Air-Stable Li 2 S Cathodes Enabled by an In Situ-Formed Li + Conductor for Graphite-Li 2 S Pouch Cells.

Using Li 2 S cathodes instead of S cathodes presents an opportunity to pair them with Li-free anodes (e.g., graphite), thereby circumventing anode-related issues, such as poor reversibility and safety, encountered in Li-S batteries. However, the moisture-sensitive nature of Li 2 S causes the release of hazardous H 2 S and the formation of insulative by-products, increasing the manufacturing difficulty and adversely affecting cathode performance. Here, Li 4 SnS 4 , a Li + conductor that is air-stable according to the hard-soft acid-base principle, is formed in situ and uniformly on Li 2 S particles because Li 2 S itself participates in Li 4 SnS 4 formation. When exposed to air (20% relative humidity), the protective Li 4 SnS 4 layer maintains its components and structure, thus contributing to the enhanced stability of the Li 2 S@Li 4 SnS 4 composite. In addition, the Li 4 SnS 4 layer can accelerate the sluggish conversion of Li 2 S because of its favorable interfacial charge transfer, and continuously confine lithium polysulfides owing to its integrity during electrochemical processes. A graphite-Li 2 S pouch cell containing a Li 2 S@Li 4 SnS 4 cathode is constructed, which shows stable cyclability with 97% capacity retention after 100 cycles. Hence, combining a desirable air-stable Li 2 S cathode and a highly reversible Li-free configuration offers potential practical applications of graphite-Li 2 S full cells.