Abstract
Reversible intercalation reactions provide the basis for modern battery electrodes. Despite decades of exploration of electrode materials, the potential for materials in the nonoxide chemical space with regards to intercalation chemistry is vast and rather untested. Transition metal fluorides stand out as an obvious target. To this end, we report herein a new family of iron fluoride-based perovskite cathode materials AxK1–xFeF3 (A = Li, Na). By starting with KFeF3, approximately 75% of K+ ions were subsequently replaced by Li+ and Na+ through electrochemical means. X-ray diffraction and Fe X-ray absorption spectroscopy confirmed the existence of intercalation of alkali metal ions in the perovskite structure, which is associated with the Fe2+/3+ redox couple. A computational study by density functional theory showed agreement with the structural and electrochemical data obtained experimentally, which suggested the possibility of fluoride-based materials as potential intercalation electrodes. This study inc...
Highlights
The study of the intercalation chemistry of alkali ions in transition metal-containing hosts has fascinated scientists for the past 40 years
Measurements at the Fe L-edge are very sensitive to changes in valence electronic structure since the 2p excitation probes the Fe 3d states, which are directly involved in the redox reactions in batteries.[52]
A small pre-edge feature can be observed in the spectra of both the materials in the pristine and oxidized state due to the slightly distorted FeF6 octahedra in the structures
Summary
The study of the intercalation chemistry of alkali ions in transition metal-containing hosts has fascinated scientists for the past 40 years. The ability to intercalate large amounts of lithium enabled the advent of modern Li-ion batteries. It is at the core of many efforts to develop next-generation energy storage solutions to enable the integration of renewable energy sources for a modern sustainable society. With major challenges arising from the difficulties in achieving high energy density and extended life at a reasonable cost, new battery chemistries for electric vehicles and smart grids applications are a field of intensive research.[1] Searching for new electrode materials to replace the oxide cathodes used currently in Li-ion technologies is the fastest way toward leaps in storage capability. The highly ionic nature of the transition metal-fluoride bonds elevates the redox potential of the material and renders fluorides interesting in the search for an intercalation cathode that enables devices with high energy density. A few binary iron fluoride phases FeF3·xH2O)[2−7] and mixed-anion compounds ((FFeeOF3F, )F8−eF102,haanvde been studied as active materials for electrochemical energy storage.[2,5,11−15] most of these phases were found to react through conversion mechanisms, where the fluoride compounds are reduced all the way to metallic particles and
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