Abstract
Manganese oxide, MnO2, excels as a hybrid electrical energy storage material: The manganese centers in MnO2 are capable, of undergoing a reduction from 4+ to 3+ balanced by the intercalation of lithium ions to form LixMnO2 while its conductive surfaces simultaneously store energy as an electrical double layer capacitor: The highest capacitance and power performance for MnO2 has been obtained for ensembles of nanowires that are 200 nm or less in width and many microns in length. Typically such MnO2 nano-wires are attached to a current collector at just one end, and electrical conductivity of the nanowire is therefore required in order to maintain a consistent redox and charge state along its axis. The electrical conductance of the nanowire therefore plays a very important role, and yet this parameter has been measured in few previous studies. In this work, we directly measure the electrical conductance of delta-MnO2 nanowires in situ in M LiClO4, acetonitrile as a function of the equilibrium Li content for nanowires with varying lateral dimensions. This measurement is accomplished using arrays of 200 MnO2 nanowires that are 40-60 nm in height and 275-870 nm in width and which span a 10 mu m gap between two gold contacts. Nanowires of fully oxidized MnO2 are first prepared at +0.60 V vs MSE in acetonitrile. As the equilibrium electrode potential is decreased from 0.60 V to -0.80 V and lithium is intercalated, the electrical:conductivity of MnO2 nanowires in by up to 1 order of magnitude. The measured change in conductivity is dependent on the equilibrium potential, which in turn is related to the Li content, and also depends on the width of nanowires. After doping at -0.80 V vs MSE, the conductivity increases by 30% for a 870 nm wide nano-wire array and 880% for a 275 nm wide nanowire array. TEM investigations implicate the nanowire porosity in this difference.