Air-Stable Li₂S Cathodes Enabled by an In Situ-Formed Li⁺ Conductor for Graphite-Li₂S Pouch Cells.

Abstract

Using Li₂S 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 Li₂S causes the release of hazardous H₂S and the formation of insulative by-products, increasing the manufacturing difficulty and adversely affecting cathode performance. Here, Li₄SnS₄, a Li⁺ conductor that is air-stable according to the hard-soft acid-base principle, is formed in situ and uniformly on Li₂S particles because Li₂S itself participates in Li₄SnS₄ formation. When exposed to air (20% relative humidity), the protective Li₄SnS₄ layer maintains its components and structure, thus contributing to the enhanced stability of the Li₂S@Li₄SnS₄ composite. In addition, the Li₄SnS₄ layer can accelerate the sluggish conversion of Li₂S because of its favorable interfacial charge transfer, and continuously confine lithium polysulfides owing to its integrity during electrochemical processes. A graphite-Li₂S pouch cell containing a Li₂S@Li₄SnS₄ cathode is constructed, which shows stable cyclability with 97% capacity retention after 100 cycles. Hence, combining a desirable air-stable Li₂S cathode and a highly reversible Li-free configuration offers potential practical applications of graphite-Li₂S full cells.