Flat Li-ion battery (LP603450)
LP603450 Li-polymer · 3.7 V · 1200 mAh · protection board (cuts at 4.28 V / 2.5 V / 3 A) · JST-PH 2.0 lead

Try it on the benchThe lab drops it on an empty bench and opens its datasheet card.
Pins
- − (black lead)
- + (red lead)