Salt Additives for Improving Cyclability of Polymer-Based All-Solid-State Lithium–Sulfur Batteries

Lithium–sulfur batteries are attracting extensive attention for energy storage owing to their high theoretical energy density. However, their practical implementation is hindered because of inherent issues of the technology such as the shuttling effect of the polysulfide intermediates and the formation of dendritic lithium metal (Li0) deposits during battery operation leading to the short cycle life of the cell. It is generally accepted that the formation of robust solid electrolyte interphase (SEI) layers on the surface of the Li0 anode is an effective way to mitigate these issues. Herein, the use of salt additives, lithium (difluoromethanesulfonyl)­(trifluoromethanesulfonyl)­imide {Li­[N­(SO2CF2H)­(SO2CF3)], LiDFTFSI} and lithium tricyanomethanide [LiC­(CN)3, LiTCM], added to the classical solid polymer electrolyte (SPE) comprising lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI) and poly­(ethylene oxide) (PEO) is proposed, with the aim to improve the quality of the SEI layer on the Li0 anode. Through this approach, SEI layers with good mechanical integrity and Li-ion conductivity are formed thanks to the beneficial anion chemistry of these salt additives, allowing the PEO-based all-solid-state lithium–sulfur cells to be cycled for more than 100 cycles with good rate capability and Coulombic efficiency. These results attest to the great importance of electrolyte additives, even at small doses, to improve the battery performance through the selective modification of SEI components.