RESUMEN
Polyanionic Na2FePO4F is one of the most important cathode materials for sodium-ion batteries. The orthorhombic ß-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 ß-Na2FePO4F. Then, we compared the difference charge densities, magnetism, binding energies, and electrostatic potentials of α-Na2FePO4F and ß-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.
RESUMEN
Reducing charge overpotential is of great significance to enhance the efficiency and cyclability of Li-O2 batteries. Here, a dramatically reduced charge overpotential via boron-doped graphene as a catalytic substrate is successfully predicted. By first-principles calculations, from the perspective of reaction thermodynamics and kinetics, the results show that the electrochemical oxidation of the Li2O2+ cation is easier than the chemical oxidation of the neutral Li2O2 molecule, and the oxidation of (Li2O2)0,+ is facilitated by boron-doping in pristine graphene. More importantly, the results reveal the oxidation mechanism of (Li2O2)0,+: two-step dissociation with the LiO2 molecule as a reactive intermediate has advantages over one-step dissociation; the rate-determining step for the dissociation of (Li2O2+)G is the oxygen evolution process, while the lithium removal process is the rate-determining step for the dissociation of (Li2O20)G, (Li2O20)BG, and (Li2O2+)BG.
RESUMEN
The Li-O2 battery is an ideal energy storage device due to its highest theoretical energy density; however, its high charge overpotential limits its practical application. Herein, through ab initio calculations, we systematically investigated the structural and electronic properties of small (Li2O2)nm+ (n = 1, m = 0, 1 and n = 2, m = 0, 1, and 2) clusters and calculated the reaction energies of various decomposition reactions. Results show that the (Li2O2)1 monomer has a low spin, whereas the (Li2O2)2 dimer has a high spin. The analysis of bond length, molecular orbitals, and projected density of states reveals that the interaction of O-O is stronger in the cationic cluster than in the neutral one, whereas the interaction of O-Li is weaker in the cationic cluster than in the neutral one; this facilitates the decomposition of cationic lithium peroxide cluster. Furthermore, the calculated reaction energies indicate that the peroxide lithium decomposition preferentially favors two-step reaction over one-step reaction. Finally, the lowest-energy reaction pathway for the decomposition of (Li2O2)2 dimer was predicted to be (Li2O2)2 â Li2O2 â (Li2O2)+ â LiO2 â O2, and the rate-determining step was predicted to be the first step.
