Rapid synthesis of MgCo₂O₄ and Mg_2/3Ni_4/3O₂ nanocrystals in supercritical fluid for Mg-ion batteries

Magnesium metal complex oxides are potential electrode materials for magnesium ion batteries with high specific capacities. However, the strong electrostatic interaction between Mg²⁺ and the host lattice due to its divalency induces slow intercalation kinetics of Mg ions within the crystal lattices. Thus, nanocrystalline particles with shortened Mg ion diffusion distance enable the insertion/extraction of Mg ions and improve the specific capacities of the batteries. Herein, we report the facile rapid production of crystalline MgCo₂O₄ and Mg_2/3Ni_4/3O₂ nanocrystals by rapid supercritical fluid processing. The phase transition from spinel to rocksalt during the Mg²⁺ ion intercalation has been confirmed by high-resolution transmission electron microscopy. The nanosheets of Mg_2/3Ni_4/3O₂ rocksalt nanocrystals were controllably synthesized for the first time, which are active materials for magnesium-ion batteries.