Sulfur Polymers as Flexible Interfacial Additives for Low Stack‐Pressure Solid‐State Lithium‐Ion Batteries

Abstract Solid‐state batteries (SSBs) fabricated using sulfide solid electrolytes (SSEs) typically require cell stack‐pressures in the range of tens to hundreds of megapascals to maintain effective interfacial contact and lithium‐ion mobility across the full cell stack. These relatively high cell stack‐pressures necessarily require additional cell components that reduce the delivered volumetric and gravimetric capacities of SSBs. This work has developed a novel sulfur polymer (polyS) that improves lithium‐ion conductivity in SSBs at low cell stack‐pressures, thereby directly targeting this technological limitation. Specifically, this work shows that polyS can be combined with the argyrodite Li₆PS₅Cl (LPSC) to form a stable composite SSE. By combining LPSC particles with a flexible additive that enhances interfacial contact at low pressures, this new polyS LPSC composite SSE material greatly improves ionic conductivity at cell stack‐pressures below 2.0 MPa in comparison to conventional LPSC composites. Furthermore, this polyS LPSC composite can be used to fabricate a full SSB that cycles reversibly at only 1.6 MPa. Finally, this composite SSE exhibits self‐healing behavior when combined with a lithium metal electrode, wherein lithium dendrites are oxidized to form passivating Li₂S species that recover the cell from shorting.