Density Functional Theory Study of Redox Potential Shifts in LixMnyFe1-yPO4 Battery Electrodes

Simon Loftager, Steen Brian Schougaard, Tejs Vegge, Juan Maria García Lastra*

*Corresponding author for this work

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Olivine-structured LiMPO4 materials (M = Mn, Fe, Co, Ni, or mixtures) exhibit higher redox potentials than their layer oxide counterparts. This is due to the so-called inductive effect in the former, where the inner P–O bonds in the phosphate units make the M–O bond weaker than in the latter. A strategy to further increase the redox potentials in the olivines is to mix two metals. Along these lines, Kobayashi et al. have shown experimentally that Mn2+–Mn3+ and Fe2+–Fe3+ redox potentials approximately shift 0.1 V upon full substitution of Fe by Mn in LixMnyFe1–yPO4. Here, through density functional theory calculations, we found that the average metal–oxygen bond lengths (M = Mn, Fe) increase with increasing Mn content, resulting in a decrease in the covalency of the transition-metal–oxygen interaction. The decrease in the covalency can be linked with good qualitative agreement to the experimentally observed M2+–M3+ voltage-plateau positive shift. Finally, the impact of the Mn-content-dependent voltage plateaus and unit-cell volume on the energy densities of the active compound is discussed.
Original languageEnglish
JournalThe Journal of Physical Chemistry Part C
Issue number1
Pages (from-to)102-109
Number of pages8
Publication statusPublished - 2019


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