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 .

Tailoring Native Defects in LiFePO4: Insights from First-Principles Calculations

“…For example, the calculated formation enthalpy of LiFePO 4 at T =0 K (with respect to its constituents) is −18.853 eV per formula unit (f.u. ), 41 almost identical to that (of −18.882 eV/f.u.) obtained in our calculations.…”

“…Figure 1 shows the slice of the (µ Li , µ Fe , µ O2 ) polyhedron in the µ O2 =−4.59 eV plane, constructed with the calculated formation enthalpies (taken from Ref. 41 Chemical-potential diagram for LiFePO4 at µO 2 =−4.59 eV. The µO 2 axis extends out of the page.…”

“…Let us assume, for example, Li-deficient environments and vary µ O2 from −3.03 (where LiFePO 4 first starts to form) to −8.25 eV (where it ceases to form). 41 This amounts to choosing different cuts along the µ O2 axis in Fig. 1 to give different two-dimensional polygons of LiFePO 4 stability.…”

Evaluation of Tavorite-Structured Cathode Materials for Lithium-Ion Batteries Using High-Throughput Computing

“…In almost all cases the difference is less than 10 meV / atom. To assess the thermodynamic stability and oxygen chemical potential of each tavorite-structured compound in this paper, we compared the energy of each compound to a thermodynamic hull constructed from the compounds in our database according to the methodology outlined by Ong et al 30 Stability for a given compound is evaluated against any linear combination of compounds in the database that have the same averaged composition. To evaluate thermodynamic stability against oxygen gas, we fit the oxygen chemical potential to experimental data as outlined by Wang et al 31 and used a reference temperature of 298 K and a partial pressure of 1 atm.…”

Ab initiostudy of the stabilities of and mechanism of superionic transport in lithium-rich antiperovskites

“…To investigate the chemical stability of antiperovskites under extreme Li insertion and extraction conditions, we constructed the lithium grand potential phase diagram of Li-O-Cl, Li-O-Br, and Li-O-Cl-Br using the method outlined by Ong et al 27 . By studying the phase evolution of the LOC, LOB, and LOCB compounds with the changing chemical potential (µ Li ), we determine the equilibrium phases at bulk metallic lithium anode and charged cathode.…”