COPPER ALLOY FOIL, NEGATIVE ELECTRODE FOR LITHIUM ION SECONDARY BATTERY, LITHIUM ION SECONDARY BATTERY, METHOD OF MANUFACTURING COPPER ALLOY FOIL, AND METHOD OF MANUFACTURING NEGATIVE ELECTRODE FOR LITHIUM ION SECONDARY BATTERY

PROBLEM TO BE SOLVED: To stably maintain sufficient mechanical strength even after a heat treatment in a manufacturing process of a negative electrode for a lithium ion secondary battery.SOLUTION: A copper alloy foil includes, at least, Cr of 0.20 mass% or more and 0.40 mass% or less, Ag of 0.01 mass% or more and 0.10 mass% or less, and Sn of 0.10 mass% or more and 0.20 mass% or less, the balance being Cu, wherein a Cr solid solution index Z calculated by following formula (1): Z=(R-R)/(R-R) is 0.4 or less. Here, Rindicates an actually measured conductivity R (% IACS) of a copper alloy foil 10, Rindicates a calculated value (% IACS) of a conductivity R of the copper alloy foil 10 in a case that whole Cr are solid-soluted, and Rindicates a calculated value (% IACS) of a conductivity R of the copper alloy foil 10 in a case that whole Cr are precipitated.