Copper wire instead of Nickel strips

Has anyone tried spot welding copper wire to cells instead of nickel strips?

Neigh impossible. Copper is extremely difficult to spot weld.

+1 on Korishan

Spot welding works well on steel / nickel, because they have fairly high electrical resistance, so the current can heat up and momentarily melt the material.
Copper has much much lower electrical resistance, so the electric current passes through without heating up the material much. Also, copper conducts heat extremely well, so it's very difficult to create localized heating.

Could always try to do ultrasonic welding. Though, that may be pricey for the equipment. Not sure though.

You don't spot-weld copper, you solder it.
And yes, it's fine as long as you know what you're doing and don't expose the cell to overheating.