Do I really need to do spot weld with nickel ?
Simple questions do not have simple answers.
Most common cells (except the very old C and D type ZnCl2 cells) have contacts that are Ni plated. They are easier to solder and still easier to weld.
Soldering is easy if you have a relatively high power soldering iron - but do it quickly. Use lots of flux. For this, copper is the preferred metal because solder wets copper easily (tin forms an alloy with copper).
Tin also dissolves in Nickel (forms an alloy) and if the contacts are Ni plated you can solder them without much problem. But you should do it quickly.
For spot welding, you apply far less energy for a shorter time in a very small area. The metal melts and fuses but only at the point of contact.
For spot welding, Ni foils is the most suitable because Cu can corrode easily (Cu is also not easy to spot weld). So if you have the setup, go for spot welding.
The spot welding setup need not be complicated: a bank of capacitors and a high current SCR may be all that is needed.
If you are planning to do this on a regular basis, I suggest that you go for a spot welding setup.