Higher-order polysulfides induced thermal runaway for 1.0 Ah lithium sulfur pouch cells

Comprehensive analyses on thermal runaway mechanisms are critically vital to achieve the safe lithium–sulfur (Li–S) batteries. We found the reactions between dissolved higher-order polysulfides and Li metal were the origins for thermal runaway of 1.0 Ah cycled Li–S pouch cells. 16-cycle pouch cell indicates high safety, heating from 30 to 300 °C without thermal runaway, while 16-cycle pouch cell with additional electrolyte undergoes severe thermal runaway at 147.9 °C, demonstrating the key roles of electrolyte on the thermal safety of batteries. On the contrary, thermal runaway does not occur for 45-cycle pouch cell despite the addition of electrolyte. It is found that the higher-order polysulfides (Li2Sx≥6) are discovered in 16-cycle electrolyte while the sulfur species in 45-cycle electrolyte are Li2Sx≤4. In addition, strong exothermic reactions are discovered between cycled Li and dissolved higher-order polysulfide (Li2S6 and Li2S8) at 153.0 °C, driving the thermal runaway of cycled Li–S pouch cells. This work uncovers the potential safety risks of Li–S batteries and negative roles of the undesirable polysulfide shuttle for Li–S batteries from the safety view.