On the Utility of Spinel Oxide Hosts for Magnesium-Ion Batteries

There is immense interest to develop Mg-ion batteries, but finding suitable cathode materials has been a challenge. The spinel structure has many advantages for ion insertion and has been successfully used in Li-ion batteries. We present here findings on the attempts to extract Mg from MgMn₂O₄-based spinels with acid (H₂SO₄) and with NO₂BF₄. The acid treatment was able to fully remove all Mg from MgMn₂O₄ by following a mechanism involving the disproportionation of Mn³⁺, and the extraction rate decreased with increasing cation disorder. Samples with additional Mg²⁺ ions in the octahedral sites (e.g., Mg_1.1Mn_1.9O₄ and Mg_1.5Mn_1.5O₄) also exhibit complete or near complete demagnesiation due to an additional mechanism involving ion exchange of Mg²⁺ by H⁺, but no Mg could be extracted from MgMnAlO₄ due to the disruption of Mn–Mn interaction/contact across shared octahedral edges. In contrast, no Mg could be extracted with the oxidizing agent NO₂BF₄ from MgMn₂O₄ or Mg_1.5Mn_1.5O₄ as the electrostatic repulsion between the divalent Mg²⁺ ions prevents Mg²⁺ diffusion through the 16c octahedral sites, unlike Li⁺ diffusion, suggesting that spinels may not serve as potential hosts for Mg-ion batteries. The ability to extract Mg with acid in contrast to that with NO₂BF₄ is attributed to Mn dissolution from the lattice and the consequent reduction in electrostatic repulsion. The findings could provide insights toward the design of Mg hosts for Mg-ion batteries