On the Feasibility of Pairing Pyrochlore Iron(III) Hydroxy Fluoride Cathode with Argyrodite Li6PS5Cl Solid-State Electrolyte for Low-Cost All-Solid-State Batteries

As demand for low-cost, high-energy-density all-solid-state batteries continues to rise, exploring novel cathodes composed of earth-abundant elements is imperative. Iron hydroxy fluorides with the pyrochlore structure (Pyr-IHF) emerge as compelling cathode materials due to abundant natural reserves of their constituent elements, high energy density, and rate capability. In this work, we explore the viability of Pyr-IHF as a cathode material in all-solid-state batteries when paired with argyrodite-type Li₆PS₅Cl (LPSCl) solid-state electrolyte. Our findings show that the Pyr-IHF/LPSCl cathode delivers a high initial charge capacity of 172 mAh g⁻¹ at a 0.1 C rate, with ca. 65 % capacity retention after 50 cycles. Advanced characterization techniques, including focused ion beam-scanning electron microscopy, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, and X-ray absorption spectroscopy, indicate a pronounced redox reaction between Pyr-IHF and LPSCl upon cell preparation, resulting in significant capacity contributions from the sulfur redox of LPSCl decomposition products during electrochemical cycling.