Li-Fe-P-O2 Phase Diagram from First Principles Calculations - Shyue Ping Ong, Lei Wang, Byoungwoo Kang, and Gerbrand Ceder - Abstract

Oct 26, 2026

Efficient ab initio calculations are used to construct the phase diagram of the quaternary Li-Fe-P-O2 system. Ground-state energies of known compounds are obtained with density functional theory using GGA and DFT+U, while gaseous-phase entropy is included to express stability versus oxidation conditions through the oxygen chemical potential, μO2, linking temperature and oxygen partial pressure. A modified Ellingham diagram incorporates experimental gas entropy, reproducing key reduction and oxidation products. The results guide optimization of LiFePO4 synthesis.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages10

Reading Time25 min

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Introduction

Lithium transition metal phosphates and LiFePO4 background

Motivation for optimizing LiFePO4 synthesis routes

Aliovalent doping studies and conductivity explanations

Motivation for first-principles phase diagram calculation

Chem. Mater. XXXX, xxx, 000–000 A Li-Fe-P-O2 Phase Diagram from First Principles Calculations Shyue Ping Ong,† Lei Wang,† Byoungwoo Kang, and Gerbrand Ceder* Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts AVenue, Cambridge, Massachusetts 02139 ReceiVed August 16, 2007. ReVised Manuscript ReceiVed NoVember 26, 2007 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, µO2, 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 µO2 ) -11.52 eV and the last of the Fe-containing phosphates to be reduced at µO2 ) -16.74 eV. Lower µO2 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 (Fe7(PO4)6 and Fe2 P2O7) commonly observed under moderately reducing conditions during LiFePO4 synthesis and the formation of iron phosphides (Fe2P) 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 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-O2 system and serve as a guide for future experimental efforts in materials processing, in particular, for the optimization of synthesis routes for LiFePO4. 1. Introduction In recent years, lithium transition metal phosphates have emerged as promising cathodes for rechargeable lithium batteries.1–5 Of these, olivine LiFePO4 has garnered the most interest because of its reasonable theoretical capacity of 170 mA h g-1, low materials cost, and low toxicity. Because of its substantial potential, much research effort has been directed toward optimizing synthesis routes for LiFePO4 cathodes. A variety of techniques have been developed to control particle size and morphology as well as improve the electrical conductivity of the electrode through coating with conducting phases (e.g., carbon or metallic iron phosphides) or aliovalent doping. One of the ﬁrst attempts to dope LiFePO4 with aliovalent cations was reported by Chung et * To whom correspondence should be addressed: Tel (617) 253-1581, Fax (617†)T2he- 34utho, ers-aoinltrgcedeibute@mitqually.edt .this work. (1) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Electrochem. Soc. 1997, 144, 1188–1194 . (2) Huang, H.; Yin, S. C.; Nazar, L. F. Electrochem. Solid State Lett. 2001, 4, A170–A172 . (3) Morcrette, M.; Wurm, C.; Masquelier, C. Solid State Sci. 2002, 4, 239–246. (4) Barker, J.; Saidi, M. Y.; Swoyer, J. L. Electrochem. Solid State Lett. 2003, 6, A53–A55 . (5) Morgan, D.; Ceder, G.; Saidi, M. Y.; Barker

FAQs

What computational approach is used to calculate the Li-Fe-P-O2 phase diagram?

The diagram is built using ab initio methods where ground-state energies are calculated with DFT using GGA and the DFT+U extension.

How is temperature and oxygen partial pressure incorporated into the phase diagram?

Only the entropy of gaseous phases is considered, and stability is expressed as a function of oxidation conditions using the oxygen chemical potential, μO2.

Which phases does the study predict for different oxidation environments of LiFePO4?

It predicts LiFePO4 stability over a wide range of oxidation environments, identifies Fe-containing phosphates upon reduction at specific μO2 values, and for Li-deficient or Li-excess compositions predicts iron phosphate/iron phosphide or Li3PO4 and iron oxides accordingly.

Oct 26, 2026

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