Simultaneous Redox Mediation and Interfacial Engineering Enabled by Diphenyl Diselenide for High‐Performance All‐Solid‐State Li–S Batteries

All‐solid‐state lithium–sulfur batteries (ASSLSBs) are emerging as promising candidates for next‐generation energy storage solutions, owing to their exceptional specific energy, inherent safety, and cost‐effectiveness. However, sluggish solid–solid sulfur redox kinetics hinder their rate performance and cycling longevity, posing a critical challenge for practical implementation. Herein, we introduce diphenyl diselenide (DPDSe) as a multifunctional additive that simultaneously addresses both the limitations of sulfur cathodes and interfacial issues in ASSLSBs. DPDSe serves as an efficient redox mediator, reacting with lithium polysulfides (LiPSs) to generate phenyl selenylated intermediates (LiPhSePSs), which enhance charge transfer kinetics. Furthermore, DPDSe improves the ionic conductivity of solid polymer electrolytes (SPEs) and reacts with lithium metal to form a robust solid electrolyte interphase (SEI) enriched with aromatic moieties. This aromatic‐rich SEI mitigates electrolyte degradation and suppresses lithium dendrite formation, ensuring stable interfacial contact during prolonged cycling. Remarkably, ASSLSBs incorporating DPDSe achieve an exceptional specific capacity of 400 mAh g −1 after 200 cycles at 1 C. This performance marks the highest cycling stability reported for organic mediator‐enhanced solid‐state lithium‐sulfur systems. This work establishes a novel multifunctional additive strategy to fully exploit the electrochemical potential of sulfur cathodes in solid‐state battery architectures.