Anomalous Sodium Insertion in Highly Oriented Graphite: Thermodynamics, Kinetics and Evidence for Two-Sided Intercalation.

The difficult intercalation of sodium (Na) into graphite is studied by systematic and long-time investigations (of up to 2 years) using highly oriented pyrolytic graphite (HOPG). In this way a comprehensive picture of the thermodynamics, kinetics and the atomistic situation is arrived at. The results do not only allow us to draw conclusions on the applicability in Na-based batteries, but also to understand the anomalous behavior of Na within the alkali metals as regards open circuit voltage (OCV), storage capacity, storage kinetics, and atomistic storage pattern. The round picture requires including entropic effects and space charges into the established discussion. Such considerations even give new insight into the staging mechanism as such. The storage was performed both chemically and electrochemically over a wide temperature range. Our analyses show that at room temperature higher Na concentrations may be thermodynamically possible, but they are kinetically out of reach. The sodiated samples were investigated by electrochemical tools, by chemical analysis as well as by advanced electron microscopy. The latter reveals an unexpected, striking storage pattern: Unlike the other alkali metals, Na enters HOPG predominantly in the form of bilayers before it forms larger aggregates and finally staging compounds.