Decoding Multi‐Electron Redox Pathways in Carbon‐Free Iron Selenide Cathodes: Enabling Energy‐Dense All‐Solid‐State Lithium Batteries Across Extreme Temperatures

Conversion‐type iron chalcogen cathodes, characterized by the multi‐electron redox reaction and cost‐effectiveness, represent an alternative pathway for next‐generation all‐solid‐state lithium batteries (ASSLBs). In this study, α‐FeSe as a cathode is identified that operates stably through a Fe2+/Fe0 redox reaction in a sulfide solid‐state system at 30 °C, without the need for any carbon additives. This carbon‐free α‐FeSe cathode exhibits rapid Li+/e− transfer properties and limited volume change, thus yielding high reversible capacity (564.6 mAh g−1), long‐term cycling stability (80.3% capacity retention after 800 cycles), high areal loadings (≈26 mg cm−2), and wide‐temperature operability (−20–150 °C). Apart from Fe2+/Fe0 redox reaction, extended cycling or elevated temperature induces partial electrolyte decomposition to generate S‐containing species while triggering a complementary S/S2− redox process. This dual mechanism enables exceptional cyclability (>6000 cycles at 60 °C) and a near‐doubled specific capacity of 956 mAh g−1 at 120 °C. Thereby, as‐fabricated ASSLBs deliver the ultrahigh energy densities (515.3 Wh kg−1/1874.6 Wh L−1 at 30 °C, 1568 Wh kg−1/8310 Wh L−1 at 120 °C), demonstrating the great potential of using iron selenides as the next‐generation cathode for practical applications of ASSLBs.