Overall conductivity and NCL-type relaxation behavior in nanocrystalline sodium peroxide Na2O2—Consequences for Na-oxygen batteries

Metal air batteries are considered as promising candidates for room-temperature batteries with high-energy densities. On discharge, atmospheric oxygen is reduced at the positive electrode which, in the ideal case, forms the discharge products in a reversible cell reaction. In Na-O2 batteries upon discharge either sodium peroxide (Na2O2) or sodium superoxide (NaO2) is reported to be formed. So far, the charge carrier transport remains relatively unexplored but is expected to crucially determine the efficiency of such energy storage systems. Na2O2 is predicted to be an electrical insulator wherein the transport presumably is determined by very slow hopping processes. Understanding the basic fundamental properties of the overall charge carrier transport, including also nanostructured forms of Na2O2, is key to developing high-energy metal oxygen batteries. The present study answers the question how overall, i.e., total, conductivity changes when going from microcrystalline to nanocrystalline, defect-rich Na2O2. Nanocrystalline Na2O2 was prepared via a top-down approach, viz by high-energy ball milling. Milling does not only shrink the average crystallite diameter but also introduces a large amount of defects which are anticipated to influence total conductivity. It turned out that even after vigorous mechanical treatment the conductivity of the sample is only increased by ca. one order of magnitude. The activation energy remains almost untouched. Thus, the increase seen might be attributed to an enhanced number of charge carriers. Low-temperature data reveals nearly constant loss relaxation behavior which has frequently explained in terms of strictly localized electrical relaxation processes.