It's almost impossible to spot-weld copper directly onto the 18650 cell ends. The cell tips are nickel-plated steel. However the path through the 18650 shell is thin. Nickel spot-welds easily, it's high resistance creates very hot spots precisely at the points where the welder probes are touching. Nickel melts at 2650F / 1350C
Copper of the same cross section can conduct the same amount of current while barely getting warm, but that high-conductivity / low-resistance also means it doesn't get hot when the spot-welder sends a jolt through the probes.
Here are 18650-cell bus-plates from ES member kdog
When experimenting with copper sheet, I recommend to buy a sampler pack with these
0.13mm___5-mil__36 ga (fragile, paper thin, not recommended)
0.20mm___8-mil__32 ga (minimum recommended thickness, 4 times more conductive than 0.20mm nickel)
0.25mm__10-mil__30 ga (recommended, cuts with common scissors)
0.33mm__13-mil__28 ga (acceptable for very high current)
0.40mm__16-mil__26 ga (very stiff, expensive, hard to cut with hand shears, not recommended)
Here is my recommended nickel tab shape. The split down the middle makes the spot-welds more consistent and stronger while using a lower setting. If you attach the nickel tabs to the copper sheet before you add the cells, you can use as much heat as you like for that particular connection and nothing will be damaged. I would like to try a 3V / 500A MOT-based spot-welder. For the nickel-tab to cell connection I have a kWeld spot-welder.
You can easily cut common nickel strip with an abrasive disc on a Dremel