Monoclinic α-Na2FePO4F with Strong Antisite Disorder and Enhanced Na+ Diffusion

Maria A. Kirsanova, Alexey S. Akmaev, Dmitry A. Aksyonov, Sergey V. Ryazantsev, Victoria A. Nikitina, Dmitry S. Filimonov, Maxim Avdeev, Artem M. Abakumov

Research output: Contribution to journalArticlepeer-review

3 Citations (Scopus)

Abstract

A new monoclinic α-polymorph of the Na2FePO4F fluoride-phosphate has been directly synthesized via a hydrothermal method for application in metal-ion batteries. The crystal structure of the as-prepared α-Na2FePO4F studied with powder X-ray and neutron diffraction (P21/c, a = 13.6753(10) Å, b = 5.2503(2) Å, c = 13.7202(8) Å, β = 120.230(4)°) demonstrates strong antisite disorder between the Na and Fe atoms. As revealed with DFT-based calculations, α-Na2FePO4F has low migration barriers for Na+ along the main pathway parallel to the b axis, and an additional diffusion bypass allowing the Na+ cations to go around the Na/Fe antisite defects. These results corroborate with the extremely high experimental Na-ion diffusion coefficient of (1-5)·10-11 cm2·s-1, which is 2 orders of magnitude higher than that for the orthorhombic β-polymorph ((5-10)·10-13 cm2·s-1). Being tested as a cathode material in Na- and Li-ion battery cells, monoclinic α-Na2FePO4F exhibits a reversible specific capacity of 90 and 80 mAh g-1, respectively.

Original languageEnglish
Pages (from-to)16225-16237
Number of pages13
JournalInorganic Chemistry
Volume59
Issue number22
DOIs
Publication statusPublished - 16 Nov 2020

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