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

The FeS(2) has abundant reserves and a high specific capacity (894 mAh g(−1)), commonly used to fabricate Li‐FeS(2) primary batteries, like LiM(x)‐FeS(2) thermal batteries (working at ≈500 °C). However, Li–FeS(2) batteries struggle to function as rechargeable batteries due to serious issues such as pulverization and polysulfide shuttling. Herein, highly reversible solid‐state Li‐FeS(2) batteries operating at 300 °C are designed. Molten salt‐based FeS(2) 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)‐FeS(2) thermal batteries. Most importantly, these high‐temperature Li–FeS(2) solid‐state batteries achieve FeS(2) conversion to Li(2)S and Fe at discharge and further back to FeS(2) at charge, unlike room‐temperature Li‐FeS(2) batteries where FeS and S act as oxidation products. Therefore, these new‐type Li‐FeS(2) batteries have a lower operating temperature than Li‐FeS(2) 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.