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September 10, 2025

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

Key Points

This cathode achieves an impressive capacity retention of 80.3% after 800 cycles, showcasing long-term stability.
The iron selenide cathodes deliver energy densities of 515.3 Wh kg −1 at 30 °C and 1568 Wh kg −1 at 120 °C, indicating substantial potential.
The dual redox mechanisms, including Fe 2+ /Fe 0 and S/S 2 − reactions, enhance cyclability allowing over 6000 cycles at elevated temperatures.
The carbon-free nature of α-FeSe facilitates rapid ion transfer, leading to a reversible capacity of 564.6 mAh g −1.

Abstract

Abstract 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 Fe 2+ /Fe 0 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 Fe 2+ /Fe 0 redox reaction, extended cycling or elevated temperature induces partial electrolyte decomposition to generate S‐containing species while triggering a complementary S/S 2 − 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.

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