alloy being 50% or less, and the average crystal grain size of the crystal grains constituting the Al matrix being 5 μm or less.

[0010] In the aluminum-air battery negative electrode material according to the present invention, the Sn-enriched region is preferably a phase that exists in the form of islands on the surface or cross section of the aluminum alloy. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide an anode material for an aluminum-air battery that exhibits a uniform, full-surface dissolution morphology and is stable at a low potential in a chloride aqueous solution having a pH of 5 to 9. Therefore, according to the present invention, it is possible to increase and stabilize the discharge voltage of an aluminum-air battery that uses a chloride aqueous solution that is less likely to cause self-corrosion of the anode material. [Brief explanation of the drawings]

[0012] [Figure 1] 1A shows a scanning electron microscope (SEM) image and an elemental map of Sn by energy dispersive X-ray spectroscopy (EDS) of (a) an aluminum-air battery negative electrode material according to an embodiment of the present invention, and (b) a comparative example (Comparative Example 4) in which the area ratio of the Al matrix is ​​large. [Figure 2] 1A is an inverse pole figure orientation map by electron backscatter diffraction (EBSD) of (a) an embodiment of the aluminum-air battery negative electrode material of the present invention, and (b) a comparative example (Comparative Example 4) in which the area ratio of the Al matrix is ​​large. [Figure 3]1 is a graph showing the change in potential over time when discharged at a current density of 1 A / m2 in a constant current anode polarization test for aluminum-air battery negative electrode materials according to embodiments of the present invention, comparative examples not containing Sn (Comparative Examples 1 and 2), and a comparative example having a large area ratio of the Al parent phase (Comparative Example 4). [Figure 4] FIG. 1 shows optical microscope photographs of the electrode surfaces of (a) an aluminum-air battery negative electrode material according to an embodiment of the present invention and (b) a comparative example (Comparative Example 4) in which the area ratio of the Al parent phase is large, after discharging at a current density of 1 A / m for 24 hours in a constant current anodic polarization test. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described based on examples. The aluminum-air battery negative electrode material according to an embodiment of the present invention is made of an aluminum alloy containing, by mass%, 0.3 to 5% Sn, with the remainder being Al and unavoidable impurities. Sn exists in the Al alloy as an Sn-enriched region. Sn destroys the oxide film of the Al matrix around the Sn-enriched region, promoting uniform dissolution of Al, thereby lowering the potential of the Al alloy. If the amount of Sn added is less than 0.3% by mass, the effect of lowering the potential of the Al alloy cannot be fully achieved. However, if too much Sn is added, self-corrosion of the Al alloy becomes more likely. For this reason, it is necessary to limit the Sn content to 0.3 to 5% by mass.

[0014] The Sn-enriched region in the present invention is a region containing 0.5% or more Sn by mass when the relative concentrations of Al, Sn, and O are quantitatively analyzed using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray analyzer (EDS). The present invention does not limit the crystal structure of the Sn-enriched region, and the region may be β-Sn having a body-centered tetragonal crystal structure or face-centered cubic Al in which Sn is supersaturated in solid solution.

[0015] The Al matrix phase of the present invention is a region in which the Sn concentration is less than 0.5% by mass when the composition is analyzed using an SEM equipped with EDS.

[0016] In the aluminum-air battery negative electrode material according to the embodiment of the present invention, the distance between adjacent Sn-enriched regions is 50 μm or less on average to ensure a uniform dissolution morphology. If the distance between adjacent Sn-enriched regions exceeds 50 μm on average, the influence of the Sn-enriched regions is weakened at positions away from the Sn-enriched regions, resulting in a non-uniform dissolution morphology of the Al matrix. Furthermore, the distance between adjacent Sn-enriched regions is preferably 20 μm or less on average, and more preferably 3 μm or less on average. It is preferable that the maximum distance between adjacent Sn-enriched regions does not exceed 70 μm.

[0017] Furthermore, in the aluminum-air battery anode material according to the embodiment of the present invention, the area ratio of the Al matrix is ​​50% or less in order to achieve a uniform dissolution morphology. If the area ratio of the Al matrix exceeds 50%, the area of ​​the Sn-enriched region decreases relatively, resulting in a wide region where uniform dissolution of the Al matrix is ​​not sufficiently promoted, which is not preferable as an air battery anode material.

[0018] Furthermore, in the aluminum-air battery anode material according to the embodiment of the present invention, the average crystal grain size of the Al matrix is ​​5 μm or less to stabilize the potential. If the average crystal grain size exceeds 5 μm, the potential of the Al alloy will be unstable and will fluctuate significantly. This is because the Al dissolution rate varies depending on the crystal grain orientation, and the non-uniformity of the dissolution rate of each crystal grain will be reflected in the potential fluctuation. By reducing the average crystal grain size to 5 μm or less, the potential fluctuation due to the dependence of the Al dissolution rate on the plane orientation will no longer be observed.

[0019] Hereinafter, the present invention will be described in detail based on examples, but the present invention is not limited to the description of the examples. [Example]

[0020] Aluminum-air battery negative electrode materials with the Sn contents (mass%) shown in Table 1 were prepared. In preparing Samples 1 to 3, the raw material Al powder and Sn powder were weighed to achieve the Sn contents shown in Table 1, and the raw material powders were mixed and pulverized using a planetary ball mill. The raw material powders used had a purity of 99.9% by mass or higher and a particle size of 200 μm or less. A stainless steel milling container was charged with the raw material powder, 1% ethanol (mass% relative to the amount of raw material powder filled), and stainless steel balls, and ball milling was performed in an Ar gas atmosphere inside the container. The mass ratio of raw material powder to balls was 1 / 5. The revolution speed in the planetary ball milling method was 500 rpm, and the processing time was 10 hours. The ball-milled powder was removed from the milling container, filled into a graphite sintering mold, and then subjected to spark plasma sintering while applying a pressure of 40 MPa. The heating temperature during sintering was 500°C, and the heating time was 15 minutes. The heating temperature is not the temperature of the powder during sintering, but the temperature measured by a thermocouple embedded in the graphite sintering mold. The distance between the thermocouple temperature measuring point and the powder being sintered is approximately 5 mm via the graphite sintering mold.

[0021] [Table 1]

[0022] Comparative Example 1 is an example produced by spark plasma sintering Al powder without mixing Sn powder or ball milling. The conditions for sintering, such as pressure, heating temperature, and heating time, were the same as those for Samples 1 to 3.

[0023] Comparative Example 2 is an example produced by ball milling Al powder without mixing Sn powder using a planetary ball mill, followed by spark plasma sintering. The ball milling and sintering conditions were the same as those for Samples 1 to 3.

[0024] Comparative Example 3 is an example produced by weighing out Sn powder so that the content of Sn powder was 0.2% by mass relative to Al powder, ball milling using a planetary ball mill, and then spark plasma sintering. The ball milling and sintering conditions were the same as those for Samples 1 to 3.

[0025] In Comparative Example 4, Sn powder was weighed out to a content of 1.0% by mass relative to Al powder, and the mixture was placed in a vinyl bag and shaken at various angles for about 30 seconds to mix. After that, the mixture was subjected to spark plasma sintering without ball milling. The sintering conditions were the same as those for Samples 1 to 3.

[0026] In Comparative Example 5, only Al powder was subjected to ball milling using a planetary ball mill. The ball milling conditions were the same as those for Samples 1 to 3. Sn powder was weighed out to a content of 1.0% by mass, and placed in a vinyl bag together with the milled Al powder, which was then mixed by shaking at various angles for about 30 seconds, followed by spark plasma sintering. The sintering conditions were the same as those for Samples 1 to 3.

[0027] The surface of each aluminum-air battery anode material was wet-polished with 4000-grit SiC paper. The polished surface of the test piece was observed using an SEM equipped with EDS, and the morphology and distribution of the Sn-enriched regions and the Al matrix were analyzed. The area ratio of the Al matrix was calculated for a 500 μm × 500 μm field of view. Figure 1 shows the observation results for Sample 1 and Comparative Example 4 among the test pieces. Table 1 also shows the average distance and area ratio between adjacent Sn-enriched regions for each test piece. As shown in Figure 1(a), Sn is distributed in an island-like pattern, and it can be seen that the Sn-enriched regions are a phase existing in an island-like pattern.

[0028] To measure the average crystal grain size of each test specimen, the electropolished test specimens were observed using an SEM equipped with an electron backscatter diffraction (EBSD) measurement device, and the crystal grain size was analyzed by defining the boundary where the misorientation between adjacent crystals was 15° or more as the grain boundary. Figure 2 shows the inverse pole figure orientation maps of the test specimens of Sample 1 and Comparative Example 4. Table 1 also shows the average crystal grain size of each test specimen.

[0029] To measure the potential of each test piece, a 5 mm × 5 mm area on the test piece was wet-polished with 4000-grit SiC paper and used as the electrode surface. The periphery of the electrode surface was then covered with epoxy resin, and a constant current anodic polarization test was performed in a non-degassed 2 mol / L NaCl aqueous solution. The pH of the solution was 7.0, and the temperature was 25°C. The current density was 1 A / m 2 The potential was measured over time while the specimen was held at 40000 mV, and the change in potential was measured. The standard for displaying the potential was a standard hydrogen electrode (SHE). Figure 3 shows the measurement results for the specimens of Sample 1, Comparative Example 1, Comparative Example 2, and Comparative Example 4. Table 1 also shows the potential value (the "Potential" column in Table 1) for each specimen 40,000 seconds after the start of polarization, and the magnitude of the potential oscillation. The potential stability was evaluated by rating potential oscillations within 30 mV as "small" and those exceeding 30 mV as "large."

[0030] To analyze the dissolution morphology of each test piece, a constant current anodic polarization test was performed at a current density of 1 A / m 2The test was performed for 24 hours at 1000 K, and the differences in dissolution patterns were compared when the amount of dissolution was equal for each test piece. After the test, the coating material was removed, and the test pieces were immersed in an aqueous solution containing dissolved H3PO4 and CrO3 to remove the corrosion products. The test piece surfaces after the corrosion products were removed were observed using an optical microscope. Figure 4 shows the surface observation results for Sample 1 and Comparative Example 4 of each test piece. The area surrounded by the dotted line in the image is the electrode surface, and the area outside the dotted line is the area coated with resin. The area ratio of the dissolved area was calculated as the proportion of the dissolved area on the electrode surface. The maximum depth of the dissolved area was also measured using an optical microscope. The maximum depth of the dissolved area was calculated as the distance from the polished surface to the deepest point of the dissolved area. Table 1 shows the area ratio of the dissolved area and the maximum depth of the dissolved area for each test piece.

[0031] As shown in Table 1, the test pieces of Samples 1 to 3 have lower potentials and smaller potential fluctuations than those of Comparative Examples 1 to 5. Furthermore, it is clear that the area ratio of the dissolved region increases significantly and the maximum depth of the dissolved region decreases. In other words, it is clear that the Al dissolution reaction did not concentrate on a part of the test piece surface, but proceeded uniformly over the entire test piece surface.

[0032] Comparative Example 2 in Table 1 was produced without adding Sn in order to analyze the effect of refining the average crystal grain size alone. By reducing the average crystal grain size to 5 μm or less, potential fluctuations were suppressed. However, because Sn was not added, the potential was high and Al dissolution was localized.

[0033] Comparative Example 3 in Table 1 is an example in which the Sn content is lower than the range of the aluminum-air battery negative electrode material according to the embodiment of the present invention. The potential is lower than that of Comparative Example 2, but is higher than that of Samples 1 to 3, and the dissolution form is not uniform.

[0034] Comparative Examples 4 and 5 in Table 1 contain 1.0% Sn by mass, but because Sn is unevenly distributed in only a small portion of the test piece, the area ratio of the Sn-enriched regions is small and the area ratio of the Al matrix is ​​large. It can be seen that although the potential is low, the dissolution morphology is not uniform. Comparative Example 5 has a refined average crystal grain size of the Al matrix, and although the potential fluctuation is small, the dissolution morphology is not uniform. Therefore, it can be seen that it is important to distribute the Sn-enriched regions over a wide area of ​​the test piece and reduce the distance between adjacent Sn-enriched regions. [Industrial Applicability]

[0035] The negative electrode material for an aluminum-air battery according to the present invention can stabilize the discharge voltage of an aluminum-air battery using an aqueous chloride solution at a high value, and is of great industrial benefit.

Claims

1. An aluminum alloy containing 0.3 to 5 wt% Sn, with the remainder being Al and unavoidable impurities; The aluminum alloy has an Al matrix and a plurality of Sn-enriched regions, each containing 0.5 wt% or more of Sn, the distance between adjacent Sn-enriched regions is 50 μm or less on average, the area ratio of the Al matrix is ​​50% or less on the surface or cross section of the aluminum alloy, and the average crystal grain size of the Al matrix is ​​5 μm or less. Anode material for aluminum-air batteries.

2. 2. The aluminum-air battery negative electrode material according to claim 1, wherein the Sn-enriched region is a phase present in the form of islands on the surface or cross section of the aluminum alloy.

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