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. The reactions between dissolved higher-order polysulfides and Li metal were found to be the origins for the 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 the electrolyte on the thermal safety of batteries. On the contrary, thermal runaway does not occur for 45-cycle pouch cell despite the addition of the electrolyte. It is found that the higher-order polysulfides (Li₂S_{x ≥ 6}) are discovered in 16-cycle electrolyte while the sulfur species in 45-cycle electrolyte are Li₂S_{x ≤ 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 polysulfide shuttle for Li–S batteries from the safety view.