Operando X-ray Spectroscopy Unveils Light-Driven Redox Selectivity for Photo-Assisted Li-S Batteries

Photo-assisted lithium-sulfur batteries (PALSBs) can accelerate the sluggish redox kinetics of sulfur cathodes. However, the introduced light field inevitably complicates interfacial reactions, necessitating in situ evidence under realistic operating conditions. Here, we construct a TiO₂/FePS₃ (TF) p-n junction bifunctional photoelectrode and employ a multiphysics-coupled in situ x-ray spectroscopic technique to elucidate light-regulated catalysis from the interface into the bulk. Operando low-energy XPS identifies potential interfacial catalytic sites. High-energy operando XAFS is, for the first time, applied in PALSBs to track the K-edge position of catalytic centers throughout cycling. The results show that the reversible dynamic valence evolution synchronizes with the stepwise sulfur redox process, revealing that photogenerated carriers and electrocatalytic electrons act cooperatively to promote polysulfide conversion. DFT calculations corroborate, from thermodynamic and kinetic perspectives, that illumination strengthens polysulfide anchoring and lowers the energy barriers of key conversion steps, consistent with the operando spectroscopic observations. Benefiting from this photoelectrochemical co-regulation, the TF-based PALSB maintains excellent reversible capacity and cycling stability under high sulfur loading and low electrolyte content. This work establishes a characterization paradigm for the rational design of high-performance photo-assisted Li-S cathodes.