Rapid synthesis of MgCoO and MgNiO 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 Mg 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 MgCoO and MgNiO nanocrystals by rapid supercritical fluid processing. The phase transition from spinel to rocksalt during the Mg ion intercalation has been confirmed by high-resolution transmission electron microscopy. The nanosheets of MgNiO rocksalt nanocrystals were controllably synthesized for the first time, which are active materials for magnesium-ion batteries.