NaV_1.25Ti_0.75O₄: 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 Mg 2+ 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 NaV 1.25 Ti 0.75 O 4 as a Mg cathode. The Na + ions, which lie in the ion migration pathway, need to be removed or exchanged with Mg 2+ to allow Mg 2+ 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. Mg 2+ ion exchange at moderate temperature (80 °C) resulted in the formation of Na 0.19 Mg 0.41 V 1.25 Ti 0.75 O 4; however, phase transformation was observed when higher temperatures were applied to attempt complete ion exchange. Such phenomena point to the instability of the CF lattice when the tunnel is empty or occupied by a small ion (Mg 2+ ). Thus, while the low migration barrier predicted by computation is partly based on the relative metastability of the theoretical CF-Mg x V 1.25 Ti 0.75 O 4 lattice, the difficulty in stabilizing it also renders the material synthetically nonaccessible, hindering this post-spinel’s application as an electrode material.