Lithium-desorption mechanism in LiMn2O4, Li1.33Mn1.67O4, and Li1.6Mn1.6O4 according to precisely controlled acid treatment and density functional theory calculations

Abstract

Spinel-type lithium manganese oxides (LMOs) are the most promising lithium-adsorption materials. LMOs can be in the form of solid solutions of LiMn₂O₄, Li_1.33Mn_1.67O₄, and Li_1.6Mn_1.6O₄. However, uncertainty about the lithium-desorption mechanism restricts material development. The synthesis of intermediate products of spinel-type LMOs during acid treatment is important to investigate such a mechanism. In this work, precisely controlled acid treatment experiments were performed to successfully obtain the intermediate products of LiMn₂O₄, Li_1.33Mn_1.67O₄, and Li_1.6Mn_1.6O₄. For LiMn₂O₄, lithium was desorbed with the dissolution of manganese. For Li_1.33Mn_1.67O₄ and Li_1.6Mn_1.6O₄, the lithium-desorption mechanism was an ion-exchange reaction, where lithium ions (Li⁺) in the 8a sites were prioritized for the ion-exchange reaction compared with Li⁺ in the 16d sites. Density functional theory (DFT) calculation results confirmed the experimental results and explained that the prioritization of Li⁺ in the 8a sites arose from its lower reaction-energy barrier, thereby revealing that Li⁺ preferred to move to the nearest vacant 8a sites when lithium was desorbed because of the lower diffusion-energy barrier. This work is the first to clarify the lithium-desorption mechanism through experiments and DFT calculations, and thus lays a foundation for the further exploitation of LMOs.