Li−Fe−P−O2Phase Diagram from First Principles Calculations.

We present an efficient way to calculate the phase diagram of the quaternary Li−Fe−P−O 2system using ab initio methods. The ground-state energies of all known compounds in the Li−Fe−P−O 2system 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 4to 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 2represents 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) 6and Fe 2P 2O 7) commonly observed under moderately reducing conditions during LiFePO 4synthesis and the formation of iron phosphides (Fe 2P) under highly reducing conditions are consistent with the predictions from our phase diagram. Our diagrams also predict the formation of Li 3PO 4and 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 3Fe 2(PO 4) 3and Fe 2O 3. The predicted carbothermal reduction temperatures for LiFePO 4from 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 2system and serve as a guide for future experimental efforts in materials processing, in particular, for the optimization of synthesis routes for LiFePO 4. [ABSTRACT FROM AUTHOR]