Monoclinic α-Na2FePO4F with strong antisite disorder and enhanced Na+ diffusion
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Date
2020-11-02
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Publisher
American Chemical Society
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. © 2020 American Chemical Society
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Keywords
Crystal structure, Sodium, Electrochemical cells, Monoclinic lattices, Neutron diffraction, X-ray diffraction
Citation
Kirsanova, M. A., Akmaev, A. S., Aksyonov, D. A., Ryazantsev, S. V., Nikitina, V. A., Filimonov, D. S., Avdeev, M., & Abakumov, A. M. (2020). Monoclinic α-Na2FePO4F with strong antisite disorder and enhanced Na+ diffusion. Inorganic Chemistry, 59(22), 16225-16237. doi:10.1021/acs.inorgchem.0c01961