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

doi:10.1021/acs.inorgchem.0c01961

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

Inorg Chem. 2020 Nov 16;59(22):16225-16237. doi: 10.1021/acs.inorgchem.0c01961. Epub 2020 Nov 2.

Authors

Maria A Kirsanova¹, Alexey S Akmaev², Dmitry A Aksyonov², Sergey V Ryazantsev², Victoria A Nikitina², Dmitry S Filimonov³, Maxim Avdeev⁴, Artem M Abakumov²

Affiliations

¹ Advanced Imaging Core Facility, Skolkovo Institute of Science and Technology, Bolshoy Boulevard 30, bld. 1, 121205 Moscow, Russia.
² Center for Energy Science and Technology, Skolkovo Institute of Science and Technology, Nobel str. 3, 121205 Moscow, Russia.
³ Department of Chemistry, Moscow State University, 1 Leninskiye Gory, 119991 Moscow, Russia.
⁴ Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, New South Wales 2232, Australia.

PMID: 33137251
DOI: 10.1021/acs.inorgchem.0c01961

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.