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

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