Real-Time Monitoring of Multiphase Evolution in All-Solid-State Li–S Batteries via In-Situ Synchrotron X-ray Absorption Spectroscopy

All-solid-state lithium–sulfur batteries (ASSLSBs) can overcome polysulfide shuttle problem and offer an economic, high-energy-density solution for next-generation energy storage. However, its solid-phase reaction mechanism remains unclear due to complex multiphase evolution, which is difficult to analyze using existing operando methods. Here, we design an in-situ cell enabling real-time sulfur K-edge X-ray absorption spectroscopy under practical conditions. Meanwhile, to address the complexity in spectra analysis, we introduce a hybrid method combining principal component analysis and constrained optimization, allowing robust quantitative spectral deconvolution despite severe peak overlap. Our results reveal a two-step discharge pathway: sulfur directly reduces to Li 2 S 2 and then to Li 2 S. Additionally, we found that a P 2 S 5 additive can react in-situ with Li 2 S to form the solid electrolyte Li 3 PS 4, enhancing ionic conduction and promoting more complete discharge. This work establishes a general framework for real-time analysis of multiphase evolution during an electrochemical process and elucidates sulfur redox mechanisms in ASSLSBs.