Pouch Film Stack and Secondary Battery

A pouch film stack according to the present invention includes a base layer, a gas barrier layer, and a sealant layer stacked in that order. The sealant layer includes a first sealant layer located adjacent to the gas barrier layer and a second sealant layer stacked on the first sealant layer. A percentage difference in yield strength between the first and second sealant layers (ΔY) is 66% to 120%, where ΔY is defined as: {(Yield strength of second sealant layer−Yield strength of first sealant layer)/Yield strength of first sealant layer}×