Soldering 18650; is an FX-888D good enough?
Use a BIG ASS tip like the T18-C5 Shape-5C. You want large surface contact and lots of mass. Short and fat, not long or thin.

Chisel or bevel tips would work. No coned. They aren't good for anything in my experience.

Bevel tips with only the cut surface tinned are really good for other things too like SMD where the tip carries the solder rather than you feeding it. However, the T18-C5 is tinned around the edges and the largest cut face tinned is 4mm compared to the T18-C5's 5mm. Tinning around the edges is beneficial for tasks (like your batteries) where you have to feed solder to the workpiece rather than pre-loading solder onto the tip (as you would for SMD).