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
Due to issues welding the EMS Sigma 60 directly to the cells(requires too much weld energy, so you can’t do a lot of welds in a short time), nickel strips need to be welded to the cells and the sigma 60.
In MSXIV they found that thinner nickel strips were better – 3 criteria were considered: Power loss and heat dissipated, Manufacturability, Safety
Power loss was marginally higher with the thinnest strips(0.1mm vs 0.15 and 0.20mm) but since a low weld energy was needed to be safer and allow lots of welds to be done, the 0.1mm nickel was chosen