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

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Abstract Olivine-type LiFePO 4 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 NaFePO 4 , has a low power density as an electrode material for Na ion batteries. To understand the large difference of power density between LiFePO 4 and NaFePO 4 , 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 LiFePO 4 and NaFePO 4 . On the other hand, the migration energy of sodium ion in NaFePO 4 is 0.05 eV higher than that of lithium ion in LiFePO 4 , which may account for slow kinetics in NaFePO 4 electrode. Further studies of the phase stability and alkaline ion migration at the interfaces between the two phases of (Li/Na)FePO 4 and FePO 4 suggest that the difference in rate performance between LiFePO 4 and NaFePO 4 is related to the formation of this interface.

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