Supercharging a MnO₂ Nanowire: An Amine-Altered Morphology Retains Capacity at High Rates and Mass Loadings

The influence of hexamethylenetetraamine (HMTA) on the morphology of δ-MnO₂ and its properties for electrical energy storage are investigated-specifically for ultrathick δ-MnO₂ layers in the micron scale. Planar arrays of gold@δ-MnO₂, core@shell nanowires, were prepared by electrodeposition with and without the HMTA and their electrochemical properties were evaluated. HMTA alters the MnO₂ in three ways: First, it creates a more open morphology for the MnO₂ coating, characterized by "petals" with a thickness of 6 to 9 nm, rather than much thinner δ-MnO₂ sheets seen in the absence of HMTA. Second, the electronic conductivity of the δ-MnO₂ is increased by an order of magnitude. Third, δ-MnO₂ prepared in HMTA shows a (001) interlayer spacing that is expanded by ≈30% possibly accelerating Li transport. The net effect of "HTMA doping" is to dramatically improve high rate performance, culminating in an increase in the specific capacity for the thickest MnO₂ shells examined here by a factor of 15 at 100 mV/s.