A device for suppressing fires generated by lithium-ion batteries exhibiting thermal runaway. The device can include a thermally insulating enclosure for housing one or more battery cells. A fluid delivery system having a heat/temperature sensitive tube passes through each enclosure and contains an agent capable of extinguishing fires. When the cell or battery starts to undergo thermal runaway, the increase in temperature or the eruption of open flame causes the fluid delivery system to rupture inside the enclosure. The agent leaks out of the rupture and is transported into the enclosure and onto the malfunctioning cell or battery. Any fire is suppressed and the cell or battery is cooled down by the agent.