Ultrahigh Performance All Solid-State Lithium Sulfur Batteries: Salt Anion’s Chemistry-Induced Anomalous Synergistic Effect

With a remarkably higher theoretical energy density compared to lithium-ion batteries (LIBs) and abundance of elemental sulfur, lithium sulfur (Li–S) batteries have emerged as one of the most promising alternatives among all the post LIB technologies. In particular, the coupling of solid polymer electrolytes (SPEs) with the cell chemistry of Li–S batteries enables a safe and high-capacity electrochemical energy storage system, due to the better processability and less flammability of SPEs compared to liquid electrolytes. However, the practical deployment of all solid-state Li–S batteries (ASSLSBs) containing SPEs is largely hindered by the low accessibility of active materials and side reactions of soluble polysulfide species, resulting in a poor specific capacity and cyclability. In the present work, an ultrahigh performance of ASSLSBs is obtained via an anomalous synergistic effect between (fluorosulfonyl)­(trifluoromethanesulfonyl)­imide anions inherited from the design of lithium salts in SPEs and the polysulfide species formed during the cycling. The corresponding Li–S cells deliver high specific/areal capacity (1394 mAh g_sulfur^–1, 1.2 mAh cm^–2), good Coulombic efficiency, and superior rate capability (∼800 mAh g_sulfur^–1 after 60 cycles). These results imply the importance of the molecular structure of lithium salts in ASSLSBs and pave a way for future development of safe and cost-effective Li–S batteries.