Patent Application
20120193001
Aluminum is known to have a relatively high electrochemical capacity, and therefore, is highly attractive for use as an anode in batteries, including aluminum-air batteries, in which the aluminum reacts with oxygen from the air. However, the use of such anodes is limited due to the corrosion of the anode, which occurs mainly at open circuit voltage and at low current density by reaction of the aluminum anode (Al-anode) with the electrolyte. Such corrosion causes the consumption of the Al-anode, without the generation of electrical power, thus causing the deterioration of the battery and highly limiting the shelf life thereof.
Several attempts have been made to suppress the corrosion of the Al-anodes, including changing the metallurgical properties of the Al-anode and adding corrosion inhibitors to the electrolyte. One such attempt is that of changing the metallurgic properties of the anode by alloying the Al with other elements. However, these attempts were not very successful.
In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, modules, units and/or circuits have not been described in detail so as not to obscure the invention.
Embodiments of the invention are directed to a method for preparing an aluminum (Al) based anode. The anode according to this embodiment is prepared by smelting an alloy from aluminum and at least one alloying element, such as magnesium, so as to provide a supersaturated solid solution metastable phase, producing strips of the smelted alloy, treating the strips by solid solution heat treatment and decomposing the metastable supersaturated solid solution phase by artificial aging, plastic deformation, or both, providing a Al based anode.
According to some embodiments, an Al—Mg anode is prepared. According to these embodiments, the amount of the magnesium in the Al—Mg alloy does not exceed its maximum solubility concentration in the supersaturated solid solution metastable phase, and therefore, the amount of the magnesium in the alloy is 0.5 to 10% w/w. In some embodiments, the percentage of Mg may be 2-4% w/w.
In some embodiments, the means used for producing strips of the smelted alloy may include hot rolling, cold rolling, stamping, pressing and machining. In some embodiments, the solid solution heat treatment may include heating the strips to a temperature of 400-500° C.; maintaining this temperature for 1-5 hours; and quenching into a liquid media. In some embodiments the decomposition of the metastable supersaturated solid solution phase may be performed by rolling.
Further embodiments of the invention are directed to a an Al based anode prepared by smelting an alloy from aluminum and at least one alloying element so as to provide a supersaturated solid solution metastable phase, producing strips of the smelted alloy, treating the strips by solid solution heat treatment and decomposing the metastable supersaturated solid solution phase by artificial aging, plastic deformation, or both.
Further embodiments are directed to Al based anodes, wherein the alloying element is Mg, having a coulombic efficiency of at least 85% at a temperature of 40-50° C. Further embodiments are directed to Al based anodes, wherein the alloying element is Mg, having a coulombic efficiency of 87-91% at a temperature of 40-50° C.
Various aspects of the invention are described in greater detail in the following Examples, which represent embodiments of this invention, and are by no means to be interpreted as limiting the scope of this invention.
The results presented below are based in the statistical analysis of the results of several experiments.
1.5 kg of Al—Mg 2.5% alloy was smelted from 1.462 kg of aluminum (purity 99.99%) and 0.038 kg of magnesium (purity 99.999%) in a graphite crucible in an induction furnace under a protective atmosphere. Magnesium and other alloying elements were wrapped up in Aluminum foil and plunged into already melted Al. The melt was poured out into a steel mould of 150×15×260 size. Before casting the melt was vigorously stirred by graphite rod. The same procedure (besides the alloy's composition) was used for smelting of all the mentioned below Al base alloys.
Casting stress relief annealing was carried out at 350° C. for two hours, cooled down to room temperature and then the strips were rolled in a duo rolling mill to a thickness of 3.5 mm. This annealing procedure is optional and may be performed to reduce internal stress and to homogenize the structure. SSHT of the strips was carried out in an electric batch type furnace with circulating air. The strips were heated up to 415° C., maintained at this temperature for four hours and quenched in water to room temperature. A rolling duo mill having a roll diameter of 300 mm was used for rolling the ingots with different rates of deformation.
The test samples had a size of 30 mm diameter and 2.5 mm of thickness. Aluminum samples were machined directly from the ingots while alloy samples were machined from the strips and later subjected to solid solution heat treatment, and optionally an artificial aging process. The artificial aging process was carried out in a batch furnace at 150-200° C., depending on alloy composition, under an air atmosphere (the specific temperatures used during the artificial aging process for each alloy are presented in Table III below).
The corrosion value, coulombic efficiency and polarization tests were carried out in electrochemical half-cells in 4M KOH at 50° C. The corrosion value at OCV and coulombic efficiency in galvanostatic experiments were measured by weight loss. Here and further all the potentials were measured vs. Hg/HgO reference electrode with IR drop correction. Before each test the sample's working surface was polished by the SiC abrasive paper grit 600, followed by a fine alumina suspension AP-A polishing.
The corrosion rate at OCV for Al and Al based alloys after solid solution heat treatment is as follows:
As shown in Table I, performing solid solution heat treatment for Al—Mg alloys having an Mg content of less than 4% results in a significant decrease in the corrosion rate in comparison to pure Al, as well as other Al based alloys. It was further found that the corrosion products of Al—Mg alloys having up to 4% Mg completely dissolve in an alkaline solution, and therefore, the working (corroded) surface of these alloys is smooth and clean. In contrast, it was found that the other alloys, including the Al—Mg alloy with 6% Mg, form a porous layer of corroded product on the working surface of anode. This porous layer can notably increase the anodic polarization, as will be shown below. Additionally, the corroded products may migrate into the electrolyte to form a very fine suspension, further disrupting the efficiency of the anode.
The alloys prepared according to the procedure detailed above were additionally subjected to artificial aging. The corrosion rate .vs. the time of aging of the various alloys is shown below in Table II.
1.5 kg of an Al—Mg 3.4% alloy was prepared according to the procedure described in Example 1. The ingot of size 150×15×260 mm was rolled to the strips having thickness 4.5 mm. The solid solution heat treatment for these strips was carried out as follows: heating up to 415° C., maintaining at this temperature for 4 hours and quenching in water at room temperature. After quenching the strips were rolled from the thickness of 4 mm to 1.1-1.2 mm and then some samples (Group A) were electrochemically tested and some of them (Group B) were subjected to aging process at 150° C., before electrochemical testing.
The results show that the average corrosion rate for Group A samples was 0.33 mg/cm2·min The results of the corrosion test for samples of Group B are summarized in Table III
From comparing the results presented in Examples 2 and 3 (Group A), it can be concluded that solid solution heat treatment+plastic deformation by rolling of the Al—Mg alloy, having supersaturated solid solution structure, results in notably lower corrosion rate as compare to the solid solution heat treatment+aging (see Table II). It should be also emphasized that a plastic deformation by rolling is much less time and labor consuming compared to the low temperature, long time aging process. Further, the rolling also provides the flattening of the strips, which are deformed after the solid solution heat treatment. By comparing the results of Group A (including plastic deformation with no artificial aging) and Group B (including both plastic deformation and artificial aging) it is concluded that once plastic deformation is performed, the additional artificial aging process does not change the corrosion rate.
Polarization data for the Al—Mg alloys with Mg content 2.5-4.0% after solid solution heat treatment+aging or solid solution heat treatment+deformation do not differ markedly. However, when comparing a pure aluminum anode with an Al—Mg 2.5% anode and an Al—Si 1.2% anode (both prepared according to the procedure described in Example 2), it is shown (see Table IV) that the current density of the Al—Mg 2.5% anode is highly improved.
As shown in Table IV, the Al—Mg alloy provides markedly more negative potentials as compared to the pure Al, i.e., the presence of Mg in the crystalline structure of the Al—Mg alloys provides more negative anode potentials. The polarization of the Al—Si alloy is much higher than both the pure Al and the Al—Mg anode, which may be caused by the formation of a corroded product on the working surface of the anode.
The anode coulombic efficiency, % for pure Al (99.99), for an Al—Mg 3.4% alloy prepared using solid solution heat treatment+75% deformation and for an Al—Mg2.7%—Cr0.19%—Mn0.04% prepared without any solid solution heat treatment or deformation is as presented in Table V:
As shown in Table V, at small current densities (or at more negative potentials) the coulombic efficiency of heat treated alloy is notably higher than for commercial Al of high purity. This may be explained by the heat treated alloy having a much lower level of parasitic corrosion.
The measurement of the coulombic efficiency was carried out in a single cell having technical parameters as follows:
Air electrode size: 7.5×7.3 cm
Number of air electrodes: 2
Numbers of anodes: 1
Anode thickness: 0.24 cm
Anode working area 52.2 cm2 (7.35×7.1 cm)
Distance between air electrodes: 0.7 cm
Total volume of circulating electrolyte: 300 ml
The anode material was Al—Mg2.5% alloy after Solid Solution Heat Treatment and 65% deformation by rolling.
The parameters and the test results are as follows:
Discharge current: 10.5 A
Discharge current density: 100 mA/cm2
Discharge average voltage: 1.3V
Time of discharge: 1.1 h
Temperature of electrolyte: 41-43° C.
Total discharge capacity: 11.5 Ah
The anode weight loss during the test was 4.324 g which corresponds to the capacity of:
4.32 g×2.98 Ah/g=12.88 Ah.
The coulombic efficiency is calculated as:
[1−(12.88 Ah−11.5 Ah):12.88 Ah]×100=89.3%.
As shown from the test results there is a very good correlation between the coulombic efficiency of the Al—Mg anode in the half cell test (88% in table 5) and in the real Al-Air cell (89.3%—taking into account the difference in the temperature of electrolyte: 50° C. in a half cell vs. 43° C. in Al-Air cell).
While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
