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

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, mu(o2), 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 LiFePO4 to be stable over a wide range of oxidation environments, being the first Fe2+-containing phase to appear upon reduction at mu(o2) = -11.52 eV and the last of the Fe-containing phosphates to be reduced at mu(o2) = -16.74 eV. Lower mu(o2) 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 LiFePO4. For Li-deficient stoichiometries, the formation of iron phosphate phases (Fe-7(PO4)(6) and Fe-2 P2O7) Commonly observed under moderately reducing conditions during LiFePO4 synthesis and the formation of iron phosphides (FeP) under highly reducing conditions are consistent with the predictions from our phase diagram. Our diagrams also predict the formation of Li3PO4 and iron oxides for Li-excess stoichiometries under all but the most reducing conditions, again in agreement with experimental observations. For stoichiometric LiFePO4, the phase diagram gives the correct oxidation products of Li3Fe2(PO4)(3) and Fe2O3. The predicted carbothermal reduction temperatures for LiFePO4 from the Ellinoham diagram are also within the range observed in experiments (800-900 degrees 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 LiFePO4.