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.