| Summary: | <p>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<small><sub>2</sub></small>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<small><sub>4</sub></small>Fe<small><sub>2</sub></small> oxygen coordination with a preference for polar <em>cis</em>-OLi<small><sub>4</sub></small>Fe<small><sub>2</sub></small> over non-polar <em>trans</em>-OLi<small><sub>4</sub></small>Fe<small><sub>2</sub></small> 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 <em>trans</em>-OLi<small><sub>4</sub></small>Fe<small><sub>2</sub></small> oxygen coordination and corresponding long-range crystallographic order. The absence of long-range order in Li<small><sub>2</sub></small>FeSO can therefore be attributed to the relative stability of <em>cis</em>-OLi<small><sub>4</sub></small>Fe<small><sub>2</sub></small> and other non-OLi<small><sub>4</sub></small>Fe<small><sub>2</sub></small> 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 that similar anion-polarisation-directed short-range-ordering may be present in other heterocationic materials that contain cations with different formal charges. Our analysis 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.</p>
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