Density functional studies of olivine-type LiFePO4 and NaFePO4 as positive electrode materials for rechargeable lithium and sodium ion batteries

• Comparative DFT calculations have been performed for LiFePO4 and NaFePO4 cathode materials for Li and Na ion battery.
• Electronic and Li and Na ionic migration in the bulk does not show large difference between LiFePO4 and NaFePO4.
• The formation of interface between Li or Na-rich and -poor phases provides fast Li ionic conduction pathways.

Olivine-type LiFePO4 is a positive electrode material for rechargeable Li ion batteries with high power density (i.e. ability of fast charge–discharge rates). However, its Na alternative, olivine-type NaFePO4, has a low power density as an electrode material for Na ion batteries. To understand the large difference of power density between LiFePO4 and NaFePO4, their ion and electron transport properties are investigated by first-principles density functional theory (DFT). In the present DFT studies, no significant difference is obsereved in electronic migration energies in both bulk LiFePO4 and NaFePO4. On the other hand, the migration energy of sodium ion in NaFePO4 is 0.05 eV higher than that of lithium ion in LiFePO4, which may account for slow kinetics in NaFePO4 electrode. Further studies of the phase stability and alkaline ion migration at the interfaces between the two phases of (Li/Na)FePO4 and FePO4 suggest that the difference in rate performance between LiFePO4 and NaFePO4 is related to the formation of this interface.