PNNL

Abstract

Solid-state Li-S batteries (SSLSBs) are made of low cost and abundant materials free of supply chain concerns. Due to their high theoretical energy densities, they are highly desirable for electrical vehicles1-3. However, the development of SSLSBs has been historically plagued by the insulating nature of sulfur4,5 and the poor interfacial contacts induced by its large volume change during cycling6,7, impeding charge transfer among different solid components. Here, we report a S9.3I molecular crystal with I2 inserted in the crystalline S structure, which shows a semiconductor-level electrical conductivity (~5.9×10-7 S cm-1) at 25 oC, an 11 order of magnitude increase over S itself. Iodine introduces new states into the band gap of S and promotes the formation of reactive polysulfides during electrochemical cycling. Further, the material features a low melting point of ~65 oC, which enables repairing damaged interfaces due to cycling by periodical re-melting of the cathode material. As a result, a Li-S9.3I battery demonstrates 400 stable cycles with a specific capacity retention of 87%. The design of this conductive, low melting-point sulfur iodide material represents a substantial advancement in the chemistry of S materials, and opens the door to the practical realization of SSLSBs.