First-principles study of olivine AFePO4 (A = Li, Na) as a positive electrode for lithium-ion and sodium-ion batteries

In this paper, we present the first principles of calculation on the structural and electronic stabilities of the olivine LiFePO4 and NaFePO4, using density functional theory (DFT). These materials are promising positive electrodes for lithium and sodium rechargeable batteries. The equilibrium lattice constants obtained by performing a complete optimization of the structure for all the LiFePO4 and NaFePO4 structures were found to be in good agreement with the available experimental and theoretical data to within 5%. In addition, the electronic properties show that the 3d and 2p orbitals of Fe and O contribute significantly to the Fermi level LiFePO4 and NaFePO4. We found that LiFePO4 and NaFePO4 show intercalation potentials of 3.37 V and 2.75 V, which compare well with experimental and previous calculations at 4% and 5%, respectively. On the other hand, the migration energy of sodium ion in NaFePO4 is 0.78 eV higher than that of lithium-ion in LiFePO4 (0.55 eV), this difference in migration energy could potentially explain the slower kinetics observed in the NaFePO4 electrode compared to the LiFePO4 electrode.