Modulation of the Oxidation End‐Product Toward Polysulfides‐Free and Sustainable Lithium‐Pyrite Thermal Batteries

Abstract The FeS₂ has abundant reserves and a high specific capacity (894 mAh g⁻¹), commonly used to fabricate Li‐FeS₂ primary batteries, like LiM_x‐FeS₂ thermal batteries (working at ≈500 °C). However, Li–FeS₂ batteries struggle to function as rechargeable batteries due to serious issues such as pulverization and polysulfide shuttling. Herein, highly reversible solid‐state Li‐FeS₂ batteries operating at 300 °C are designed. Molten salt‐based FeS₂ slurry cathodes address the notorious electrode pulverization problem by encapsulating pulverized particles in time with e⁻ and Li⁺ flow conductors. In addition, the solid electrolyte LLZTO tube serves as a hard separator and fast Li⁺ channel, effectively separating the molten electrodes to construct a liquid–solid–liquid structure instead of the solid–liquid–solid structure of LiM_x‐FeS₂ thermal batteries. Most importantly, these high‐temperature Li–FeS₂ solid‐state batteries achieve FeS₂ conversion to Li₂S and Fe at discharge and further back to FeS₂ at charge, unlike room‐temperature Li‐FeS₂ batteries where FeS and S act as oxidation products. Therefore, these new‐type Li‐FeS₂ batteries have a lower operating temperature than Li‐FeS₂ thermal batteries and perform highly reversible electrochemical reactions, which can be cycled stably up to 2000 times with a high specific capacity of ≈750 mAh g⁻¹ in the prototype batteries.