How to solder wires on Li-ion cell phone batteries?
Don't.

The battery manufacturers specifically tell you not to do this, as the batteries are quite heat-sensitive. If you manage to set one on fire, you can't extinguish it either: it reacts explosively with water, so you have to bury it in sand or powder.

The correct solution is to get spring-loaded contacts or "pogo pins", attach them to a PCB or piece of Veroboard, then attach the wiring to that. You can then either build a battery holder nicely or use tape or elastic bands to hold it in place.

(The manufacturers use electric or ultrasonic spot welding to attach wires to battery terminals internally.)