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

A new monoclinic alpha-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 alpha-Na2FePO4F studied with powder X-ray and neutron diffraction (P2(1)/c, a = 13.6753(10) angstrom, b = 5.2503(2) angstrom, c = 13.7202(8) angstrom, beta = 120.230(4)degrees) demonstrates strong antisite disorder between the Na and Fe atoms. As revealed with DFT-based calculations, alpha-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) cm(2).s(-1), which is 2 orders of magnitude higher than that for the orthorhombic beta-polymorph ((5(-10)).10(-13) cm(2).s(-1)). Being tested as a cathode material in Na- and Li-ion battery cells, monoclinic alpha-Na2FePO4F exhibits a reversible specific capacity of 90 and 80 mAh g(-1), respectively.