Vacancy Ordering in Layered Metal Oxide Na-Ion Battery Cathodes

Current state of the art Na-ion battery cathodes are selected from the broad chemical space of layered first row transition metal (TM) oxides. Unlike their lithium ion counterparts, seven first row layered TM oxides can intercalate Na ions reversibly. Their voltage curves also indicate significant and numerous reversible phase transformations during electrochemical cycling. These transformations are not yet fully understood, but arise from Na-ion vacancy ordering and metal oxide slab glide.

In this study, we investigate the nature of vacancy ordering transformations within the O3 lattice framework. We generate predicted electrochemical voltage curves for each of the Na-ion intercalating layered single TM oxides using a high-throughput framework of density functional theory (DFT) calculations. We determine a set of vacancy ordered phases appearing as ground states in seven O3-Na_xMO₂systems (M = Ti, V, Cr, Mn, Fe, Co, Ni) and investigate the effect on phase stability of ordering interactions between adjacent sodium layers.

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