First-Principles Studies on the Structural and Electronic Properties of α-Na 2 FePO 4 F with Strong Antisite Disorder.

Polyanionic Na 2 FePO 4 F is one of the most important cathode materials for sodium-ion batteries. The orthorhombic β-Na 2 FePO 4 F material has been studied extensively and intensively since it was proposed. In this article, a novel monoclinic sodium phosphate fluoride α-Na 2 FePO 4 F is concerned. Kirsanova's experiment showed that Na and Fe ions in α-Na 2 FePO 4 F are prone to antisite, leading to strong antisite disorder. Through first-principle calculations, we show that the steric effect, the magnetic exchange and superexchange interactions between transition-metal cations are shown to be the main driving forces for Na + /Fe 2+ antisite disorder. We first calculated the crystal structures, electronic properties, and cohesive energies of all the 10 antisite phases of α-Na 2 FePO 4 F and β-Na 2 FePO 4 F. Then, we compared the difference charge densities, magnetism, binding energies, and electrostatic potentials of α-Na 2 FePO 4 F and β-Na 2 FePO 4 F materials in the antisite and pristine phases. In α-Na 2 FePO 4 F, the binding energy of the antisite phase with the lowest binding energy is almost degenerate with that of the pristine phase. Moreover, only small differences of the electrostatic potential and the charge density distribution are found between the antisite (with lowest energy) and the pristine phases of α-Na 2 FePO 4 F, which also helped elaborate the facile formation of Na + /Fe 2+ antisite in the α-Na 2 FePO 4 F material. Our research contributes to the understanding of the mechanism of Na + /Fe 2+ antisite and the development of high-performance polyanionic cathode materials.