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

The FeS2 has abundant reserves and a high specific capacity (894 mAh g-1 ), commonly used to fabricate Li-FeS2 primary batteries, like LiMx -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 LiMx -FeS2 thermal batteries. Most importantly, these high-temperature Li-FeS2 solid-state batteries achieve FeS2 conversion to Li2 S 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.