Keywords lithium-ion thermal runaway trace hydrogen gas sensor battery safety Introduction The major safety concern with lithium-ion cells is the possibility of thermal runaway1,2,3,4 (Figure 1). Lithium-ion cells stressed by high temperature, overcharging, mechanical deformation, or defects can develop internal exothermic chemical reactions, causing the cell temperature to rise,1,5,6,7,8,9,10,11 degrading the cell components.