Resolving the Influence of Structural Water on Li-Ion Diffusion in Pyrochlore Iron Hydroxy Fluoride Cathodes

Lithium-ion batteries composed of cost-effective cathode materials exhibit substantial potential for large-scale stationary storage of electricity. Iron (III) fluorides are appealing low-cost energy storage materials due to the virtually unlimited supply of the constituting elements and high energy densities.¹ The pyrochlore modification is of particular interest because its 3D interconnected channels may potentially enable fast Li-ion diffusion.² However, the prohibitively high cost of synthesis and cathode architecture currently prevent their commercial use in low-cost Li-ion batteries. Moreover, amorphization upon prolonged cycling makes structural characterization of the Li-ion intercalation behavior challenging. Inspired by the synthesis of porous materials, such as metal organic frameworks, zeolithes and Prussian blue analogues, which often employ dissolved precursor building blocks that condense into the desired porous structure in the presence of a suitable templating agent, we reasoned that a similar approach could be used to synthesize the pyrochlore type structure at ambient conditions.<br/>Herein, we extend this synthetic paradigm to access pyrochlore iron (III) hydroxy fluoride (Pyr-IHF) from soluble iron (III) fluoride precursors.³ This facile dissolution-precipitation synthesis of Pyr-IHF allows the production at low costs (10-20 $ kg^-1), competitive with Lithium iron phosphate. Guided by <i>operando</i> X-ray diffraction experiments, we selectively vary the solvent content inside the channels of Pyr-IHF by heat-treatment to study the effect of structural water on the Li-ion diffusion within the channels of Pyr-IHF. Rate capability tests of Pyr-IHF cathodes of different solvent content confirm this hypothesis, thereby providing the first experimental evidence for Li-ion diffusion occurring through the 3D channels of Pyr-IHF. Without the need for elaborate cathode designs, we demonstrate outstanding capacity retention of &gt;80% after 600 cycles at high current densities of 1 A g^-1.<br/><br/><br/>1. Wu, F. & Yushin, G. Conversion cathodes for rechargeable lithium and lithium-ion batteries. <i>Energy Environ. Sci. </i><b>10, </b>435–459 (2017).<br/>2. Li, C.<i> et al. </i>An FeF₃* 0.5 H₂O polytype: a microporous framework compound with intersecting tunnels for Li and Na batteries. <i>J. Am. Chem. Soc. </i><b>135, </b>11425–11428 (2013).<br/>3. Baumgärtner, J. F.<i> et al. </i>Dissolution Precipitation Synthesis of Pyrochlore Type Iron Hydroxy Fluoride for Low-Cost Lithium-Ion Batteries. Manuscript Submitted.