NaV1.25Ti0.75O4: A Potential Post-Spinel Cathode Material for Mg Batteries

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...