JP2020026560A - Aluminum foil for battery collector and method for manufacturing the same - Google Patents

Abstract

To provide an aluminum foil for battery collector having sufficient strength and ductility even after heat treatment.SOLUTION: An aluminum alloy foil for battery collector contains Fe: 1.2 mass% or more and 1.8 mass% or less, Si:0.08 mass% or more and 0.15 mass% or less, Cu:more than 0.005 mass% and 0.01% or less, and the balance Al with inevitable impurities, and has tensile strength before heat treatment after rolling of 170 MPa or more and elongation of 5.0% or more, and has tensile strength after first heat treatment at 120°C for 1 minute of 160 MPa or more and tensile strength after second heat treatment at 200°C for 1 minute after the first heat treatment of less than 100 MPa and elongation of 8.0% or more.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an aluminum alloy foil for a battery current collector and a method for producing the same.

  In recent years, for the purpose of increasing the capacity of a lithium ion battery, it has been required to reduce the thickness of an aluminum foil or a copper foil as an electrode current collector and a separator. The aluminum foil used as the current collector of the positive electrode is thinned, so that the aluminum foil is easily broken in a battery production line. Therefore, when the thickness of the aluminum foil is reduced, high strength and high elongation are generally required to suppress breakage. There is a step in which heat is applied to the current collector during the manufacture of the battery electrode (for example, see Patent Literatures 1 to 3). In many cases, thermal drying is performed at a temperature of about 120 to 140 ° C., followed by pressing to increase the density of the active material layer, and then heat treatment at a high temperature of 180 to 200 ° C.

JP 2010-150637 A JP 2011-241410 A JP 2017-186630A

  However, since the active material applied to the current collector repeatedly expands and contracts during charge and discharge, the aluminum foil as the current collector must be soft and have high ductility to prevent peeling and breakage of the electrode. Can be However, when an aluminum foil having a low recrystallization temperature, which easily softens even at a low temperature, is used, the strength is reduced during low-temperature heat treatment in an electrode production line, which causes wrinkles and sometimes breaks. In particular, many lithium-ion battery production lines are provided with a pressing step after heat drying.At this time, a strong force is applied to the aluminum foil as a current collector. The risk of breakage is particularly high.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an aluminum foil for a battery current collector having sufficient strength and ductility even after heat treatment and a method for producing the same.

  That is, in the aluminum alloy foil for a battery current collector of the present invention, the first mode is Fe: 1.2% by mass to 1.8% by mass, and Si: 0.08% by mass to 0.15% by mass. Hereinafter, Cu: contains more than 0.005% by mass and 0.01% or less, the balance has a composition consisting of Al and inevitable impurities, and after rolling, the tensile strength is 170 MPa or more and the elongation is 5.0% or more. Yes, after the first heat treatment at 120 ° C. × 1 minute, the tensile strength is 160 MPa or more, and after the second heat treatment at 200 ° C. × 1 minute after the first heat treatment, the tensile strength is less than 100 MPa. The elongation is 8.0% or more.

  Another aspect of the invention of an aluminum alloy foil for a battery current collector is the invention of the above aspect, further comprising Mn: less than 0.01% by mass.

Another aspect of the invention of an aluminum alloy foil for a battery current collector is the invention of the above aspect, wherein the crystal grain size in the thickness direction of the foil after cold rolling is 2.0 μm or less, and the equivalent circle diameter is 1 μm or more. It is characterized in that Al-Fe based intermetallic compounds of 3 μm or less are distributed in a number of 8.0 × 10 ³ or more per 1 mm ² .

Among the methods for manufacturing an aluminum alloy foil for a battery current collector of the present invention, the first mode is a method for manufacturing the aluminum alloy foil for a battery current collector according to any of the above modes,
The ingot of the aluminum alloy having the composition described in the above form is subjected to homogenization treatment at 420 to 520 ° C for 8 hours or more, and after the homogenization treatment, the rolling finish temperature is 230 ° C or more and less than 300 ° C. Hot rolling is performed, and during the subsequent cold rolling, the intermediate annealing at 300 ° C. to 400 ° C. is performed at a thickness at which the cold rolling ratio after hot rolling becomes 60% or more, and after the intermediate annealing. The final cold rolling reduction is 95% or more.

  Hereinafter, the conditions of the components and the like defined in the present invention will be described. In addition, below, component content is shown by mass%.

-Fe: 1.2 to 1.8%
Fe is an element that can refine the crystal grains of aluminum and improve the strength and elongation of the foil. If the Fe content is less than 1.2%, the refinement of the crystal grains is insufficient and the elongation value is low. If the Fe content exceeds 1.8%, Al-Fe-based and Al-Fe-Si-based crystallization products are obtained. Are coarsened, resulting in pinholes, breakage during rolling, and reduced elongation. For this reason, the Fe content is determined within the above range. For the same reason, it is desirable to set the lower limit of the Fe content to 1.3% and the upper limit to 1.6%.

-Si: 0.08 to 0.15%
Si has a function of accelerating the precipitation of Fe. By adding Si in a certain amount or more, the recrystallization temperature of the aluminum foil is lowered, and the foil is easily softened even by a low-temperature heat treatment. When the Si content is less than 0.08%, Fe precipitation is suppressed, and the foil is hardly softened by heat treatment at 180 to 200 ° C. On the other hand, if the content exceeds 0.15%, pinholes, breakage during rolling, and a decrease in elongation occur due to coarse Al-Fe-Si crystals formed during casting. For this reason, the Si content is determined within the above range. For the same reason, it is desirable to set the lower limit of the Si content to 0.08% and the upper limit to 0.13%.

Cu: more than 0.005% by mass and 0.015% by mass or less Cu is an element capable of improving the strength of the foil and increasing the recrystallization temperature. When the content is 0.005% or less, even if it is contained, it hardly contributes to the strength improvement, and the strength of the foil may be reduced even by a low-temperature heat treatment at a temperature of 120 to 140 ° C. When the content exceeds 0.015%, the elongation after rolling is reduced, and the recrystallization temperature is increased, and the foil is hardly softened by heat treatment at 180 to 200 ° C. Determine. For the same reason, it is desirable to set the upper limit of the Cu content to 0.01%.

-Mn: contained less than 0.01% (regulation)
Mn is an element generally capable of improving the strength of a foil and also improving ductility when added to an Al-Fe alloy. However, the content is preferably restricted to a range of less than 0.01% in order to greatly raise the recrystallization temperature of aluminum even if the content is very small. If the content exceeds 0.01%, the recrystallization temperature increases, and the foil is hardly softened by heat treatment at 180 to 200 ° C.

-Tensile strength after rolling of 170 MPa or more, elongation of 5.0% or more Breaking in a battery production line can be suppressed by setting the tensile strength before heat treatment to 170 MPa or more and elongation to 5.0% or more. If the tensile strength is less than 170 MPa and the elongation is less than 5.0%, there is a concern that problems such as breakage and wrinkles may occur in the production line.

The tensile strength after the first heat treatment at 120 ° C. for 1 minute is 160 MPa or more. The heat treatment in the electrode manufacturing process is generally rapid heating, and is held for at least 1 minute. In the heat treatment at 120 ° C. corresponding to the heat drying, the material hardly recovers, and the tensile strength of the foil is 160 MPa or more, so that wrinkles and breakage in a subsequent pressing step or the like can be prevented.
If the tensile strength is less than 160 MPa or the elongation is less than 4.0% after the first heat treatment, there is a high risk of wrinkling or breaking in a pressing step or the like. Desirably, it has a tensile strength of 160 MPa or more even after the heat treatment at 140 ° C.

The tensile strength is less than 100 MPa and the elongation is 8.0% or more after the second heat treatment at 200 ° C. for 1 minute. After the heat treatment at 200 ° C. corresponding to the high-temperature heat treatment after pressing, the foil undergoes recrystallization and the tensile strength is less than 100 MPa. When the elongation is 8.0% or more, it is possible to prevent deterioration of the electrode during charging and discharging. This second heat treatment may be performed for several hours if it is long. Desirably, the tensile strength after heat treatment at 180 ° C. is desirably 100 MPa or less. Further, a heat treatment exceeding 200 ° C. is not desirable because it causes deterioration of the active material.

・ The aluminum foil having a crystal grain size of 2.0 μm or less in the thickness direction of the foil after cold rolling is so thin that it is broken as soon as local deformation occurs. Therefore, when the crystal grain size is reduced, the foil is easily deformed uniformly, and excellent elongation characteristics can be obtained. Here, the grain boundary refers to a large-angle grain boundary having a misorientation of 15 ° or more. Although sub-grain boundaries having a misorientation of 2 ° to 15 ° also contribute to the elongation characteristics, the degree is extremely small as compared with the large-angle grain boundaries. By setting the average crystal grain size in the thickness direction to 2.0 μm or less, high elongation characteristics can be obtained even with a thin aluminum alloy foil having a thickness of 15 μm or less.

-Al-Fe-based intermetallic compounds having a circle equivalent diameter of 1 μm or more and 3 μm or less are distributed in a number of 8.0 × 10 ³ or more per 1 mm ² . Grain subdivision is remarkable, and the crystal grains are refined. Further, the intermetallic compound becomes a nucleation site for recrystallization during heat treatment at 200 ° C. after the intermediate annealing or the final cold rolling, so that the recrystallized grains are refined and high elongation is obtained. A distribution of 8.0 × 10 ³ or more per 1 mm ² is preferable, but the recrystallized grain size does not change extremely after this number. If the equivalent circle diameter is less than 1 μm, it hardly becomes a nucleus during recrystallization. Conversely, an intermetallic compound exceeding 3 μm not only leads to pinholes and breaks during rolling, but also causes a decrease in elongation.

  According to the present invention, since the electrode slurry is applied during the production of the electrode, and has sufficient strength and ductility even after heat drying at a temperature of 120 to 140 ° C., it is difficult to break even in a line or in a pressing step after drying, and Since the foil is sufficiently softened by the high-temperature heat treatment at a temperature of 180 to 200 ° C., there is an effect that deterioration of the electrode due to expansion and contraction of the active material during charge and discharge can be prevented.

It is a flow figure showing an example of a manufacturing process for obtaining aluminum alloy foil for battery current collectors of one embodiment of the present invention.

  An aluminum alloy having the above composition is cast by a semi-continuous casting method or a continuous casting method, and the obtained ingot is subjected to a homogenization treatment at 420 to 520 ° C for 8 hours or more. If the temperature is kept lower than 420 ° C. or less than 8 hours, the segregation generated at the time of casting is not eliminated and Fe is not sufficiently deposited, so that it is difficult to complete the recrystallization of the foil at 200 ° C. On the other hand, if the temperature is higher than 520 ° C., the precipitation of Fe becomes insufficient, and it is also difficult to complete recrystallization of the foil at 200 ° C. The upper limit of the holding time is not particularly defined, but is preferably 24 hours or less in consideration of productivity. More preferred retention time is 10 hours or more and less than 16 hours.

  After the melting of the alloy, hot rolling is performed as shown in FIG. In the hot rolling, the finishing temperature is desirably 300 ° C. or less in order to prevent recrystallization of the material. When the temperature exceeds 300 ° C., recrystallization occurs partially, and the fiber structure and the recrystallized grain structure are mixed, and the recrystallized grain size during the intermediate annealing becomes nonuniform. Since it leads to uniformity, the elongation of the foil is reduced. If the temperature is too low, the productivity is reduced due to the occurrence of side cracks during hot rolling.

  After the hot rolling, cold rolling is performed as shown in FIG. In cold rolling, it is desirable to perform intermediate annealing. The intermediate annealing can be performed at a temperature of 300 to 400 ° C. for 3 hours or more. Intermediate annealing not only softens the hardened material by repeating cold rolling and restores the rollability, but also has the role of promoting the precipitation of Fe and controlling the recrystallization temperature of the material. In performing this intermediate annealing, it is desirable that the cold rolling reduction from after hot rolling to before annealing is 60% or more. When the cold rolling reduction is less than 60%, not only precipitation of Fe becomes insufficient, but also the recrystallized grains after annealing tend to become coarse, leading to a decrease in final elongation.

  The final cold rolling reduction after the intermediate annealing is desirably 95% or more. It is known that crystal grains of an aluminum alloy are divided and refined only by rolling (grain subdivision). The higher the rolling reduction, the finer the crystal grains become. Therefore, by setting the final cold rolling reduction at the time of cold rolling to 95% or more, higher elongation characteristics can be obtained. Further, since the recrystallization temperature of the foil decreases as the strain accumulated in the cold rolling increases, the recrystallization may not be completed by the heat treatment at 200 ° C. when the final cold rolling reduction is less than 95%. Here, the final cold rolling refers to cold rolling from the thickness at which intermediate annealing was performed during the rolling process to the final thickness.

After cold rolling, the aluminum alloy foil for a battery current collector of the present embodiment has a tensile strength of 170 MPa or more and an elongation of 5.0% or more. Moreover,
After cold rolling, 8.0 × 10 ³ Al-Fe intermetallic compounds having a crystal grain size in the thickness direction of the foil of 2.0 μm or less and an equivalent circle diameter of 1 μm or more and 3 μm or less per 1 mm ^2. It is distributed in the above numbers.
The crystal grain size and the distribution of the intermetallic compound can be controlled by the homogenization treatment, the setting of the hot finishing temperature, the setting of the intermediate annealing temperature, and the setting of the cold rolling reduction.

As shown in FIG. 1, the aluminum alloy foil for a battery current collector after cold rolling is subjected to a first heat treatment such as thermal drying at a temperature of 120 to 140 ° C. for 1 to 2 minutes after the application of the slurry. Is done. When the first heat treatment is performed at 120 ° C. × 1 minute, after the first heat treatment, in the present embodiment, the aluminum alloy foil for a battery current collector has a characteristic of a tensile strength of 160 MPa or more.
Furthermore, the aluminum alloy foil for a battery current collector of the present embodiment is subjected to press forming, and thereafter, the foil is sufficiently softened by, for example, a second heat treatment at a temperature of 180 to 200 ° C. × 1 minute to several hours. Thus, deterioration of the electrode due to expansion and contraction of the active material during charging and discharging is prevented. When the second heat treatment is performed at 200 ° C. for 1 minute, the aluminum alloy foil for a battery current collector of the present embodiment has a property that the tensile strength is less than 100 MPa and the elongation is 8.0% or more.
Due to the above characteristics, deterioration of the electrode can be prevented during heat treatment and during the heat treatment.

Hereinafter, examples of the present invention will be described.
An ingot of an aluminum alloy having each composition shown in Table 1 (remainder Al and other unavoidable impurities) was homogenized under the conditions shown in Table 1, and then hot-rolled into a 3 mm plate material. Thereafter, a sample of an aluminum alloy foil having a thickness of 15 μm and a width of 1200 mm was produced through cold rolling, intermediate annealing and final cold rolling under the conditions shown in Table 1.
In the intermediate annealing, batch annealing at 360 ° C. × 5 hours was performed.
The following property tests were performed on each test piece of aluminum foil having a thickness of 15 μm, and the results are shown in Table 2.

-Mechanical properties (tensile strength, elongation): Tensile test All were measured by a tensile test. In the tensile test, a JIS No. 5 test piece was sampled from a sample so that the elongation in the direction parallel to the rolling direction could be measured, and a universal tensile tester (AGS-X 10 kN manufactured by Shimadzu Corporation) was used at a pulling speed of 2 mm / min. The test was performed. The calculation of the elongation is as follows. First, before the test, two lines are marked in the longitudinal center of the test piece in the vertical direction of the test piece at intervals of 50 mm, which is the gauge length. After the test, the distance between the marks was measured by associating the fracture surfaces of the aluminum alloy foil, and the elongation (mm) obtained by subtracting the gauge length (50 mm) therefrom was divided by the gauge length (50 mm). (%) Was determined.

Heat treatment After the cold-rolled foil was subjected to heat treatment at 120 ° C for 1 minute and 200 ° C for 1 minute, the mechanical properties were measured by a tensile test. The heat treatment method is not particularly limited as long as the rate of temperature rise is rapid heating, and examples thereof include a method of immersing the foil in an oil bath and a method of placing the foil on a plate heated to a predetermined temperature and heating. Here, a method of immersing the foil in an oil bath for one minute was used.

-Grain size in thickness direction The RD-ND surface of the aluminum alloy foil was cut with a cross section policer (CP), and the cut surface was analyzed by the SEM-EBSD method. The entire thickness of the foil was observed at a magnification of × 2000 and at a magnification of × 3000 when actually measuring the particle size. In the obtained orientation mapping image, the crystal grain size in the thickness direction of the foil was calculated by a line segment method from a grain map indicating a grain boundary having an orientation difference of 15 ° or more. In addition, observation of × 3000 was performed in three visual fields for one sample, and the crystal grain size was an average value.

-Intermetallic compound The intermetallic compound is obtained by cutting a parallel cross section (RD-ND plane) of the foil with a cross section policer (CP) and observing it with a field emission scanning electron microscope (FE-SEM: NVVision 40 manufactured by Carl Zeiss). Was done. Regarding the “Al-Fe-based intermetallic compound having a particle diameter (equivalent circle diameter) of 1 μm or more and 3 μm or less”, five visual fields observed at a magnification of × 2000 were image-analyzed and the density was calculated.

As described above, according to the material of the present invention, high strength is maintained after the first heat treatment, and high elongation after the second heat treatment. In Example 9, the rolling reduction before annealing was low, the softening was difficult, the strength at 200 ° C. was high, and the elongation was low.
Comparative Example 10 has a low Fe content and coarsened crystal grains, while Comparative Example 11 has a high Fe content, reduced elongation, and reduced rollability.
In Comparative Example 12, the Si content was low and the softening temperature was increased. In Comparative Example 13, the Si content was high, the elongation was reduced, and the rollability was reduced.
In Comparative Example 14, the Cu content was high, the elongation was reduced, and the softening temperature was increased. In Comparative Example 15, the Cu content was low, and the strength was reduced even in the heat treatment at 120 ° C.
In Comparative Example 16, the homogenization temperature was high and did not soften at 200 ° C.
In Comparative Example 17, the hot rolling finish temperature was high and the elongation was low.
In Comparative Example 18, the rolling reduction before annealing was low, and the softening temperature increased.
In Comparative Example 19, the final cold rolling reduction was low, and the elongation was low.

  As described above, the present invention has been described based on the above embodiments and examples. However, the present invention is not limited to the contents described in the above embodiments and examples, and the present invention is not limited thereto unless it departs from the scope of the present invention. Appropriate changes to the embodiments and examples are possible.

Claims (4)

  Fe: 1.2% by mass to 1.8% by mass, Si: 0.08% by mass to 0.15% by mass, Cu: more than 0.005% by mass and 0.01% or less, with the balance being Al After rolling, the tensile strength is 170 MPa or more, the elongation is 5.0% or more, and after the first heat treatment at 120 ° C. × 1 minute, the tensile strength is 160 MPa or more, An aluminum alloy foil for a battery current collector, wherein after the second heat treatment at 200 ° C. for 1 minute after the first heat treatment, the tensile strength is less than 100 MPa and the elongation is 8.0% or more.   The aluminum alloy foil for a battery current collector according to claim 1, further comprising Mn: less than 0.01% by mass. The crystal grain size in the thickness direction of the foil after cold rolling is 2.0 μm or less, and 8.0 × 10 ³ Al-Fe intermetallic compounds having an equivalent circle diameter of 1 μm or more and 3 μm or less per 1 mm ^2. The aluminum alloy foil for a battery current collector according to claim 1 or 2, wherein the aluminum alloy foil is distributed in the above number. A method for producing an aluminum alloy foil for a battery current collector according to claim 1,
An ingot of the aluminum alloy having the composition according to claim 1 or 2 is subjected to a homogenization treatment at 420 to 520 ° C for 8 hours or more, and after the homogenization treatment, a rolling finish temperature is 230 ° C or more and 300 ° C or more. Perform hot rolling such that the temperature is lower than 0 ° C., and perform intermediate annealing at 300 ° C. to 400 ° C. at a thickness such that the cold rolling reduction after hot rolling is 60% or more during the subsequent cold rolling. A method for producing an aluminum alloy foil for a battery current collector, wherein the final cold rolling reduction after annealing is 95% or more.

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