I use a basic Weller 80W iron for stained glass to solder my battery cells (SPG80L). I got a spot welder for a while to rebuild some dewalt packs but my results were mixed, more of a hassle, took up too much space etc... so I sold it. My experience with spot welds is that they didn't stand up to vibration and shock as well as cells I soldered. I know that a "good" spot weld is of course better than solder ... but I wasn't always getting good spot welds in factory purchased packs or by doing them myself.
The issue with soldering cells is that the heat can degrade the cell, so the key for the least impact to the cell's integrity is to balance getting a good connection with the least amount of thermal transfer (this is why spot welding is supposed to be better).
I scruff the surface with sandpaper, wipe with alcohol and then tin each surface. The all important cell top takes some practice but also needs a hot enough iron to allow quick local heat and without enough heat no amount of practice will help. The cell SHOULD NOT get hot except right where the solder goes. A good tin will have a nice smooth even transition from edges. Not hot enough, too quick or too much solder on tip leaves more of a ball type tinned spot. If _ is the surface and / or ( is the tinned solder spot, _/ is a good edge and this _( is not. I couldn't get a good tin patch of solder on cells without getting the cell hot with my fancy 35w iron stations (regardless of tip used), until I doubled the watts of the iron. I would suggest nothing less than 65w.
Another thing that worked for me is that I used fine flux core solder usually for IC to board type work. I touch the solder end back and forth between the surface and the iron tip because a little solder on the tip helps speed up the thermal transfer. As soon as the solder I'm trying to feed flows onto the surface vs. the iron tip, I pull the iron and feed the needed solder before the patch cools. Sort of a melt, no melt, melt, no melt, melt, melt & feed & pull'it deal. I found it more difficult when using thick solder to find good timing of the tip removal.