In this presentation, we will discuss the prospect of oxide materials for Mg2+ de/intercalation. One such cathode candidate is the “calcium ferrite” (CF) –type structured oxide in which a low Mg2+ migration barrier has been predicted by first principles calculations.4 A sodium vanadium-titanium oxide compound with the CF structure was obtained by solid state synthesis. Na+ ions reside in the spacious tunnels of the CF structure in this material, and need to be either removed or replaced by Mg2+ in order to allow Mg2+ de/intercalation. Desodiation (Fig. 1a) was carried out with both electrochemical and chemical methods, resulting in ~ 55% and 85% Na+ extraction, respectively. The chemically desodiated material exhibits ~ 85 mAh g-1 discharge capacity in a Mg full cell (Fig. 1b), showing a promising Mg2+ intercalation capability in the structure, although only half of this is reversible in these preliminary studies. XAS studies suggest that vanadium is the redox centre. We also examined Mg2+ ion exchange for the CF material, finding that ~ half of the Na+ ions were replaced with Mg2+ (Fig. 1a). Interestingly, we observe a phase transformation reaction accompanying major desodiation or ion exchange. This demonstrates a trend towards the rearrangement of the polyhedral framework in the structure when the large ion tunnel is emptied or occupied by the smaller Mg2+ ion, the origins of which will be discussed in this talk. Nevertheless, the promising electrochemistry of the desodiated material upon Mg2+ de/intercalation, as well as the ability to partially exchange Na+ with Mg2+, suggests moderately good Mg2+ diffusion in the CF structure, and sheds light on the possibility of iso-structural compounds as cathode materials for Mg batteries that are stable or metastable upon Mg2+de/intercalation. Other promising cathode materials for Mg batteries, not strictly limited to oxides, will also be the topic of the presentation.
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