Distinct Redox Chemistry in Solid-State Li–S Batteries Induced by Solid Electrolytes

The sulfur redox mechanism in solid-state lithium–sulfur (Li–S) batteries remains unclear, as it has been reported to vary under different operating conditions due to sluggish reaction kinetics. Herein, we investigate sulfur reactions in solid-state batteries using solid electrolytes with high ionic conductivities to mitigate kinetic limitations. A solid-state Li–S cell employing Li 5.5 PS 4.5 Cl 1.5 exhibits an asymmetric voltage profile at 25 °C, with two voltage plateaus during discharge and poorly separated oxidation reactions during charge. Ex situ X-ray absorption spectroscopy (XAS) elucidates that these poorly separated oxidation reactions consist of overlapping conversion reactions, in contrast to the clearly distinguishable two-step conversion from S 8 to Li 2 S via Li 2 S x during discharge. In addition, operando impedance evolution, analyzed using a combined distribution of relaxation times (DRT) and distribution of phasances (DOP) model, reveals distinct relaxation processes during discharge and charge that arise from differences in the transport properties of the reaction products. This work demonstrates an intrinsic asymmetric sulfur redox mechanism in solid-state batteries that is difficult to resolve by conventional electrochemical measurements alone but can be clarified by combining XAS with impedance analysis using the DRT-DOP model.