A new direction for the performance improvement of rechargeable lithium/sulfur batteries

In this work we introduce a new direction for the performance improvement of rechargeable lithium/sulfur batteries by employing an electrolyte that promotes Li anode passivation in lithium polysulfide solutions. To examine our concept, we assemble and characterize Li/Li2S9 liquid cells by using a porous carbon electrode as the current collector and a 0.25 m Li2S9 solution as the catholyte. Results show that Li/Li2S9 liquid cells are superior to conventional Li/S cells in specific capacity and capacity retention. We also find that use of LiNO3 as a co-salt in the Li2S9 catholyte significantly increases the cell's Coulombic efficiency. More importantly, the cells with LiNO3 have a ∼2.5 V voltage plateau before the end of charging and demonstrate a steep voltage rise at the end of charging. The former is indicative of the formation of elemental sulfur from soluble lithium polysulfides on the carbon electrode, and the latter provides a distinct signal for full charging. Electrochemical analyses on Li plating and stripping in Li2S9 catholyte solutions indicate that LiNO3 participates in the formation of a highly protective passivation film on the Li metal surface, which effectively prevents the Li anode from chemical reaction with polysulfide anions in the electrolyte and meanwhile prevents polysulfide anions from electrochemical reduction on the Li surface.

► A new direction for improving Li/S batteries by in situ protecting Li anode.
► LiNO3 promotes formation of a highly protective passivation film on Li surface.
► Li/Li2Sx liquid cell offers better capacity retention than conventional Li/S cells.
► LiNO3 increases specific capacity and cycling efficiency of Li/S cells.