Li−Fe−P−O2 Phase Diagram from First Principles Calculations

Abstract

We present an efficient way to calculate the phase diagram of the quaternary Li−Fe−P−O 2 system using ab initio methods. The ground-state energies of all known compounds in the Li−Fe−P−O 2 system were calculated using the generalized gradient approximation (GGA) approximation to density functional theory (DFT) and the DFT+U extension to it. Considering only the entropy of gaseous phases, the phase diagram was constructed as a function of oxidation conditions, with the oxygen chemical potential, μ O 2, capturing both temperature and oxygen partial pressure dependence. A modified Ellingham diagram was also developed by incorporating the experimental entropy data of gaseous phases. The phase diagram shows LiFePO 4 to be stable over a wide range of oxidation environments, being the first Fe 2+ -containing phase to appear upon reduction at μ O 2 = −11.52 eV and the last of the Fe-containing phosphates to be reduced at μ O 2 = −16.74 eV. Lower μ O 2 represents more reducing conditions, which generally correspond to higher temperatures and/or lower oxygen partial pressures and/or the presence of reducing agents. The predicted phase relations and reduction conditions compare well to experimental findings on stoichiometric and Li-off-stoichiometric LiFePO 4 . For Li-deficient stoichiometries, the formation of iron phosphate phases (Fe 7 (PO 4 ) 6 and Fe 2 P 2 O 7 ) commonly observed under moderately reducing conditions during LiFePO 4 synthesis and the formation of iron phosphides (Fe 2 P) under highly reducing conditions are consistent with the predictions from our phase diagram. Our diagrams also predict the formation of Li 3 PO 4 and iron oxides for Li-excess stoichiometries under all but the most reducing conditions, again in agreement with experimental observations. For stoichiometric LiFePO 4, the phase diagram gives the correct oxidation products of Li 3 Fe 2 (PO 4 ) 3 and Fe 2 O 3 . The predicted carbothermal reduction temperatures for LiFePO 4 from the Ellingham diagram are also within the range observed in experiments (800–900 °C). The diagrams developed provide a better understanding of phase relations within the Li−Fe−P−O 2 system and serve as a guide for future experimental efforts in materials processing, in particular, for the optimization of synthesis routes for LiFePO 4 .