Li-ion batteries a superior in terms of energy density but like lead acid batteries(chlorine and hydrogen gas danger) they have problems. Thermal runaway in the main has been contained by the stack housing and battery management system but the gasses given off during cell melt down are apparently very toxic.

Li-Ion batteries are superior in terms of energy density, but like Lead-Acid batteries  (chlorine and hydrogen gas danger) they have problems. 

Thermal runaway in the main has been contained by the stack housing and battery management system, but the gasses given off during cell melt down are apparently very toxic.