Exceptional Flux Growth and Chemical Transformation of Metastable Orthorhombic LiMnO2 Cuboids into Hierarchically-Structured Porous H1.6Mn1.6O4 Rods as Li Ion Sieves

Selective lithium uptake from seawater and lake brine is an important challenge in energy and environmental science. H 1.6 Mn 1.6 O 4 with pseudospinel-type structure is a highly selective adsorbent for Li ions, but it is difficult to prepare large, highly crystalline H 1.6 Mn 1.6 O 4 crystals with porous structure due to its thermodynamic metastability. Herein we demonstrate simple chemical processes that transform flux-grown, idiomorphic orthorhombic LiMnO 2 (o-LiMnO 2 ) cuboids of micrometer size into hierarchically structured H 1.6 Mn 1.6 O 4 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-LiMnO 2 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-LiMnO 2 cuboids. The metastability of o-LiMnO 2 is confirmed by ab initio density functional theory calculation in comparison with other lithium manganates such as LiMn 2 O 4 and Li 2 MnO 3 . The successive calcination and acid treatment allow the transformation of o-LiMnO 2 into H 1.6 Mn 1.6 O 4 rods with porous structure. The resultant H 1.6 Mn 1.6 O 4 shows high Li + adsorption capacity (∼5.6 mmol g –1 ), high Li + /Na + selectivity, and good durability compared with existing H 1.6 Mn 1.6 O 4 adsorbents.