Exceptional Flux Growth and Chemical Transformation of Metastable Orthorhombic LiMnO₂ Cuboids into Hierarchically-Structured Porous H_1.6Mn_1.6O₄ Rods as Li Ion Sieves

Selective lithium uptake from seawater and lake brine is an important challenge in energy and environmental science. H_1.6Mn_1.6O₄ with pseudospinel-type structure is a highly selective adsorbent for Li ions, but it is difficult to prepare large, highly crystalline H_1.6Mn_1.6O₄ crystals with porous structure due to its thermodynamic metastability. Herein we demonstrate simple chemical processes that transform flux-grown, idiomorphic orthorhombic LiMnO₂ (o-LiMnO₂) cuboids of micrometer size into hierarchically structured H_1.6Mn_1.6O₄ rods. We have optimized the flux growth conditions such as the Mn source, holding temperature, and solute concentration in order to yield large, single-phase o-LiMnO₂ particles. The use of MnO under very low solute concentration (1 mol %) and high temperature (1000 °C) is critical to obtaining the single-phase, idiomorphic o-LiMnO₂ cuboids. The metastability of o-LiMnO₂ is confirmed by ab initio density functional theory calculation in comparison with other lithium manganates such as LiMn₂O₄ and Li₂MnO₃. The successive calcination and acid treatment allow the transformation of o-LiMnO₂ into H_1.6Mn_1.6O₄ rods with porous structure. The resultant H_1.6Mn_1.6O₄ shows high Li⁺ adsorption capacity (∼5.6 mmol g^-1), high Li⁺/Na⁺ selectivity, and good durability compared with existing H_1.6Mn_1.6O₄ adsorbents.