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Fast lithium-ion conductors with oxide frameworks are key materials for high performance solidstate rechargeable batteries. This study reveals fast Li+ ion diffusion in the recently discovered pyrochlore-type lithium lanthanum niobium oxyfluoride, Li2–xLa(1+x)/3□(2x–1)/3Nb2O6F (□ = vacancy), using pulsed-field gradient nuclear magnetic resonance (NMR) and impedance measurements. These analyses confirm that fast Li+ ion diffusion is the origin of the high ionic conductivity. Moreover, 7Li and 19F-NMR data suggest that local disorder at the Li+ ion sites facilitate fast diffusion. Chemical shifts of the 19F NMR can be explained by the number of La, Li and vacancies around fluorine. The Arrhenius plot exhibits a slight bending at approximately 200 K. The thermal expansion coefficient also changes from negative to positive at 200 K. These results suggest that Li+ ions in pyrochlore-type oxyfluorides undergo an order–disorder phase
transition. The insights provided by this study into the mechanism of fast Li+ ion diffusion in pyrochlore-type oxyfluorides pave the way for fabricating solid electrolytes with improved performance over conventional solid electrolytes.

Rights:

• Creative Commons BY Attribution 4.0 International
  

Keyword: Solid-state battery, Ionic conductor, Diffusion, Pyrochlore-type oxyfluoride, PFG-NMR

Date published: 2025-06-07

Publisher: Elsevier BV

Journal:

• Solid State Ionics (ISSN: 01672738) vol. 428 116924

Funding:

• National Institute for Materials Science JPMXP1224NM5390
• Government of Japan Ministry of Education Culture Sports Science and Technology JPMXP0219207397
• Japan Society for the Promotion of Science 24H02205

Manuscript type: Publisher's version (Version of record)

MDR DOI:

First published URL: https://doi.org/10.1016/j.ssi.2025.116924

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Updated at: 2025-06-12 16:30:29 +0900

Published on MDR: 2025-06-12 16:25:29 +0900