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9 5 I H I G H L I G H T S 2 0 2 3

PRINCIPAL PUBLICATION AND AUTHORS

Pyrochlore Type Iron Hydroxy Fluorides as Low-Cost Lithium-Ion Cathode Materials for Stationary Energy Storage, J.F. Baumgärtner (a,b), M. Wörle (a), C.P. Guntlin (a), F. Krumeich (a), S. Siegrist (a), V. Vogt (a), D.C. Stoian (c), D. Chernyshov (c), W. van Beek (c), K.V. Kravchyk (a,b), M.V. Kovalenko (a,b), Adv. Mater. 2304158 (2023); https:/doi.org/10.1002/adma.202304158 (a) Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Zürich (Switzerland) (b) Laboratory for Thin Films and Photovoltaics, Empa Swiss Federal Laboratories for Materials Science and Technology, Dübendorf (Switzerland) (c) Swiss Norwegian beamlines, ESRF

REFERENCES

[1] F. Wu, G. Yushin, Energy Environ. Sci. 10, 435 (2017). [2] C. Li et al., J. Am. Chem. Soc. 135, 11425 (2013).

X-ray scattering (SAXS) signals indicated morphological changes in Pyr-IHF particles during the release of disordered water within the channels, which was further confirmed by transmission electron microscopy (TEM). Furthermore, despite a loss in crystallinity during heat-treatment, Pyr-IHF retained its local structure, as confirmed by X-ray absorption spectroscopy (XAS) obtained on BM31 showing highly similar features in the spectra before and after heat-treatment.

Finally, the impact of structural water on the electrochemical performance of Pyr-IHF was studied by preparing a series of cathodes with varying water content and cycling them at different current rates (Figure 73). Pyr-IHF samples with similar porosities with less water content exhibited significantly higher capacities, indicating that that the presence of water inside the Pyr-IHF channels negatively affected both Li-ion storage and diffusion, reminiscent of an intercalation-type mechanism for Li-ion storage.

In summary, this study presents a cost-effective and scalable synthesis of Pyr-IHF with well-defined morphology and narrow particle size distribution. The dissolution-precipitation process enhances Li-ion conducting channels from 1D to 3D. Heat-treatment selectively modifies the morphology and solvent content within the 3D channels. Operando XRD measurements revealed that changes in surface morphology are linked to the removal of solvent molecules from the Pyr-IHF crystal structure during heating. Batteries with heat-treated Pyr-IHF cathodes exhibited exceptional cycling stability, showcasing its potential as a low-cost cathode material. Additionally, removing water of crystallisation from the channels significantly improves capacity, highlighting the role of Pyr-IHF s channels in facilitating Li-ion diffusion. This work provides the first evidence of Li-ion intercalation behaviour in Pyr-IHF, warranting further investigation into structural changes during Li-ion intercalation.

Fig. 73: Influence of morphology and water of crystallisation on the electrochemical performance of Pyr-IHF cathodes. a) Rietveld refinements and TEM micrographs (100 nm scale bars) for the Pyr-IHF samples of different crystalline H2O content. b) Cycling stability of the same Pyr-IHF cathodes. c) Schematic representation of the effect of H2O inside the channels of Pyr-IHF on the Li-ion storage ability.