"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.