Synthesis and Mg2+ deintercalation in manganese spinel nanocrystals

Transition metal oxides are promising materials as cathodes for divalent batteries with high capacity under high voltage. A major challenge is the sluggish kinetics of intercalation of divalent cations into the oxide host. Tailoring of the size, composition and crystal structure of oxides is a necessary strategy to alleviate these barriers, which challenges our control of their synthesis. Here, we selectively synthesize spinel-type MgxMn3-xO4 nanocrystals with different Mg/Mn ratios and investigate their ability to electrochemically shuttle Mg2+ ions. Crystal-chemical characterization of the reaction outcomes was conducted with X-ray diffraction, X-ray absorption spectroscopy, electron microscopy, and elemental analysis. Both 5 ​nm thick MgMn2O4 nanosheets and 10 ​nm Mg0.41Mn2.59O4 nanocubes underwent reversible Mg2+ deintercalation, yet no obvious reaction was observed in 60 ​nm Mg1.2Mn1.8O4 nanocubes. Our results suggest that both the size of the spinel nanocrystals and the Mg/Mn ratios play a role in the observed behaviour. The advances in the synthesis of spinel oxide nanocrystals achieved, and their correlation with Mg2+ deintercalation, pave the way toward the precise synthesis of multivalent cathode materials that fundamentally overcome barriers to practical application.