JP2011249150A - Aluminum foil for power storage device collector and power storage device collector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an aluminum foil for a power storage device connector in which a through-hole can be easily formed by etching and stability with respect to fluorine is improved.SOLUTION: An aluminum foil for a power storage device collector has a composition where Ni is contained for 20 to 300 ppm in mass% and the remaining part is comprised of Al in 99% or more and inevitable impurities, and has a thickness of less than 50 μm. In the aluminum foil for power storage device collector, a through-hole is easily formed by etching by producing an Al-Ni sludge in a base, and stability with respect to fluorine is improved by making the concentration of Al 99% or more. It is preferable for the aluminum foil that a ratio of crystal grains through a cross section in a thickness direction is 10% or more and that the through-hole having a diameter of 0.5 to 500 μm is present in an area ratio of 5 to 50%.

Description

  The present invention relates to an aluminum foil for a current collector used in an electricity storage device using Li ions.

  An electricity storage device such as a secondary battery is basically composed of a positive electrode, a negative electrode, a separator that insulates the positive electrode from the negative electrode, and an electrolyte solution that allows ions to move between the positive electrode and the negative electrode. . The positive electrode and the negative electrode are obtained by applying various active materials to the surface of a current collector made of a metal foil. For example, in a lithium secondary battery, a positive electrode in which an active material containing lithium cobaltate or the like is applied to a current collector made of an aluminum foil is used, while a non-graphitizable carbon or the like is used as a negative electrode. A material obtained by applying an active material containing copper to a current collector made of copper foil is used. And as electrolyte solution, what melt | dissolved the lithium compound as a solute in the nonaqueous solvent is used.

An electricity storage device using Li ions has been proposed in which a metal lithium foil and a negative electrode are brought into electrochemical contact in order to improve energy density. By bringing the metal lithium foil and the negative electrode into contact, it becomes possible to dope lithium ions into the negative electrode. It is important to form a through hole having a diameter of 0.5 to 500 μm for the positive electrode current collector and the negative electrode current collector in order to smoothly carry out doping of lithium ions to the stacked electrodes (for example, (See Patent Document 1).
In order to form a through hole in the current collector, an etching method is usually applied. In addition, a method has been proposed in which a thin metal plate is rolled to obtain a comb-like perforated current collector (Patent Document 2).

Japanese Patent No. 3485935 JP-A-11-97035

However, in order to create a through hole by an etching method, it is necessary to pass a direct current or an alternating current in an acid solution. In the case of direct current, power is supplied to the aluminum foil, so that a sufficient foil thickness is required. If the foil thickness is less than 50 μm, wrinkles, cutting, etc. occur due to heat generated by the power supply, making production difficult. In the case of an alternating current, an alternating current is applied to the electrodes, and the aluminum foil is transported between them, so that no stress due to power feeding occurs. However, in the cathode state, the aluminum foil is oxidized to form a film, so that there is a problem that it is difficult to form through holes.
In addition, in the case of an electricity storage device using Li ions, when an electrolytic solution to which a fluorine-based additive fluoroethylene carbonate is added for the purpose of suppressing a reduction in storage capacity is used, the current collector has stability against fluorine. Desired.
An object of the present invention is to provide an aluminum foil for an electricity storage device current collector that is easy to form a through hole by etching and is excellent in stability against fluorine.

In the present invention, Ni is contained in order to promote the formation of through holes. Ni together with Al produces Al—Ni-based precipitates in the matrix. Since the Al—Ni-based precipitate is more noble than the base made of Al, the Al—Ni-based precipitate forms a local battery with the base, and thus acts as a discharge site for electric charges. . As a result, the solubility of Al constituting the base can be improved, and uniform through holes can be formed by etching with an acid.
Next, when an electrolytic solution to which a fluorine-based additive (for example, fluoroethylene carbonate) is added is used, fluorine in the additive reacts with Al constituting the current collector, and the current is collected on the surface of the current collector. A film made of aluminum fluoride (AlF ₃ ) is formed. This aluminum fluoride film improves the stability of the current collector with respect to fluorine. However, it has been found that the presence of Al-Fe-based, Al-Ni-based precipitates in the current collector (aluminum foil) causes defects in the aluminum fluoride film and decreases the stability of the current collector to fluorine. did.
The aluminum foil for electricity storage device current collector of the present invention based on the above has a composition comprising 20 to 300 ppm of Ni in a mass ratio (hereinafter the same), with the balance being 99% or more of Al and inevitable impurities. The thickness is less than 50 μm.
In the aluminum foil for a power storage device current collector of the present invention, the ratio of crystal grains penetrating the cross section in the thickness direction is preferably 10% or more, and the through hole having a diameter of 0.5 to 500 μm is 5 to 50. An aluminum foil present in an area ratio of% is preferable as the electricity storage device current collector.

  According to the present invention, it is easy to form through-holes by etching by generating Al-Ni-based precipitates in the matrix, and stability to fluorine can be achieved by making the Al concentration 99% or more. An excellent aluminum foil for an electricity storage device current collector is provided.

An example of the current-potential curve obtained in the example is shown.

Hereinafter, the present invention will be described more specifically.
[Chemical composition]
<Ni: 20 to 300 ppm>
In the present invention, Ni is contained in order to promote formation of a through hole by etching with an acid. However, when the amount of Ni is less than 20 ppm, the amount of Al—Ni-based precipitates is insufficient, the solubility is lowered, and it is difficult to promote the formation of uniform through holes.
On the other hand, when the amount of Ni exceeds 300 ppm, the amount of generated Al—Ni-based precipitates increases. If this Al—Ni-based precipitate is exposed on the surface of the aluminum foil, that portion becomes a defect in which an aluminum fluoride film is not formed, which causes a decrease in the stability of the current collector to fluorine.
A desirable amount of Ni is 50 to 200 ppm, and a more desirable amount of Ni is 50 to 100 ppm.

<Al: 99% or more>
Al constitutes a base, but as described above, for example, Al—Fe-based precipitates formed by being combined with Fe, as described above, improve the stability of the current collector against fluorine. This causes a defect in the aluminum fluoride film. Therefore, in order to reduce especially Fe contained as impurities, the present invention makes the Al content (purity) 99% or more. The desirable Al content is 99.5% or more, and the more desirable Al content is 99.7% or more.

<Other impurity elements>
In the aluminum foil of the present invention, elements other than Ni and Al, such as Si, Fe, Cu, Mn, Mg, Zn and Ti, are regulated as impurities. These elements treated as impurities are usually Si ≦ 0.25%, Fe ≦ 0.4%, Cu ≦ 0.05%, assuming that Al is 99% or more and Ni is 20 to 300 ppm. Mn ≦ 0.05%, Mg ≦ 0.05%, Zn ≦ 0.05%, and Ti ≦ 0.03% (all by mass%) are regulated. In this, it is desirable to regulate the amount of Fe that becomes an Al—Fe-based precipitate and impairs the soundness of the aluminum fluoride film to 0.2% or less, more preferably 0.1% or less.

[Degree of penetration of crystal grains]
The aluminum foil of the present invention forms through holes by etching using an acid solution. In order to grow through holes during etching, it is desirable that there are few grain boundaries present in the aluminum foil. This is because the etching pits tend to stop growing at the grain boundaries. Therefore, it is ideal that the aluminum foil of the present invention has more crystal grains grown to the extent that it penetrates the thickness direction of the foil.
However, it is not easy to specify the length of crystal grains in the thickness direction of the aluminum foil. Therefore, the present invention estimates that the crystal grain penetrates in the thickness direction of the foil by observing the size of the crystal grain on the surface of the aluminum foil. That is, if the minor axis (Ds ((μm))) of the crystal grain measured by observing the surface of the aluminum foil with a microscope is equal to or greater than the thickness (tμm) of the aluminum foil, the crystal grain is in the thickness direction of the foil. For example, if the thickness of the aluminum foil is 50 μm, if Ds is 50 μm or more, the crystal grains are considered to penetrate the thickness direction of the foil.
In the present invention, the area ratio (degree of penetration) of crystal grains satisfying Ds> t is preferably 10% or more, more preferably 20% or more, and further preferably 30% or more.

[Thickness of aluminum foil]
The aluminum foil according to the present invention desirably has a thickness of less than 50 μm. This is because when the thickness is 50 μm or more, when the current collector is stacked or wound, the surface area per volume becomes small, and the capacity of the electricity storage device becomes small. However, if the foil is too thin, the strength is lost, and defects such as cutting in lamination or winding processing frequently occur. A more desirable foil thickness is 10 to 30 μm, and even more desirably 15 to 25 μm.

[Through hole]
When the aluminum foil of the present invention is used as a current collector, a through hole is formed by etching.
It is desirable that the through holes having a diameter (hereinafter simply referred to as a diameter) in the range of 0.5 to 500 μm are formed with an area ratio of 5 to 50%.
In order to grow a hole until it becomes a through hole, a certain size diameter is required. When the diameter is less than 0.5 μm, holes are generated, but the rate of growth into through holes is reduced. Therefore, extra etching is required, and the production efficiency is lowered. Further, when the diameter exceeds 500 μm, the strength is significantly lowered, and problems such as cutting in lamination or winding processing frequently occur. A more preferable diameter of the through hole is 1.0 to 200 μm, and a more preferable diameter of the through hole is 3.0 to 50 μm.
The through hole having the above diameter is desirably formed with an area ratio of 5 to 50%. If it is less than 5%, it is difficult to achieve the purpose of smoothly performing doping of lithium ions due to insufficient through holes, and if it exceeds 50%, the strength as a current collector is reduced. A more desirable through-hole area ratio is 10 to 40%, and a more desirable through-hole area ratio is 20 to 30%. In addition, the area ratio of the through-hole in this invention is a ratio of the sum total of the opening area of a through-hole with a diameter of 0.5-500 micrometers which occupies for the surface area which functions as a collector. The same applies to the following embodiments.

[Production method]
The aluminum foil of the present invention can be produced according to a conventional method. That is, an aluminum ingot having the above-described composition is produced, subjected to homogenization treatment, and then subjected to hot rolling and cold rolling to obtain an aluminum foil having a desired thickness. In this case, the homogenization can be performed by heating the ingot before the hot rolling. Moreover, the aluminum foil of desired thickness can also be obtained through continuous casting rolling and cold rolling. Annealing (intermediate annealing) can be performed in the middle of cold rolling, and annealing (final annealing) can be performed after the end of cold rolling.

<Generation of Al-Ni-based precipitate>
In the present invention, in order to promote the formation of through-holes, it is important that Al—Ni-based precipitates are generated based on the base, and particularly present in the surface layer portion of the foil. Therefore, it is desirable to concentrate Ni in the surface layer part of foil. Concentration of Ni to the surface layer part can be sufficiently realized by annealing shown below.

<Crystal grain penetration>
In addition, as described above, in the present invention, it is desirable that there are many crystal grains penetrating in the thickness direction of the foil. In order to realize this, it is important to sufficiently promote the growth of recrystallized grains by annealing.
In the case of aluminum having a composition targeted by the present invention, recrystallization starts from a temperature of 200 to 250 ° C., and the higher the annealing temperature, the more crystal grains that penetrate the thickness direction of the foil. However, when the annealing temperature is high, there is a high possibility that blocking occurs where the foils adhere to each other. In particular, in the case of an aluminum foil having a thickness of less than 50 μm, which is the subject of the present invention, when the final annealing is performed at a temperature exceeding 500 ° C., the wound aluminum foil is blocked, which hinders the subsequent manufacturing process. There is a high risk of inviting. On the other hand, when the annealing temperature is low, the proportion of crystal grains penetrating through the thickness direction of the foil decreases. As described above, in producing the aluminum foil of the present invention, it is desirable to select the annealing temperature from a range of more than 300 ° C. and less than 500 ° C. By adopting this annealing temperature, Ni can also be concentrated in the surface layer of the foil. A more desirable annealing temperature is 350 to 450 ° C.
The annealing holding time is set according to the thickness of the aluminum foil to be annealed in addition to the temperature, and can be selected from a range of approximately 1 to 10 hours.

  In order to have many crystal grains penetrating through the thickness direction of the foil, in addition to specifying the annealing temperature, the thickness of the aluminum foil at the time of annealing is important. That is, even if annealing is performed under the same conditions (temperature, time), the amount of crystal grains penetrating through the thickness direction of the foil is different if the thickness of the aluminum foil is different. For example, even if an aluminum foil having a thickness of 200 μm is annealed at 400 ° C. for 4 hours, recrystallized grains cannot be grown until it penetrates the thickness direction of the foil. The aluminum foil for the current collector targeted by the present invention has a thickness of less than 50 μm, and even if it is subjected to intermediate annealing in a thick state as described above, it is recrystallized until it penetrates the thickness direction of the foil. Since the grains cannot be grown, this point needs to be taken into consideration when performing the intermediate annealing. In the present invention, in order to penetrate the crystal grains in the thickness direction of the aluminum foil most efficiently in the present invention, after the cold rolling is finished so as to have a thickness of less than 50 μm, preferably 300 to 500 ° C., more preferably 350 to The final annealing is performed at 450 ° C.

[Example]
Aluminum ingots having the components shown in Table 1 were rolled (hot and cold) according to a conventional method to obtain an aluminum foil having a thickness of 20 μm. A sample was obtained by subjecting this aluminum foil to annealing (final annealing) under the conditions shown in Table 1 in an N ₂ atmosphere. However, no. No. 24 was subjected to final annealing shown in Table 1 with a final foil thickness of 100 μm by cold rolling. For No. 25, the final annealing shown in Table 1 was performed with a final foil thickness of 200 μm by cold rolling.
These samples were evaluated for the degree of crystal grains penetrating through the thickness direction of the foil, the degree of occurrence of through holes, and the stability against fluorine. The results are also shown in Table 1.

The degree of occurrence of the through hole was determined by immersing the sample in a mixed acid solution of 1 mol hydrochloric acid + 3 mol sulfuric acid at 60 ° C. for 60 seconds to form a through hole. The area ratio of through-holes having a diameter in the range of 0.5 to 500 μm was calculated from the obtained image and evaluated. Most of the observed through holes had a diameter in the range of 0.5 to 5 μm.
Regarding the degree of penetration of crystal grains, the surface of the sample was subjected to constant voltage electrolysis at 40 V in a solution of ethanol perchlorate (20 vol% perchloric acid + 80 vol% ethanol) for 10 seconds, and then 40 V in a 5 vol% -hydrofluoric acid solution. Was conducted for 60 seconds, and the surface of the sample was observed with an SEM. A crystal grain having a minor axis of the observed crystal grain having a foil thickness (50 μm) or more was determined to be a through grain, and an area ratio of a crystal grain having a minor axis ≧ 50 μm was calculated by image analysis.

For the stability against fluorine, the current value generated during polarization measurement was used as an index for evaluation of leakage current. That is, after generating the through-hole according to the above, the sample was immersed in a 5 vol% hydrofluoric acid solution at 40 ° C. for 4 hours, washed with water and dried, and the leakage current of the sample in which aluminum fluoride was generated on the surface was measured. It was measured. When an aluminum fluoride film having high stability and excellent uniformity is formed, the leakage current is reduced.
For leakage current, a special grade reagent was used, a solution of boric acid 40 g / l (liter) + tetraboric acid 20 g / l (liter) was prepared, the sample was immersed at room temperature, and an electrochemical measurement system (Hokuto Denko ( HZ-3000) was used, and it was changed from the natural potential at 750 mV / min, and the current value generated at that time was measured. An example of the obtained current-potential curve is shown in FIG. 1. The current at which a current of 10 μA / cm ² or more is generated is the film breaking current, and the current value at the time when the value suddenly increases is leaked. The current was used.

From Table 1, the aluminum foil according to the example does not have an extremely large diameter through-hole, and the leakage current of the aluminum fluoride film formed in the hydrofluoric acid solution is sufficiently small and stable against fluorine. It turns out that it is excellent in property.
On the other hand, when Ni is not contained or when the content of Ni is lower than that of the present invention, the solubility by etching is low and the amount of through holes formed is small (Comparative Examples No. 20 and 22). If the purity of aluminum is lowered to improve the solubility, a through hole is formed, but the leakage current is increased and the stability against hydrofluoric acid is lowered (Comparative Example No. 21). When the Ni content is increased, a sufficient amount of through-holes can be formed, but the leakage current increases and the stability against hydrofluoric acid decreases (Comparative Example No. 23). On the other hand, if the foil is too thick, through grains cannot be formed (Comparative Examples No. 24 and 25).

Claims (3)

In mass ratio,
Containing 20 to 300 ppm of Ni,
The balance is composed of 99% or more of Al and inevitable impurities,
An aluminum foil for a power storage device current collector, wherein the thickness is less than 50 μm. The proportion of crystal grains penetrating the cross section in the thickness direction is 10% or more.
The aluminum foil for electrical storage device electrical power collectors of Claim 1. A through hole having a diameter of 0.5 to 500 μm exists at an area ratio of 5 to 50%.
The electrical storage device electrical power collector using the aluminum foil of Claim 1 or 2.

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