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arXiv · 2212.13485

Anion-polarisation-directed short-range-order in antiperovskite Li$_2$FeSO

Samuel W. Coles·Viktoria Falkowski·Harry S. Geddes·Gabriel E. Pérez·Samuel G. Booth·Alexander G. Squires·Conn O'Rourke·Kit McColl·Andrew L. Goodwin·Serena A. Cussen·Simon J. Clarke·M. Saiful Islam·Benjamin J. Morgan
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Abstract

Short-range ordering in cation-disordered cathodes can have a significant effect on their electrochemical properties. Here, we characterise the cation short-range order in the antiperovskite cathode material Li$_2$FeSO, using density functional theory, Monte Carlo simulations, and synchrotron X-ray pair-distribution-function data. We predict partial short-range cation-ordering, characterised by favourable OLi$_4$Fe$_2$ oxygen coordination with a preference for polar cis-OLi$_4$Fe$_2$ over non-polar trans-OLi$_4$Fe$_2$ configurations. This preference for polar cation configurations produces long-range disorder, in agreement with experimental data. The predicted short-range-order preference contrasts with that for a simple point-charge model, which instead predicts preferential trans-OLi$_4$Fe$_2$ oxygen coordination and corresponding long-range crystallographic order. The absence of long-range order in Li$_2$FeSO can therefore be attributed to the relative stability of cis-OLi$_4$Fe$_2$ and other non-OLi$_4$Fe$_2$ oxygen-coordination motifs. We show that this effect is associated with the polarisation of oxide and sulfide anions in polar coordination environments, which stabilises these polar short-range cation orderings. We propose similar anion-polarisation-directed short-range-ordering may be present in other heterocationic materials that contain cations with different formal charges. Our analysis also illustrates the limitations of using simple point-charge models to predict the structure of cation-disordered materials, where other factors, such as anion polarisation, may play a critical role in directing both short- and long-range structural correlations.

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Samuel W. Coles, Viktoria Falkowski, Harry S. Geddes, Gabriel E. Pérez, Samuel G. Booth, Alexander G. Squires, Conn O'Rourke, Kit McColl, Andrew L. Goodwin, Serena A. Cussen, Simon J. Clarke, M. Saiful Islam, Benjamin J. Morgan. 2022-12-27. Anion-polarisation-directed short-range-order in antiperovskite Li$_2$FeSO. https://doi.org/10.1039/d2ta10037a

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