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

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