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EP-4435122-B1 - ROLLED COPPER FOIL FOR SECONDARY BATTERIES, SECONDARY BATTERY NEGATIVE ELECTRODE USING SAME, AND SECONDARY BATTERY

EP4435122B1EP 4435122 B1EP4435122 B1EP 4435122B1EP-4435122-B1

Inventors

  • OKABE, Fumiya

Dates

Publication Date
20260506
Application Date
20220929

Claims (8)

  1. A rolled copper foil for a secondary battery, comprising 0.05 to 0.15% by weight of Zr, 0.05% by weight or less of oxygen, and the balance consisting of Cu and unavoidable impurities, wherein a tensile strength in a direction parallel to rolling after a heat treatment at 350 °C for 3 hours is 500 MPa or more, and a rate of change in the tensile strength in the direction parallel to rolling before and after the heat treatment is 15% or less.
  2. The rolled copper foil for a secondary battery according to claim 1, wherein the tensile strength in the direction parallel to rolling after the heat treatment is 550 MPa or more.
  3. The rolled copper foil for a secondary battery according to claim 1 or 2, wherein a number of Zr inclusions with a major diameter of 1 µm to 5 µm per Zr concentration within a range of 5000 µm 2 on a surface of the copper foil is 0.05 / (5000 µm 2 /weight ppm) or less.
  4. The rolled copper foil for a secondary battery according to any one of claims 1 to 3, wherein a number of Zr inclusions with a major diameter exceeding 5 µm per Zr concentration within a range of 5000 µm 2 on a surface of the copper foil is 0 / (5000 µm 2 /weight ppm).
  5. The rolled copper foil for a secondary battery according to any one of claims 1 to 4, having a thickness of 10 µm or less.
  6. The rolled copper foil for a secondary battery according to any one of claims 1 to 5, wherein a conductivity after the heat treatment is 60% IACS or more.
  7. A secondary battery negative electrode, comprising the rolled copper foil for a secondary battery according to any one of claims 1 to 6.
  8. A secondary battery, comprising the rolled copper foil for a secondary battery according to any one of claims 1 to 6.

Description

FIELD OF THE INVENTION The present invention relates to a rolled copper foil for a secondary battery, and a secondary battery negative electrode and secondary battery using the same. BACKGROUND OF THE INVENTION Secondary batteries, especially lithium-ion secondary batteries, have the characteristics of high energy density and the ability to obtain relatively high voltage, and are often used in small electronic devices such as notebook computers, video cameras, digital cameras, and mobile phones. In addition, lithium-ion secondary batteries have begun to be used as power sources for large devices such as electric vehicles and distributed power sources in general households, and because they are lighter and have higher energy density than other secondary batteries, they are widely used in a variety of devices that require power sources. The electrode body of a lithium ion secondary battery generally has a wound structure or a stacked structure in which each electrode is laminated. It is common that the positive electrode of a lithium ion secondary battery is composed of a current collector made of aluminum foil and a positive electrode active material made of a lithium composite oxide such as LiCoO2, LiNiO2, and LiMn2O4 provided on the surface of the current collector, and the negative electrode is composed of a current collector made of copper foil and a negative electrode active material made of carbon or the like provided on the surface of the current collector. For example, Patent Literature 1 (Japanese Patent Application Publication No. 2016-191139) discloses a rolled copper foil for a secondary battery, characterized in that it comprises a total of 100 to 500 ppm by weight of one or more selected from the group of Ti and Zr, an oxygen concentration is 50 ppm or less by weight, a tensile strength parallel to a rolling direction according to JIS-Z2241 after heat treatment at 350 °C for 1 hour is 350 MPa or more, and an electrical conductivity after the heat treatment is 90% IACS or more, and before and after the heat treatment, a rate of change in the tensile strength is 10% or less, and Zr or Ti inclusions with a major diameter of 1 µm to 5 µm within a range of 1000 µm2 on the surface of the copper foil is 10 or less. It is disclosed that this rolled copper foil for a secondary battery has excellent strength, heat resistance, and electrical conductivity. Furthermore, Patent Literature 2 (Japanese Patent Application Publication No. 2019-077891) discloses a rolled copper foil for a secondary battery, wherein in a direction parallel, a tensile strength to rolling is 600 MPa or more and an elongation at break is 2.0% or more, and in a direction perpendicular to rolling, a tensile strength is 640 MPa or more and an elongation at break is 3.5% or more. It is disclosed that this rolled copper foil for a secondary battery can satisfactorily suppress breakage of the copper foil due to stress generation due to volume change of the active material. PRIOR ART Patent Literature [Patent Literature 1] Japanese Patent Application Publication No. 2016-191139[Patent Literature 2] Japanese Patent Application Publication No. 2019-077891 SUMMARY OF THE INVENTION On the other hand, next-generation lithium ion secondary batteries are desired to have a higher concentration of Si-based active materials, and depending on the application, a device with a significantly higher concentration of Si-based active materials than current batteries is required. Since the Si-based active material has a large rate of volume change during charging and discharging, a solvent-based binder is generally used to make it adhere strongly to the current collector copper foil. Depending on the type of the solvent-based binder, a drying temperature can reach up to 350 °C, and a drying time can take as long as about 2 to 3 hours, so conventional rolled copper foil for a secondary battery cannot maintain its strength if heat resistance is insufficient. Further, as a result, the rolled copper foil for a secondary battery becomes soft or has insufficient strength, which causes wrinkles and bulges in the appearance of the battery during the early stages of charging and discharging, or deterioration of characteristics (battery capacity deterioration due to changes in shape and dimensions, and Li precipitation) after the charging and discharging cycles. Therefore, it is necessary to suppress the decrease in strength of rolled copper foil for a secondary battery by improving heat resistance. The present invention was created in view of the above-mentioned problems, and in one embodiment, an object of the present invention is to provide a rolled copper foil for a secondary battery having heat resistance that maintains high strength even after a heat treatment. In another embodiment, an object of the present invention is to provide a secondary battery negative electrode and a secondary battery using such a rolled copper foil for a secondary battery. As a result