Current-collector metal foil, current collector, and current-collector-metal-foil manufacturing method
A current-collector metal foil has at least at least one roughened surface and numerous recessed parts are present on the roughened surface. Each recessed part has an edge part that surrounds a bottom-surface part and is raised above the bottom-surface part. The average Feret diameter L ave of the recessed parts is 0.5-50 μm. The current-collector metal foil is suitable for use, e.g., as an electrode current collector for a lithium-ion secondary battery, a sodium secondary battery, an electric double-layer capacitor, or a lithium-ion capacitor.
1. A method for shaping a metal foil for use as a current collector of a battery, comprising:
forming a roughened surface on at least a first roll,
plating a chrome-plate film on the roughened surface of the first roll,
flattening peak parts of protruding parts on the chrome-plated film to form flat surfaces on the protruding parts, and
passing the metal foil between a pair of rolls, at least one of which is the first roll, to transfer a contour of the first roll to a first surface of the metal foil, thereby forming recesses in the first surface of the metal foil,
wherein the protruding parts having the flat surfaces on the first roll are configured such that:
(i) after passing the metal foil between the pair of rolls, the recesses in the first surface of the metal foil have an average Feret diameter L ave of 0.5 μm or more and 50 μm or less, and
(ii) a maximum depth of the recesses in the first surface of the metal foil is 2.5 μm or less and is less than the average Feret diameter L ave of the recesses in the first surface of the metal foil.
2. The method according to claim 1 , wherein:
the roughened surface is formed by one of a mechanical method, a chemical method, or a physical method,
after plating the chrome-plate film and before flattening, the peak parts each have a substantially spherical shape, and
the substantially spherical shapes of the peak parts are flattened to form the flat surfaces by rolling the first roll with a load applied, thereby smashing the substantially spherical shapes.
3. The method according to claim 1 , wherein the protruding parts having the flat surfaces on the first roll are configured such that, after passing the metal foil between the pair of rolls, 90% or more of the recesses in the first surface of the metal foil have a depth of 0.5 μm or more.
4. The method according to claim 1 , further comprising:
forming a roughened surface on a second roll,
plating a chrome-plate film on the roughened surface of the second roll, and
flattening peak parts of protruding parts on the chrome-plated film of the second roll to form flat surfaces on the protruding parts,
wherein the metal foil is passed between the first and second rolls to transfer the contour of the first roll to the first surface of the metal foil and to transfer a contour of the second roll to a second surface of the metal foil, thereby forming recesses in the first and second surfaces of the metal foil, and
the protruding parts having the flat surfaces on the second roll are configured such that, after passing the metal foil between the first and second rolls, the recesses in the second surface of the metal foil also have an average Feret diameter L ave of 0.5 μm or more and 50 μm or less and a maximum depth of 2.5 μm or less.
5. The method according to claim 1 , wherein the metal foil has a thickness of 5-35 μm.
6. The method according to claim 1 , wherein the metal foil has a thickness of 8-18 μm.
7. The method according to claim 1 , wherein the metal foil is composed of copper or a copper alloy.
8. The method according to claim 1 , wherein the metal foil is composed of aluminum or an aluminum alloy.
9. The method according to claim 1 , wherein the protruding parts having the flat surfaces on the first roll are configured such that, after passing the metal foil between the pair of rolls, 30-90% of a surface area of the metal foil is occupied by bottom surfaces of the recesses in the first surface of the metal foil.
10. The method according to claim 1 , further comprising:
after passing the metal foil between the pair of rolls, applying an active material to at least the first surface of the metal foil, the active material having an average aggregated particle size d ave of 0.5 μm or more and 50 μm or less.
11. The method according to claim 10 , wherein the average aggregated particle size d ave of the active material is equal to or greater than the average Feret diameter L ave of the recesses in the first surface of the metal foil.
12. The method according to claim 11 , wherein the active material is composed of a material selected from the group consisting of silicon, tin, germanium, and oxides or alloys thereof; a carbon material; and lithium titanate.
13. The method according to claim 11 , wherein the active material is composed of a material selected from the group consisting of LiCoO 2 , LiMn 2 O 4 , LiMnO 2 , LiNiO 2 , LiCo 1/3 Ni 1/3 Mn 1/3 , and LiFePO 4 .
14. The method according to claim 4 , wherein the protruding parts having the flat surfaces on the first roll and on the second roll are configured such that, after passing the metal foil between the first and second rolls:
30-90% of a surface area of the metal foil is occupied by bottom surfaces of the recesses in the first and second surfaces of the metal foil.
15. The method according to claim 14 , wherein:
the metal foil has a thickness of 8-18 μm; and
the metal foil is composed of copper, a copper alloy, aluminum or an aluminum alloy.