Pyrochlore-Type Iron Hydroxy Fluorides as Low-Cost Lithium-Ion Cathode Materials for Stationary Energy Storage

Pyrochlore-type iron (III) hydroxy fluorides (Pyr-IHF) are appealing low-cost stationary energy storage materials due to the virtually unlimited supply of their constituent elements, their high energy densities, and fast Li-ion diffusion. However, the prohibitively high costs of synthesis and cathode architecture currently prevent their commercial use in low-cost Li-ion batteries. Herein, a facile and cost-effective dissolution-precipitation synthesis of Pyr-IHF from soluble iron (III) fluoride precursors is presented. High capacity retention by synthesized Pyr-IHF of >80% after 600 cycles at a high current density of 1 A g^-1 is obtained, without elaborate electrode engineering. Operando synchrotron X-ray diffraction guides the selective synthesis of Pyr-IHF such that different water contents can be tested for their effect on the rate capability. Li-ion diffusion is found to occur in the 3D hexagonal channels of Pyr-IHF, formed by corner-sharing FeF_6-x (OH)_x octahedra.