Composites of sodium manganese oxides with enhanced electrochemical performance for sodium-ion batteries: Tailoring properties via controlling microstructure

Composites of Na0.44MnO2, Na0.7MnO2.05, and Na0.91MnO2 were synthesized by facile solid-state reaction, ball milling, and annealing methods. Two different composites of identical overall composition but drastically different morphologies and microstructures were synthesized. A composite of a hierarchical porous microstructure with primary and secondary particles (i.e., a “meatball-like” microstructure) achieved an excellent stable capacity of 126 mA h g-1 after 100 cycles. The rate capability of the composite could be dramatically enhanced by another round of high-energy ball milling and reannealing; subsequently, a composite that was made up of irregular rods was obtained, for which the capacity was improved by more than 230% to achieve ~53 mA h g-1 at a particularly high discharge rate of 50C. This study demonstrated the feasibility of tailoring the electrochemical performance of electrode materials by simply changing their microstructures via facile ball milling and heat treatments, which can be particularly useful for optimizing composite electrodes for sodium-ion batteries.