The main current-carrying conductors within the pack.
In MS1V the busbars connected the cells together in parallel and series arrangement of 24P2S per module. They were laser cut from EMS Sigma 60 material, which is a layer of nickel, the stainless steel, copper, stainless steel, nickel
Copper is the main conductor(EMS sigma 60 is 60% copper), but it is difficult to spot weld, which is why there is nickel and stainless steel
Information about EMS Sigma 60:
https://www.emsclad.com/fileadmin/Data/Divisions/EMS/Sigma_Clad_60.pdf