CN113518832A

CN113518832A - Rolled copper foil for secondary battery negative electrode collector, secondary battery negative electrode collector and secondary battery using same, and method for producing rolled copper foil for secondary battery negative electrode collector

Info

Publication number: CN113518832A
Application number: CN202080018556.7A
Authority: CN
Inventors: 冈部史弥
Original assignee: JX Nippon Mining and Metals Corp
Current assignee: JX Nippon Mining and Metals Corp
Priority date: 2019-03-04
Filing date: 2020-02-21
Publication date: 2021-10-19
Also published as: KR20240025712A; CN117165811A; TWI745864B; JP6790153B2; JP2020143313A; TW202033782A; KR20210133278A; WO2020179515A1

Abstract

The invention provides a rolled copper foil for a secondary battery negative electrode collector, which can well inhibit plastic deformation and fracture of the copper foil caused by stress and the like generated along with volume change of an active material. The rolled copper foil for a secondary battery negative electrode collector contains 0.2-2.0 mass% of Sn, and has a tensile strength of 650MPa or more and an elongation at break of 1.0% or more.

Description

Rolled copper foil for secondary battery negative electrode collector, secondary battery negative electrode collector and secondary battery using same, and method for producing rolled copper foil for secondary battery negative electrode collector

Technical Field

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

Background

Lithium ion secondary batteries have characteristics of high energy density and high voltage compared to other secondary batteries. Therefore, development is underway in various small-sized electronic device batteries, driving power sources for large-sized devices such as electric vehicles, and the like.

The lithium ion secondary battery is composed of a positive electrode, a negative electrode, and a separator. The positive electrode is formed of an aluminum foil collector and a lithium oxide-based active material coated on the surface thereof, and the negative electrode is formed of a copper foil 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 lithium ions move in an electrolyte therebetween to perform charge and discharge.

In recent years, high capacity of lithium ion secondary batteries is required, and various components have been developed. For a negative electrode material which is one of the members, replacement of a conventional carbon-based active material with a new active material such as a silicon-based active material has been studied. These novel active materials are characterized by a large battery capacity and a large volume change rate during charge and discharge. Therefore, when the current collector is repeatedly used, the active material is easily detached from the current collector, and the cycle characteristics are deteriorated. This is presumably because the copper foil as a current collector is plastically deformed and cracked with expansion and contraction of the active material during charge and discharge.

As a method for avoiding such a problem, there is disclosed a copper alloy foil containing 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, wherein the total content of tin and silver is 0.20% by mass or less when both tin and silver are contained, and the balance is copper and unavoidable impurities (patent document 1). Patent document 1 discloses a method for producing a copper alloy foil, which includes continuously performing cold rolling with a degree of working of 60% or less 1 time a predetermined number of times so that the total degree of working is 95% or more. In the present invention, since the copper alloy foil has not only a predetermined tensile strength but also a predetermined elongation, cracking of the copper alloy foil, which cannot be suppressed by the copper alloy foil having a predetermined tensile strength, can be suppressed.

Documents of the prior art

Patent document

Patent document 1: japanese patent No. 5739044

Disclosure of Invention

Technical problem to be solved by the invention

However, as the capacity of secondary batteries increases, large-capacity active materials are used, and accordingly, rolled copper foils for secondary battery negative electrode collectors that can withstand greater changes in volume are required.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a rolled copper foil for a secondary battery negative electrode collector, which can suppress plastic deformation and cracking of the copper foil due to stress or the like caused by a volume change of an active material.

Means for solving the problems

As a result of investigations, the inventors have found that plastic deformation and cracking of a copper foil due to volume change of an active material can be suppressed by increasing the Sn content, tensile strength, and elongation at break of a rolled copper foil for a secondary battery negative electrode collector.

Further, the inventors have found that, in the production of a rolled copper foil for a secondary battery negative electrode collector, after hot rolling of an ingot, final cold rolling with a minimum workability of 24% or more per 1 pass and a total workability of 99.9% or more is performed using a work roll having a predetermined diameter, whereby both strength and elongation are improved by work hardening of the copper foil, and therefore plastic deformation and cracking of the copper foil due to volume change of an active material can be suppressed.

Accordingly, the present invention is as follows.

(1) A rolled copper foil for a secondary battery negative electrode collector contains 0.2 to 2.0 mass% of Sn, and has a tensile strength of 650MPa or more and an elongation at break of 1.0% or more.

(2) A secondary battery negative electrode current collector having the rolled copper foil for a secondary battery negative electrode current collector as described in (1).

(3) A secondary battery negative electrode comprising the rolled copper foil for a secondary battery negative electrode current collector as described in (1).

(4) A secondary battery comprising the rolled copper foil for a collector of a negative electrode of a secondary battery as described in (1).

(5) The method for producing a rolled copper foil for a negative electrode collector of a secondary battery according to (1), comprising a final cold rolling step of hot-rolling an ingot and then finishing the ingot to a predetermined thickness, wherein in the final cold rolling step, a relation of η × r ≦ 250 is satisfied between a degree of working η at a time point when each pass is finished and a diameter r (mm) of a work roll used in the pass, as shown in the following formula, and the minimum degree of working per 1 pass of the final cold rolling step is 24% or more and the total degree of working exceeds 99.9%.

η＝ln(T₀/T_n)

In the formula, T₀: thickness of ingot before carrying out the final cold-rolling step, T_n: ingot thickness at the point where the pass ended.

(6) The method for producing a rolled copper foil for a secondary battery negative electrode collector according to (5), characterized in that, prior to the final cold rolling step, the cold rolling treatment and the annealing treatment are further performed on the hot-rolled ingot, followed by the final cold rolling step.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a rolled copper foil for a secondary battery negative electrode collector can be provided which can satisfactorily suppress plastic deformation and cracking of the copper foil associated with a volume change of an active material, and is expected to contribute to improvement of charge-discharge cycle characteristics and realization of high capacity of a secondary battery, particularly a lithium ion secondary battery.

Drawings

Fig. 1 is a graph showing tensile strength and elongation at break of an embodiment of the present invention and the prior art.

Detailed Description

Hereinafter, embodiments of the present invention will be described in detail.

(composition of rolled copper foil)

The material of the rolled copper foil for a secondary battery negative electrode collector of the present invention is preferably oxygen-free copper in accordance with JIS-H3100-C1020 standard. Since the composition is close to pure copper, the conductivity of the copper foil is not lowered, and the copper foil is suitable for a current collector. When oxygen-free copper is used, the oxygen concentration contained in the copper foil is 0.001 mass% or less.

The copper foil of the present invention is formed of industrial copper and contains inevitable impurities. P, Fe, Zr, Mg, S, Ge, and Ti, which are inevitable impurities, are preferably not contained because even if they are present in a very small amount, the crystal orientation is easily rotated by bending deformation of the copper foil, a shear band is easily introduced, and cracks and fractures are easily generated when the current collector is repeatedly bent. Therefore, the copper foil of the present invention preferably contains 1 or 2 or more kinds selected from the group consisting of P, Fe, Zr, Mg, S, Ge and Ti as inevitable impurities in a total amount of 20 mass ppm or less.

In addition, 0.2 to 2.0 mass% of Sn may be contained to improve the characteristics of the material. However, when the amount of Sn added exceeds 2.0 mass%, the recrystallization temperature rises and recrystallization annealing is difficult to be performed while suppressing surface oxidation of the copper alloy, or in the production step of the negative electrode material, the copper foil as a current collector is difficult to be recrystallized during drying after coating with an active material, and the characteristics of the present invention cannot be found. Therefore, the addition amount of Sn is preferably 2.0 mass% or less, more preferably 1.8 mass% or less, and still more preferably 1.6 mass% or less. When the addition amount of Sn is less than 0.2 mass%, the strength is insufficient. From this viewpoint, the amount of Sn added is preferably 0.2 mass% or more, more preferably 0.4 mass% or more, and still more preferably 0.6 mass% or more.

In addition, Sn is more easily oxidized than Cu, and therefore, Sn is generally added to an oxygen-free copper melt in consideration of adverse effects such as formation of an oxide in a copper foil and generation of cracks in a charge-discharge cycle test of a battery.

In this specification, the term "copper foil" alone is intended to include a copper alloy foil, and the term "oxygen-free copper" alone is intended to include a copper alloy foil based on oxygen-free copper.

(tensile Strength and elongation at Break of rolled copper foil)

One of the characteristics of the rolled copper foil of the present invention is that the tensile strength is 650MPa or more and the elongation at break is 1.0% or more.

In the conventional art, since the elongation at break is increased, in a secondary battery using a rolled copper foil as a negative electrode current collector, even if the volume of the negative electrode active material changes during charge and discharge of the secondary battery, the copper alloy foil expands or contracts with the change in the volume of the negative electrode active material.

However, even when a copper foil having a large elongation is used as the negative electrode current collector, cracks or fractures may occur in the copper foil during charge and discharge. Specifically, the active material expands and contracts due to charge and discharge, so that the copper foil as the current collector is repeatedly subjected to stress concentration and the current collector is locally subjected to bending deformation, and the bending deformation is repeated due to charge and discharge. The bending deformation alternately repeats bending and bending recovery with expansion and contraction of the active material. Under such severe conditions, cracks or fractures may be generated in the copper foil as a current collector, the coated active material may be detached and the cycle characteristics of the battery may be deteriorated.

Therefore, the present invention can suppress plastic deformation of the rolled copper foil due to stress by not only increasing the elongation at break but also increasing the tensile strength, and the suppression can effectively suppress plastic deformation and cracking of the rolled copper foil in addition to the increase in elongation at break, and is expected to contribute to improvement of charge-discharge cycle characteristics and realization of high capacity of secondary batteries, particularly lithium ion secondary batteries.

From this viewpoint, the tensile strength is preferably 660MPa or more, more preferably 670MPa or more, and still more preferably 680MPa or more. The elongation at break is preferably 1.0% or more, more preferably 1.05% or more, and still more preferably 1.1% or more. The reason for this is that, for example, there is a demand for maintaining adhesion by expansion and contraction of the active material during charge and discharge of the lithium ion secondary battery, and for following the expansion and contraction.

(thickness of rolled copper foil)

The thickness of the rolled copper foil usable in the present invention is preferably 5 to 20 μm. Although there is no particular lower limit on the thickness of the copper foil, the copper foil has poor handleability when less than 5 μm, and therefore is preferably 5 μm or more, more preferably 6 μm or more. Although there is no particular upper limit on the thickness of the foil, the energy density per unit weight of the battery decreases and the cost of the material increases as the thickness increases, and therefore, it is preferably 20 μm or less, and more preferably 10 μm or less.

In the present invention, the tensile strength is a value in the case of performing a tensile strength test at ordinary temperature (23 ℃) based on IPC-TM-650 test method 2.4.18.

The elongation at break is the elongation at break of the test piece when the tensile strength test is performed at room temperature (23 ℃) based on IPC-TM-650. The elongation at break can be determined according to the following equation. In the formula, L_oIs the sample length before the test, and L is the sample length at the time of fracture.

Elongation at break (%) - (L-L)_o)/L_o×100

(method for producing rolled copper foil)

The rolled copper foil according to the embodiment of the present invention can be produced, for example, as follows. After hot rolling an ingot cast in accordance with a predetermined composition, oxides are removed by surface grinding, and processed to a predetermined thickness by a final cold rolling step, thereby producing a copper foil. In the final cold rolling step, the total degree of processing is over 99.9%.

The total degree of working is determined by the following equation. In the formula, T₀Is subjected to a final cold rolling stepThe thickness of the pre-cast ingot, T, is the thickness of the rolled material (i.e., rolled copper foil) at the end of the cold rolling process in the final cold rolling step.

(T) total degree of processing { (T) (%)₀－T)/T₀}×100

By making the total workability exceed 99.9%, a rolled copper foil whose work hardening results in an improvement in the tensile strength and elongation at break of the rolled copper foil can be obtained.

In addition, in the rolling, the material is repeatedly passed between a pair of rolls to finish the thickness, and in this case, the material is passed between the rolls 1 time is referred to as 1 pass. In order to perform rolling at an appropriate strain rate to improve the tensile strength of the material, the degree of working per 1 pass is preferably 24% or more, more preferably 27% or more, and still more preferably 30% or more. When the degree of working per 1 pass is less than 24%, the strain rate becomes slow, and sufficient tensile strength cannot be obtained. However, if the degree of working per 1 pass is too high, the load on the rolling mill becomes too large, and therefore, it is preferably 50% or less, more preferably 45% or less, and still more preferably 40% or less. The degree of working per 1 pass can be determined by the following equation. In the formula, T_n-1Is the thickness of the ingot before rolling in that pass, T_nThe thickness of the ingot at the time point when the pass was completed.

Degree of working (%) per 1 pass { (T)_n-1－T_n)/T_n-1}×100

Further, before the final cold rolling step, the ingot after hot rolling may 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 step, the degree of working η in any rolling pass is defined as follows. In the formula, T₀Is the thickness, T, of the ingot before the final cold rolling step_nThe thickness of the ingot at the time point when the pass was completed.

η＝ln(T₀/T_n)

When η is high, work hardening leads to an increase in the strength of the material, and in order to obtain a target plate thickness, it is necessary to use a smaller diameter work roll and apply a higher pressure to the material. 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 becomes large for the necessary pressure, so it becomes difficult to obtain the necessary pressure for rolling and the load on the rolling mill becomes large, so it is necessary to reduce the work roll diameter in accordance with η in an arbitrary pass. Further, by performing the rolling step using the work rolls having a small diameter, the rolling step can be performed at a higher working degree, and the occurrence of shear bands can be suppressed. Therefore, the upper limit of the value of the product of η and the work roll diameter is 250. The upper limit of the product of η and the work roll diameter is preferably 240, more preferably 230. The shear band is a tissue in which deformation is locally concentrated, and is a portion in which strain is accumulated and dislocation density is increased. Since it is difficult to deform compared to the surrounding tissue, the elongation is deteriorated when a shear band is generated in the material. However, since the maintenance frequency increases as the work roll diameter decreases, the lower limit of the product of η and the work roll diameter r is preferably 40 from the viewpoint of manufacturability. The lower limit value of the product of η and the work roll diameter r is more preferably 70, and still more preferably 100.

Fig. 1 shows the Tensile Strength (TS) and the elongation at break of the present invention and the conventional art, in which the total degree of processing in the final cold rolling step is changed, as a diagram showing the effects of the method for producing a rolled copper foil of the present invention. In the figure, the manufacturing conditions were the same except that the total degree of working in the final cold rolling step of the present invention and the prior art was more than 99.9% and 99%, respectively. According to fig. 1, since the total degree of working in the final cold rolling step exceeds 99.9%, the tensile strength and the elongation at break can be improved.

Examples

Next, the rolled copper foil of the present invention is produced, and the properties thereof are confirmed, which will be described below. However, the description herein is for illustrative purposes only and is not intended to be limiting.

First, an ingot having a composition of Cu-0.20 mass% Sn was produced by melting, and the ingot was hot-rolled from 900 ℃ to obtain a sheet having a thickness of 100 mm. Then, as shown in Table 1 by way of example, the final cold rolling step under each pass condition of A to I was carried out to obtain a rolled copper foil having a thickness of 10 μm. The "-" in the table indicates no processing.

The test pieces thus obtained were subjected to the following characteristic evaluations. The results are shown in table 2.

< 0.2% yield strength >

A test piece having a longitudinal direction of 100mm and a width direction of 12.7mm was prepared, and a tensile test was conducted in parallel to the rolling direction using a tensile tester in accordance with IPC-TM-650 test method 2.4.18, and 0.2% yield strength was analyzed in accordance with JIS Z2241.

< conductivity >

The test piece was sampled so that the longitudinal direction of the test piece was parallel to the rolling direction, and the electrical conductivity (EC:% IACS) was measured by the 4-terminal method in accordance with JIS H0505.

< tensile Strength >

A test piece having a length direction of 100mm and a width direction of 12.7mm was prepared, and a tensile test was conducted in parallel to the rolling direction using a tensile tester in accordance with IPC-TM-650 test method 2.4.18 to measure the tensile strength.

Elongation at Break >

A test piece having a length direction of 100mm and a width direction of 12.7mm was prepared, and after stamping marks with a gap of 5mm using a stamp, a tensile test was performed in parallel to the rolling direction using a tensile tester in accordance with IPC-TM-650 test method 2.4.18, and the elongation at break was measured by measuring the gap of the marks at the portion including the broken portion of the sample after the breakage.

< evaluation of characteristics of Secondary Battery >

The characteristics of secondary batteries formed using the copper alloy foils of examples 1 to 4 and comparative examples 1 to 7 were evaluated. Specifically, as the characteristics of the secondary battery, the presence or absence of a rupture portion of the negative electrode was evaluated.

(preparation of cathode)

First, a negative electrode active material layer was formed on the front surface of each 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 a flaky graphite powder and 5 parts by mass of silicon monoxide (SIO) as negative electrode active materials, 2 parts by mass of SBR as a binder, and 20 parts by mass of an aqueous thickener solution were kneaded and dispersed to form a slurry (paste) of a negative electrode active material layer. In addition, 1 part by mass of CMC as a thickener was dissolved in 99 parts by mass of water to form a thickener aqueous solution. Next, the slurry for the negative electrode active material layer formed was applied to any one of the front surfaces (one surfaces) of the copper alloy foils of examples 1 to 4 and comparative examples 1 to 7 to a thickness of 100 μm by a doctor blade coating method. Thereafter, the copper alloy foils of examples 1 to 4 and comparative examples 1 to 7, each coated with the slurry for a negative electrode active material layer, were heated at 200 ℃ for 1 hour and dried. Thus, negative electrode active material layers having a thickness of 100 μm were formed on the copper alloy foils of examples 1 to 4 and comparative examples 1 to 7, respectively. Then, the thickness of the negative electrode active material layer was adjusted to 50 μm by applying pressure to the negative electrode active material layer. Then, the laminate of the copper alloy foil and the negative electrode active material layer is subjected to press working, thereby producing a negative electrode (negative electrode plate) having a predetermined shape.

(production of Secondary Battery)

A positive electrode plate (positive electrode) for a secondary battery was produced. Specifically, 50 parts by mass of LiCoO as a positive electrode active material₂The powder, 1 part by mass of acetylene black as a conductive additive, and 5 parts by mass of PVDF as a binder were kneaded and dispersed in water (solvent) to form a slurry (paste) for a positive electrode active material layer. Next, the slurry for the positive electrode active material layer formed was applied to any one of the front surfaces (one surface) of the aluminum foils having a thickness of 20 μm as the positive electrode current collectors by a doctor blade coating method to a thickness of 100 μm. Then, the aluminum foil coated with the slurry for the positive electrode active material layer was heated at 120 ℃ for 1 hour, and dried. Thus, 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 laminate of the aluminum foil and the positive electrode active material layer is subjected to press working, thereby producing a positive electrode (positive electrode plate) having a predetermined shape.

Coin-case-type lithium ion secondary batteries were produced using the negative electrodes, positive electrode, separator, and electrolyte of the copper alloy foils (copper foils) of examples 1 to 4 and comparative examples 1 to 7. That is, a laminate of a negative electrode, a positive electrode, and a separator was prepared by disposing the negative electrode active material layer provided in each negative electrode so as to face the positive electrode active material layer provided in the positive electrode, and inserting a separator made of a porous film made of a polypropylene resin having a thickness of 20 μm between the negative electrode active material layer and the positive electrode active material layer. Then, the laminate of the negative electrode, the positive electrode, and the separator was housed in a coin-shaped container (casing), and the positive electrode and the negative electrode were electrically connected to terminals inside the casing, respectively. Then, LiPF of 1 mol/l as an electrolyte was dissolved in a mixed solvent obtained by mixing EC of 30 vol%, MEC of 50 vol%, and methyl propionate of 20 vol%₆And an electrolyte solution of 1 mass% of VC as an additive was injected into the case, and the case was sealed to produce a secondary battery.

(evaluation of Presence of cleavage site)

In each of the secondary batteries formed using the copper alloy foils of examples 1 to 4 and comparative examples 1 to 7, the portion where the crack occurred on the copper alloy foil was visually confirmed after the charging and discharging of the secondary battery. Specifically, after 50 times of charge and discharge at 25 ℃ each, the presence or absence of cracking of the copper alloy foil was visually confirmed.

(evaluation of cycle characteristics)

The capacity retention rate after charging and discharging of the secondary batteries was measured for each of the secondary batteries formed using the copper alloy foils of examples 1 to 4 and comparative examples 1 to 7. Specifically, charge and discharge were performed at 25 ℃, and the ratio of the discharge capacity at the 50 th cycle to the discharge capacity at the 2 nd cycle was calculated, that is, (discharge capacity at the 50 th cycle/discharge capacity at the 2 nd cycle) × 100. At this time, the charging was carried out at 1mA/cm²Until the cell voltage reached 4.2V, and then at a constant voltage of 4.2V until the current density reached 0.05mA/cm²In the above-mentioned order of magnitude,the discharge is at 1mA/cm²Until the cell voltage reached 2.5V. In addition, the utilization rate of the capacity of the negative electrode was set to 90% during charging, and no metal lithium was precipitated in the negative electrode. The results of the measured capacity maintenance rates are shown in table 2. The evaluation of the capacity retention rate is shown in table 2. The evaluation was excellent, good and poor.

< evaluation result >

It was confirmed from examples 1 to 4 and comparative examples 1 to 7 that the copper alloy foil having a predetermined tensile strength and elongation can suppress cracking of the copper alloy foil due to charge and discharge of the secondary battery when used as a negative electrode current collector of the secondary battery. For example, it was confirmed that in a secondary battery using a copper alloy foil having a tensile strength of 650MPa or more and an elongation of 1.0% or more as a negative electrode current collector, plastic deformation and cracking of the copper alloy foil can be suppressed even when the secondary battery is repeatedly charged and discharged.

That is, by having a predetermined tensile strength and elongation, plastic deformation and cracking of the copper alloy due to stress caused by a volume change of the negative electrode active material during charge and discharge of the secondary battery can be suppressed. Therefore, it was confirmed that plastic deformation and cracking of the copper alloy foil could be suppressed.

[ TABLE 1-1 ]

[ TABLE 1-2 ]

[ TABLE 1-3 ]

[ TABLE 2 ]

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

In comparative example 1, the Sn concentration was insufficient, and the tensile strength was insufficient.

In comparative example 2, the Sn concentration was excessive, and the elongation was insufficient.

The final cold rolling in comparative examples 3 and 4 had insufficient total workability, and therefore the tensile strength was insufficient.

In comparative example 5, the product of the work roll diameter r and the degree of working η exceeded 250, and therefore a shear band was generated in the material and the elongation was insufficient.

In comparative examples 6 and 7, the minimum workability per 1 pass was insufficient, and therefore the strain rate was slow and the tensile strength was insufficient.

Claims

1. A rolled copper foil for a secondary battery negative electrode collector contains 0.2 to 2.0 mass% of Sn, and has a tensile strength of 650MPa or more and an elongation at break of 1.0% or more.

2. A secondary battery negative electrode collector comprising the rolled copper foil for a secondary battery negative electrode collector according to claim 1.

3. A secondary battery negative electrode comprising the rolled copper foil for a secondary battery negative electrode current collector according to claim 1.

4. A secondary battery comprising the rolled copper foil for a secondary battery negative electrode collector according to claim 1.

5. A method for producing a rolled copper foil for a negative electrode collector of a secondary battery according to claim 1, comprising a final cold rolling step of hot rolling an ingot and finishing the ingot to a predetermined thickness, wherein in the final cold rolling step, a degree of working η at a time point when each pass is completed and a diameter r (mm) of a work roll used in the pass satisfy a relationship of η x r ≦ 250, a minimum degree of working per 1 pass of the final cold rolling step is 24% or more, and a total degree of working exceeds 99.9%,

η＝ln(T₀/T_n)

6. The method for producing a rolled copper foil for a secondary battery negative electrode collector according to claim 5, wherein, prior to the final cold rolling step, the cold rolling treatment and the annealing treatment are further performed on the hot-rolled ingot, followed by the final cold rolling step.