First Principles Investigations of Reactions Mechanisms of LixMO4 (M=Fe, Mn, Ni, Co) Electrodes/Electrocatalysts for Hybrid Li-Ion/Li-O2 Cells

We recently proposed a vision for high energy, ‘all-in-one’ electrode/electrocatalyst materials for hybrid Li-ion/Li-O₂ cells [1]. Electrochemical experiments with Li₅FeO₄, which has a defect antifluorite-type structure, have demonstrated a substantially smaller voltage polarization during charge and discharge and hence a higher energy efficiency compared to standard reactions between lithium and oxygen to form Li₂O₂ [2]. Other materials in this class include Li₆MO₄ (M=Mn, Ni, Co) that offer a theoretical capacity of ~1000 mAh/g at ~3 V. The mechanisms by which these materials delithiate and lose oxygen, however, is not well understood.  The results of first principles density functional theory (DFT) calculations to establish the thermodynamic conditions for the extraction of Li/Li+O from Li₅FeO₄ and Li₆MO₄ compounds will be presented. A step-by-step, history-dependent, removal process is followed; the stability of Li and Li+O deficient samples is investigated on the basis of the extraction reaction energies. Various stages of Li/Li+O removal are identified.  Structural changes and electronic structure evolution, as well as computed characterizations of the reactions will also be reported.

[1] M. M. Thackeray, M. K. Y. Chan, L. Trahey, S. Kirklin, and C. Wolverton, Journal of Physical Chemistry Letters, 4, 3607 (2013).
[2] L. Trahey, C. S. Johnson, J. T. Vaughey, S.-H. Kang, L. J. Hardwick, S. A. Freunberger, P. G. Bruce, M. M. Thackeray, Electrochemical and Solid-State Letters, 14, A64 (2011).