First-Principles Studies on the Structural and Electronic Properties of α-Na₂FePO₄F with Strong Antisite Disorder

Polyanionic Na₂FePO₄F is one of the most important cathode materials for sodium-ion batteries. The orthorhombic β-Na₂FePO₄F material has been studied extensively and intensively since it was proposed. In this article, a novel monoclinic sodium phosphate fluoride α-Na₂FePO₄F is concerned. Kirsanova's experiment showed that Na and Fe ions in α-Na₂FePO₄F 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⁺/Fe²⁺ antisite disorder. We first calculated the crystal structures, electronic properties, and cohesive energies of all the 10 antisite phases of α-Na₂FePO₄F and β-Na₂FePO₄F. Then, we compared the difference charge densities, magnetism, binding energies, and electrostatic potentials of α-Na₂FePO₄F and β-Na₂FePO₄F materials in the antisite and pristine phases. In α-Na₂FePO₄F, 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-Na₂FePO₄F, which also helped elaborate the facile formation of Na⁺/Fe²⁺ antisite in the α-Na₂FePO₄F material. Our research contributes to the understanding of the mechanism of Na⁺/Fe²⁺ antisite and the development of high-performance polyanionic cathode materials.