JP2020143313A - Secondary battery negative electrode current collector rolled copper foil, secondary battery negative electrode current collector and secondary battery using the same, and method for producing secondary battery negative electrode current collector rolled copper foil - Google Patents

Abstract

To provide a secondary battery negative electrode current collector rolled copper foil, that can satisfactorily suppress plastic deformation and breakage of copper foil due to generation of stress accompanying volume change of active material.SOLUTION: A secondary battery negative electrode current collector rolled copper foil, containing Sn by 0.2 to 2.0 mass%, and having a tensile strength of 650 MPa or more and a breakage elongation of 1.0% or more.SELECTED DRAWING: Figure 1

Description

The present invention relates to a rolled copper foil for a secondary battery negative electrode current collector, a method for manufacturing a secondary battery negative electrode current collector and a secondary battery using the rolled copper foil, and a method for manufacturing a rolled copper foil for a secondary battery negative electrode current collector.

Lithium-ion secondary batteries have the characteristics of higher energy density and higher voltage than other secondary batteries. Therefore, development is underway for batteries for various small electronic devices and power supplies for driving large devices such as electric vehicles.

The lithium ion secondary battery is composed of a positive electrode, a negative electrode and a separator. The positive electrode is composed of an aluminum foil current collector and a lithium oxide-based active material coated on the surface thereof, and the negative electrode is composed of a copper foil current collector and a carbon-based active material coated on the surface thereof. The positive electrode and the negative electrode are insulated by a separator, and charging and discharging are performed by the movement of lithium ions in the electrolyte between them.

In recent years, there has been a demand for higher capacity lithium-ion secondary batteries, and development of various components is underway. As one of them, in the case of a negative electrode material, replacement of an existing carbon-based active material with a new active material such as a silicon-based active material is being studied. These new active materials have a large battery capacity, but at the same time, have a large volume change rate during charging and discharging. Therefore, there is a problem that the active material tends to fall off from the current collector and the cycle characteristics tend to deteriorate when used repeatedly. It is considered that this is because the copper foil, which is a current collector, is plastically deformed and broken as the active material expands and contracts during charging and discharging.

As a method for avoiding such a problem, when at least one of 0.04% by mass or more and 0.20% by mass or less of tin and 0.01% by mass or more of silver is contained, and both tin and silver are contained. Has been developed as a copper alloy foil in which the total content of tin and silver is 0.20% by mass or less, and the balance is copper and unavoidable impurities (Patent Document 1). Patent Document 1 describes a method for producing a copper alloy foil in which cold rolling in which one processing degree is 60% or less is continuously performed a predetermined number of times so that the total processing degree is 95% or more. In the present invention, the copper alloy foil has not only a predetermined tensile strength but also a predetermined elongation, so that the breakage of the copper alloy foil, which cannot be suppressed by the copper alloy foil having the predetermined tensile strength, is suppressed. be able to.

Japanese Patent No. 5739044

However, as the capacity of secondary batteries has increased, large-capacity active materials have been used, and along with this, rolled copper foil for secondary battery negative electrode current collectors that can withstand larger volume changes is required. ing.

An object of the present invention is to provide a rolled copper foil for a secondary battery negative electrode current collector that can satisfactorily suppress plastic deformation and breakage of a copper foil due to the generation of stress due to a change in the volume of an active material.

As a result of the examination, the inventor suppresses plastic deformation and breaking of the copper foil due to the volume change of the active material by increasing the Sn content, tensile strength and breaking elongation of the rolled copper foil for the negative electrode current collector of the secondary battery. I got the finding that it can be done.
In addition, the inventor has set the minimum workability per pass to 24% or more using a work roll of a specific diameter after hot rolling of the ingot in the production of rolled copper foil for the secondary battery negative electrode current collector. By performing final cold rolling with a workability of 99.9% or more, both strength and elongation are improved by work hardening of the copper foil, thereby suppressing plastic deformation and breakage of the copper foil due to volume change of the active material. I got the finding that it can be done.

Therefore, the present invention is as follows.
(1) A rolled copper foil for a secondary battery negative electrode current collector containing 0.2 to 2.0% by mass of Sn, having a tensile strength of 650 MPa or more and a breaking elongation of 1.0% or more.
(2) The secondary battery negative electrode current collector having the rolled copper foil for the secondary battery negative electrode current collector according to (1).
(3) Secondary battery negative electrode according to (1) The secondary battery negative electrode having the rolled copper foil for the current collector.
(4) The secondary battery according to (1). The secondary battery having the rolled copper foil for the negative electrode current collector.
(5) The method for manufacturing a rolled copper foil for a secondary battery negative electrode current collector according to (1), which includes a final cold rolling step of hot rolling an ingot and then finishing it to a predetermined thickness. In the process, the degree of processing η at the end of each pass shown in the following formula and the diameter r (mm) of the work roll used for the pass satisfy the relationship of η × r ≦ 250, and 1 of the final cold rolling step. A manufacturing method characterized in that the minimum processing degree per pass is 24% or more and the total processing degree exceeds 99.9%.
η = ln (T ₀ / T _n )
T ₀ : Ingot thickness before the final cold rolling process, T _n : Ingot thickness at the end of the pass.
(6) The ingot is subjected to a cold rolling treatment and an annealing treatment before the final cold rolling step and after the hot rolling, and then the final cold rolling step is performed (5). A method for manufacturing a rolled copper foil for a secondary battery negative electrode current collector according to the above.

According to the present invention, it is possible to provide a rolled copper foil for a secondary battery negative electrode current collector that can satisfactorily suppress plastic deformation and breakage of a copper foil due to a volume change of an active material, and a secondary battery, particularly lithium ion. It is expected to contribute to the improvement of charge / discharge cycle characteristics of secondary batteries and the realization of higher capacity.

FIG. 1 shows one embodiment of the present invention and the tensile strength and breaking elongation of the prior art.

Hereinafter, embodiments of the present invention will be described in detail.
(Composition of rolled copper foil)
As the material of the rolled copper foil for the negative electrode current collector of the secondary battery of the present invention, oxygen-free copper specified in JIS-H3100-C1020 is preferable. Since this composition is close to that of pure copper, the conductivity of the copper foil does not decrease, and it is suitable for a current collector. When oxygen-free copper is used, the oxygen concentration contained in the copper foil is 0.001% by mass or less.
The copper foil according to the present invention is made of industrially used copper and contains unavoidable impurities. Even if a small amount of P, Fe, Zr, Mg, S, Ge and Ti as unavoidable impurities is present, the crystal orientation is easily rotated due to bending deformation of the copper foil, and a shear band is easily formed, and the crystals are collected. It is not preferable because cracks and breaks are likely to occur when the electric body is repeatedly bent and deformed. Therefore, in the copper foil according to the present invention, one or more selected from the group consisting of P, Fe, Zr, Mg, S, Ge and Ti as unavoidable impurities is reduced to 20 mass ppm or less in total. It is preferable to control.
Further, Sn may be contained in an amount of 0.2 to 2.0% by mass in order to improve the characteristics of the material. When Sn is added to the copper foil, the material strength after the final cold rolling is increased and the handleability of the material is improved. However, when the amount of Sn added exceeds 2.0% by mass, the recrystallization temperature rises. , It is difficult to recrystallize while suppressing the surface oxidation of the copper alloy, or in the manufacturing process of the negative electrode material, it becomes difficult to recrystallize the copper foil that is the current collector when drying after coating the active material. The characteristics of the invention cannot be expressed. Therefore, the amount of Sn added is preferably 2.0% by mass or less, more preferably 1.8% by mass or less, and even more preferably 1.6% by mass or less. If the amount of Sn added is less than 0.2% by mass, the strength will be insufficient. From this viewpoint, the amount of Sn added is preferably 0.2% by mass or more, more preferably 0.4% by mass or more, and even more preferably 0.6% by mass or more.
Further, since Sn is more easily oxidized than Cu, it may have an adverse effect such as forming an oxide in the copper foil and becoming a starting point of crack generation in the charge / discharge cycle test of the battery. Therefore, in the molten metal of oxygen-free copper. It is common to add.
In the present specification, when the term "copper foil" is used alone, a copper alloy foil is also included, and when "oxygen-free copper" is used alone, a copper alloy foil based on oxygen-free copper is included.

(Tensile strength and breaking elongation of rolled copper foil)
One of the features of the rolled copper foil of the present invention is that the tensile strength is 650 MPa or more and the breaking elongation is 1.0% or more.

In the prior art, in a secondary battery in which rolled copper foil is used as a negative electrode current collector by increasing the elongation at break, even when the negative electrode active material changes in volume during charging and discharging of the secondary battery. , The copper alloy foil expands and contracts so as to follow the volume change of the negative electrode active material.
However, even if a copper foil having a large elongation is used for the negative electrode current collector, the copper foil may be cracked or broken due to charging and discharging. Specifically, as the active material expands and contracts due to charging and discharging, the copper foil, which is a current collector, receives repeated stress concentration and the current collector partially bends and deforms. Bending deformation is repeated. Bending deformation is associated with expansion and contraction of the active material, and bending and bending back are repeated alternately. Under such harsh conditions, the copper foil, which is a current collector, may crack or break, and the applied active material may fall off, deteriorating the cycle characteristics of the battery.

Therefore, the present invention suppresses the plastic deformation of the rolled copper foil due to stress by improving not only the breaking elongation but also the tensile strength, and as a synergistic effect of this and the improvement of the breaking elongation, the plastic deformation of the rolled copper foil and the plastic deformation of the rolled copper foil Breakage can be effectively suppressed, and it is expected to contribute to the improvement of charge / discharge cycle characteristics and the realization of higher capacity of secondary batteries, especially lithium ion secondary batteries.
From this viewpoint, the tensile strength is preferably 660 MPa or more, more preferably 670 MPa or more, and even more preferably 680 MPa or more. The elongation at break is preferably 1.0% or more, more preferably 1.05% or more, and even more preferably 1.1% or more. The reason for this is that, for example, it is required to maintain and follow the adhesion to the expansion and contraction of the active material during charging and discharging of the lithium ion secondary battery.

(Thickness of rolled copper foil)
The thickness of the rolled copper foil that can be used in the present invention is preferably 5 to 20 μm. There is no particular lower limit to the thickness of the copper foil, but if it is less than 5 μm, the handling of the copper foil will be poor, so 5 μm or more is preferable, and 6 μm or more is more preferable. There is no particular upper limit to the thickness of the foil, but as the thickness increases, the energy density per unit weight of the battery decreases, and the cost of the material also increases. Therefore, 20 μm or less is preferable, and 10 μm or less is more preferable.

In the present invention, the tensile strength refers to a value when a tensile strength test based on the IPC-TM-650 Test Method 2.4.18 is performed at room temperature (23 ° C.).
The elongation at break refers to the elongation at which the test piece breaks when the tensile strength test based on IPC-TM-650 is performed at room temperature (23 ° C.). The elongation at break is calculated from the following formula. In the formula, _Lo is the sample length before the test, and L is the sample length at break.
Elongation at break (%) = (L- _Lo ) / _Lo x 100

(Manufacturing method of rolled copper foil)
The rolled copper foil according to the embodiment of the present invention can be produced, for example, as follows. A copper foil is produced by hot rolling an ingot cast with a specified composition, removing oxides by surface grinding, and processing the ingot to a predetermined thickness in the final cold rolling step. In the final cold rolling process, the total workability is assumed to exceed 99.9%.
The total degree of processing can be calculated from the following formula. In the formula, T ₀ is the thickness of the ingot before the final cold rolling step, and T is the thickness of the rolled material (that is, rolled copper foil) when the cold rolling process in the final cold rolling step is completed. That's right.
Total degree of processing (%) = {(T ₀ −T) / T ₀ } × 100
When the total workability exceeds 99.9%, it is possible to obtain a rolled copper foil in which the tensile strength and breaking elongation of the rolled copper foil are improved by work hardening.
Further, in rolling, the material is repeatedly passed between a pair of rolls to finish the thickness. At this time, passing the material once between the rolls is called one pass. In order to roll at an appropriate strain rate for the purpose of increasing the tensile strength of the material, the degree of processing per pass is preferably 24% or more, more preferably 27% or more, and even more preferably 30% or more. If the degree of processing per pass is less than 24%, the strain rate is slow and sufficient tensile strength cannot be obtained. However, the degree of processing per pass is preferably 50% or less, more preferably 45% or less, still more preferably 40% or less, because if it is too high, the load on the rolling mill becomes too large. The degree of processing per pass can be calculated from the following formula. In the formula, T _n-1 is the thickness of the ingot before rolling by the pass, and T _n is the thickness of the ingot at the end of the pass.
Machining degree per pass (%) = {(T _n-1 −T _n ) / T _n-1 } × 100
Further, before the final cold rolling step, the ingot after hot rolling can be subjected to cold rolling treatment and annealing treatment. By performing the annealing treatment, the bending resistance and the like can be further improved.

In the final cold rolling process, the workability η in an arbitrary rolling path is defined as follows. In the formula, T ₀ is the thickness of the ingot before the final cold rolling step, and T _n is the thickness of the ingot at the end of the pass.
η = ln (T ₀ / T _n )
When η is high, the strength of the material increases due to work hardening, and in order to obtain the target plate thickness, it is necessary to apply a higher pressure to the material by using a work roll having a smaller diameter. When the product of η and the diameter of the work roll (hereinafter, also referred to as “work roll diameter”) r exceeds 250, the work roll diameter is larger than the required pressure, so it is difficult to obtain the pressure required for rolling. Since the load of the work roll becomes large, it is necessary to reduce the work roll diameter according to η in an arbitrary path. Further, by using a work roll having a small diameter, the rolling process can be advanced to realize higher workability rolling, and the occurrence of a shear band can be suppressed. Therefore, the upper limit of the value of the product of η and the work roll diameter is set to 250. The upper limit of the value of the product of η and the work roll diameter is preferably 240, more preferably 230. The shear band is a structure in which deformation is locally concentrated, and is a part where strain is accumulated and the dislocation density is increased. Since it is less likely to be deformed than the surrounding structure, the elongation deteriorates when a shear band is formed in the material. However, since the maintenance frequency increases as the work roll diameter becomes smaller, the lower limit value 40 is preferable for the product of η and the work roll diameter r from the viewpoint of manufacturability. The lower limit of the product of η and the work roll diameter r is more preferably 70, and even more preferably 100.

As an effect of the method for producing a rolled copper foil of the present invention, FIG. 1 shows the tensile strength (TS) and elongation at break of the present invention and the prior art in which the total workability in the final cold rolling step is changed. Has been done. In the figure, the total workability in the final cold rolling process of the present invention and the prior art is more than 99.9% and 99%, respectively, and the other manufacturing conditions are the same. According to FIG. 1, when the total workability in the final cold rolling step exceeds 99.9%, the tensile strength and the elongation at break can be improved.

Next, the rolled copper foil of the present invention was prototyped and its performance was confirmed, which will be described below. However, the description here is for the purpose of mere illustration, and is not intended to be limited thereto.

First, an ingot having a composition of Cu-0.20% by mass Sn was melted, and this ingot was hot-rolled from 900 ° C. to obtain a plate having a thickness of 100 mm. Then, a rolled copper foil having a thickness of 10 μm was finally obtained by a final cold rolling step under each of the pass conditions A to I as shown in Table 1 as an example. "-" In the table indicates that it has not been processed.
The following characteristic evaluations were performed on each of the test pieces thus obtained. The results are shown in Table 2.

<0.2% proof stress>
A test piece of 100 mm in the longitudinal direction and 12.7 mm in the width direction was prepared, and a tensile test was performed in parallel with the rolling direction by a tensile tester in accordance with IPC-TM-650 Test Method 2.4.18, and JIS Z 2241 0.2% proof stress was analyzed according to.
<Conductivity>
The test piece was sampled so that the longitudinal direction of the test piece was parallel to the rolling direction, and the conductivity (EC:% IACS) was measured by the 4-terminal method in accordance with JIS H 0505.
<Tensile strength>
A test piece of 100 mm in the longitudinal direction and 12.7 mm in the width direction was prepared, and a tensile test was performed in parallel with the rolling direction with a tensile tester in accordance with IPC-TM-650 Test Method 2.4.18, and the tensile strength was obtained. Was measured.
<Breaking elongation>
Specimens with a length of 100 mm and a width of 12.7 mm were prepared, marked with a stamp at 5 mm intervals, and then subjected to a tensile tester in accordance with IPC-TM-650 Test Method 2.4.18. A tensile test was performed in parallel with the rolling direction, and the elongation at break was measured by measuring the distance between the marks of the portion including the fractured portion of the sample after fracture.

<Characteristic evaluation of secondary battery>
The characteristics of the secondary batteries formed by using the copper alloy foils of Examples 1 to 4 and Comparative Examples 1 to 7 were evaluated. Specifically, as a characteristic of the secondary battery, the presence or absence of a broken portion of the negative electrode was evaluated.

(Preparation of negative electrode)
First, a negative electrode active material layer was formed on the main surface of any of the copper alloy foils of Examples 1 to 4 and Comparative Examples 1 to 7, and a negative electrode was produced. Specifically, 45 parts by mass of scaly graphite powder as a negative electrode active material, 5 parts by mass of silicon monoxide (SIO), 2 parts by mass of SBR as a binder, and 20 parts by mass of a thickener aqueous solution. The part and the part were kneaded and dispersed to generate a slurry (paste) of the negative electrode active material layer. The thickener aqueous solution was produced by dissolving 99 parts by mass of water in 1 part by mass of CMC as a thickener. Subsequently, the slurry for the negative electrode active material layer produced by the doctor blade method was thickened on each of the main surfaces (one side) of each of the copper alloy foils of Examples 1 to 4 and Comparative Examples 1 to 7. Was applied so as to be 100 μm. Then, each of the copper alloy foils of Examples 1 to 4 and Comparative Examples 1 to 7 coated with the slurry for the negative electrode active material layer was heated under the condition of 200 ° C. for 1 hour and dried. As a result, a negative electrode active material layer having a thickness of 100 μm was formed on each of the copper alloy foils of Examples 1 to 4 and Comparative Examples 1 to 7. Then, the thickness of the negative electrode active material layer was adjusted to 50 μm by pressurizing the negative electrode active material layer. Then, a negative electrode (negative electrode plate) having a predetermined shape was produced by punching the laminate of the copper alloy foil and the negative electrode active material layer.

(Making a secondary battery)
A positive electrode plate (positive electrode) used for a secondary battery was produced. Specifically, 50 parts by mass of LiCoO2 powder as a positive electrode active material, 1 part by mass of acetylene black as a conductive auxiliary agent, and 5 parts by mass of PVDF as a binder are placed in water (solvent). By kneading and dispersing, a slurry (paste) for the positive electrode active material layer was produced. Subsequently, the slurry for the positive electrode active material layer produced by the doctor blade method becomes 100 μm on one of the main surfaces (one side) of the aluminum foil having a thickness of 20 μm as the positive electrode current collector. Was applied. Then, the aluminum foil coated with the slurry for the positive electrode active material layer was heated under the condition of 120 ° C. for 1 hour and dried. As a result, a positive electrode active material layer having a thickness of 100 μm was formed on the aluminum foil. Then, the thickness of the positive electrode active material layer was adjusted to 50 μm by pressurizing the positive electrode active material layer. Then, a positive electrode (positive electrode plate) having a predetermined shape was produced by punching the laminate of the aluminum foil and the positive electrode active material layer.

A coin-cell type lithium ion secondary battery using each negative electrode using each copper alloy foil (copper foil) of Examples 1 to 4 and Comparative Examples 1 to 7, a positive electrode, a separator, and an electrolytic solution. Was produced. That is, the negative electrode active material layer included in each negative electrode and the positive electrode active material layer provided in the positive electrode are arranged so as to face each other, and a thickness of 20 μm is provided between the negative electrode active material layer and the positive electrode active material layer. A laminate of a negative electrode, a positive electrode, and a separator was prepared by sandwiching a separator made of a resin porous film. Then, the laminated body of the negative electrode, the positive electrode, and the separator was housed in a coin-shaped container (cell), and the positive electrode and the negative electrode were electrically connected to the terminals inside the cell, respectively. Then, LiPF6 as an electrolyte was added to 1 mol / liter as an additive in a mixed solvent produced by mixing 30% by volume of EC, 50% by volume of MEC, and 20% by volume of methyl propionate. After injecting an electrolytic solution in which 1% by volume of VC was dissolved into the cell, the cell was sealed to prepare a secondary battery.

(Evaluation of the presence or absence of breakage)
For each of the secondary batteries formed by using each of the copper alloy foils of Examples 1 to 4 and Comparative Examples 1 to 7, after charging and discharging the secondary battery, the portion where the copper alloy foil is broken is visually observed. I confirmed it in. Specifically, the presence or absence of breakage of the copper alloy foil was visually confirmed after charging and discharging were alternately performed 50 times under the condition of 25 ° C.
(Evaluation of cycle characteristics)
For each of the secondary batteries formed by using the copper alloy foils of Examples 1 to 4 and Comparative Examples 1 to 7, the capacity retention rate after charging and discharging the secondary battery was measured. Specifically, charging and discharging are performed under the condition of 25 ° C., and the ratio of the discharge capacity of the 50th cycle to the discharge capacity of the 2nd cycle, that is, (the discharge capacity of the 50th cycle / the discharge capacity of the 2nd cycle). Calculated as × 100. At that time, charging is performed at a constant current density of 1 mA / cm ² until the battery voltage reaches 4.2 V, and then charging is performed at a constant voltage of 4.2 V until the current density reaches 0.05 mA / cm ² and discharged. Was carried out at a constant current density of 1 mA / cm ² until the battery voltage reached 2.5 V. When charging, the utilization rate of the capacity of the negative electrode was set to 90% so that metallic lithium was not deposited on the negative electrode. The results of the measured capacity retention rates are shown in Table 2. Table 2 shows the evaluation of the capacity retention rate. As for the evaluation, ⊚ is particularly good, ◯ is good, and × is bad.

<Evaluation result>
From Examples 1 to 4 and Comparative Examples 1 to 7, when the copper alloy foil having a predetermined tensile strength and elongation is used as the negative electrode current collector of the secondary battery, the copper alloy foil is charged and discharged by the secondary battery. It was confirmed that the breakage could be suppressed. For example, in a secondary battery in which a copper alloy foil having a tensile strength of 650 MPa or more and an elongation of 1.0% or more is used as a negative electrode current collector, copper is used even if the secondary battery is repeatedly charged and discharged. It was confirmed that plastic deformation and breakage of the alloy foil were suppressed.

That is, it was confirmed that by having a predetermined tensile strength and elongation, it is possible to suppress plastic deformation and breakage of the copper alloy due to the stress generated by the volume change of the negative electrode active material when the secondary battery is charged and discharged. Therefore, it was confirmed that the copper alloy foil can be more suppressed from plastic deformation and breakage.

As shown in Table 2, Examples 1 to 4 contained a predetermined amount of Sn in the present invention, and by performing a predetermined final cold rolling, the tensile strength and the elongation at break could be improved.

In Comparative Example 1, the tensile strength was not sufficient because the Sn concentration was insufficient.
In Comparative Example 2, the Sn concentration was excessive, so that the elongation was insufficient.
In Comparative Examples 3 and 4, the tensile strength was not sufficient because the total workability of the final cold rolling was not sufficient.
In Comparative Example 5, since the product of the work roll diameter r and the degree of processing η exceeds 250, a shear band is generated in the material, and the elongation is insufficient.
In Comparative Examples 6 and 7, since the minimum processing degree per pass was not sufficient, the strain rate was slow and the tensile strength was not sufficient.

Claims (6)

A rolled copper foil for a secondary battery negative electrode current collector containing Sn in an amount of 0.2 to 2.0% by mass, having a tensile strength of 650 MPa or more and a breaking elongation of 1.0% or more. The secondary battery negative electrode current collector having the rolled copper foil for the secondary battery negative electrode current collector according to claim 1. The secondary battery negative electrode according to claim 1, wherein the secondary battery negative electrode has a rolled copper foil for a current collector. The secondary battery according to claim 1, which has a rolled copper foil for a negative electrode current collector. The method for producing a rolled copper foil for a secondary battery negative electrode current collector according to claim 1, which includes a final cold rolling step of hot-rolling an ingot and then finishing it to a predetermined thickness, in the final cold rolling step. , The degree of processing η at the end of each pass shown in the following formula and the diameter r (mm) of the work roll used for the pass satisfy the relationship of η × r ≦ 250, and per pass of the final cold rolling step. A manufacturing method characterized in that the minimum degree of processing is 24% or more and the total degree of processing exceeds 99.9%.
η = ln (T ₀ / T _n )
T ₀ : Ingot thickness before the final cold rolling process, T _n : Ingot thickness at the end of the pass. The fifth aspect of claim 5, wherein the ingot is subjected to a cold rolling process and an annealing process before the final cold rolling step and after the hot rolling, and then the final cold rolling step is performed. A method for manufacturing rolled copper foil for a secondary battery negative electrode current collector.

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