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

Abstract

The influence of hexamethylenetetraamine (HMTA) on the morphology of delta-MnO2 and its properties for electrical energy storage are investigated-specifically for ultrathick delta-MnO2 layers in the micron scale. Planar arrays of gold@delta 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 delta-MnO2 sheets seen in the absence of HMTA. Second, the electronic conductivity of the delta-MnO2 is increased by an order of magnitude. Third, (delta-MnO2 prepared in HMTA shows a (001) interlayer spacing that is expanded by approximate to 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.