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.