Ab initio study of lithium and sodium iron fluorophosphate cathodes for rechargeable batteries

摘要

The cathode materials Li_2FePO_4F and Na_2FePO_4F are studied by means of ab initio calculations. The generalized gradient approximation (GGA) and GGA+U are used to describe the crystal structures, and the electronic and magnetic properties of Li_2FePO_4F, Na_2FePO_4F, LiFePO_4F, and NaFePO_4F. Since only the lattice parameters are known for Li_2FePO_4F, our calculations predict the corresponding atomic positions, and reveal that the iron atom state (Fe~Ⅲ or Fe~Ⅱ) is changed during the reaction, which is of importance to understand the reactions taking place in rechargeable batteries. Finally, our calculated intercalation voltage with the GGA+U approximation is in good agreement with experimental value.