At what voltage is a 3.7 V battery dead?

3.4V

Your 3.7V lithium ion battery can be considered dead (completely discharged) at a voltage of 3.4V. Your battery mustn’t discharge beyond 2.75V (minimum safe voltage), as it could reduce its life.

hfh-tech Samsung 94AH 3.7V Li Ion Battery 6000 Cycles 20+ Yrs Life ; Battery Capacity: 94ah ; Voltage: 3.7 ; Battery Type: Lithium-Ion ; Brand: Samsung ;