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-O2 system using ab initio methods. The ground-state energies of all known compounds in the Li-Fe-P-O2 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 t...