High‐Performance Lithium‐Sulfur All‐Solid–State Batteries Enabled by Chemically Synthesized Lithium Polysulfide Cathodes

ABSTRACT To achieve high specific energy and long lifespan in lithium‐sulfur all solid‐state batteries (Li‐S ASSBs), it is pivotal to ensure high sulfur utilization and minimize volume change during cycling. In this work, we synthesize a partially lithiated sulfur material (S@Li 2 S) by mechanical mixing sulfur with lithium sulfide (Li 2 S). To our surprise, characterization techniques including Raman spectroscopy, solid‐state nuclear magnetic resonance (ssNMR), and sulfur K‐edge X‐ray absorption near edge structure spectroscopy (XANES) reveal the formation of lithium polysulfide (Li 2 S n ) species that can be stabilized at room temperature, previously thought to be unlikely in the solid‐state. With enhanced reactivity, these lithium polysulfide cathode materials show much faster kinetics than S or Li 2 S and are always present throughout the charge and discharge processes. Furthermore, molecular dynamics simulations indicate that different polysulfide species have similar densities. This explains the observed minimal volume change of S@Li 2 S during cycling. Consequently, this cathode delivers a high specific capacity of 780 mAh g −1 , more than double that of sulfur (315 mAh g −1 ), with an 85% capacity retention after 500 cycles. These mechanistic insights and cycling performance advancement contribute to the development of the high‐performance, next‐generation Li‐S ASSBs.