Magnesium Todorokite: Influence of Morphology on Electrochemistry in Lithium, Sodium and Magnesium Based Batteries

Samples of Mg-todorokite with mixed nanosheet & nanowire (referred to as nanosheets for the sake of simplicity) and pure nanowire morphology were prepared to probe the morphological impact on battery relevant electrochemistry. The samples had similar physicochemical properties where the Mg-todorokite nanosheets had a composition of Mg0.20MnO2 with a crystallite size of 9 nm and water content of 0.29 moles of H2O per formula unit, while the nanowires had a composition of Mg0.19MnO2 with a crystallite size of 14 nm and water content of 0.28. The electrochemistry of the Mg-todorokite materials was evaluated using cyclic voltammetry, galvanostatic cycling, and rate capability testing. The Mg-todorokite nanosheets showed higher discharge capacity than the nanowires in lithium, sodium and magnesium-based electrolytes. Notably, the nanowire materials exhibited excellent cycling stability compared to the nanosheets in lithium and sodium-based batteries. X-ray absorption spectroscopy (XAS) of lithiated samples suggests that Mn in the nanosheet containing material was more highly reduced to Mn3+ after the 1st full discharge explaining the higher initial capacities observed. However, a MnO-like structural distortion was observed after extended cycling in the nanosheet material consistent with its poorer capacity retention.