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

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.