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Oxides with the CaFe2O4-type structure have been predicted as being suitable hosts for reactions of intercalation of light cations such as Li and Mg because of their favorable cationic diffusion. A...

Oxides undergoing reversible electrochemical cycling of Mg2+ ions would enable novel battery concepts beyond Li+, capable of storing large amounts of energy. However, materials showing this chemical reactivity are scarce. Suitable candidates require small particles to shorten transport lengths, together with chemically complex structures that promote cation mobility, such as spinel. These goals pose a challenge for materials chemists. Here, nanocrystals of spinel-type Mg0.5Mn2.5O4 were prepared using colloidal synthesis, and their electrochemical activity is presented. Cycling in an aqueous Mg2+ electrolyte led to a reversible transformation between a reduced spinel and an oxidized layered framework. This reaction involves large amounts of capacity because of the full oxidation to Mn4+, through the extraction of both Mg2+ and, in the first cycle, Mn2+ ions. Re-formation of the spinel upon reduction resulted in enrichment with Mg2+, indicating that its insertion is more favorable than that of Mn2+. Incorpo...

Rechargeable Mg batteries are promising candidates for high energy density storage in theory, when a Mg metal anode is combined with an oxide cathode material. Despite the widely observed sluggish Mg2+ diffusion in most oxide lattices, recent first-principles calculations predicted low diffusion barriers in the calcium ferrite (CF)-type post-spinel structures. In the present work, we experimentally examine the prospect of CF-type NaV1.25Ti0.75O4 as a Mg cathode. The Na+ ions, which lie in the ion migration pathway, need to be removed or exchanged with Mg2+ to allow Mg2+ de/intercalation. Partial desodiation was achieved through chemical and electrochemical methods, as proven by X-ray diffraction and X-ray absorption spectroscopy, but deep desodiation was accompanied by partial amorphization of the material. Mg2+ ion exchange at moderate temperature (80 °C) resulted in the formation of Na0.19Mg0.41V1.25Ti0.75O4; however, phase transformation was observed when higher temperatures were applied to attempt com...