Manfred W. Breiter

Asymmetric behavior of beta″-alumina

Abstract In contrast to beta-alumina, many polycrystalline samples of beta″-alumina display an asymmetric behavior with regard to dc current flow. The voltage, measured between liquid sodium in contact with beta″-alumina on one side and a probe inside a suitable molten salt on the other side, is noticeably larger during discharge (anodic sodium dissolution) than during charge (cathodic sodium deposition). Experimental results, obtained for beta″-alumina tubes of different compositions in sodium nitrate melts and laboratory type sodium—sulfur cells, are discussed. The asymmetric behavior is attributed to a thin, passivating film of Na 2 O at the interface between beta″-alumina and liquid sodium.

Time dependence of the asymmetric resistance of polycrystalline β″-alumina

The time dependence of the asymmetric dc resistance for certain polycrystalline β″-alumina tubes was investigated by cyclic voltammetry and current pulse techniques. Various compositions were studied as representative of the different types of behavior; asymmetric resistance with time dependence (9.6% Na2O, 0.75% Li2O), asymmetric resistance without time dependence (8.6% Na2O, 1% MgO, 0.5% Li2O), and symmetric resistance (8.85% Na2O, 0.75% Li2O). Asymmetry appears to be caused by the anodic modification of an initially present film to a thin passivating film of Na2O. Film conversion occurs more slowly for the first material.

In situ evaluation of container corrosion in sodium- sulphur batteries

Recent improvements in container materials for Na/S cells (central sodium type) have led to greatly enhanced cell life and performance. Although problems with rapidly decreasing capacity are no longer observed, cells appear to exhibit a slowly increasing resistance with time. This paper describes anin situ technique for assessing the electrical stability of various container materials and determining the extent that corrosion processes influence cell resistance changes. Potentials within the sulphur electrodes of operating cells were measured over prolonged cycling times. Results indicate that chromized steel possesses excellent electrical stability compared to 347 stainless steel and that container corrosion accounts for only a fraction of the increase in total cell resistance.

Conductivity change during the replacement of Na+ ions by Ag+ ions in polycrystalline beta″ alumina

Abstract The longitudinal conductivity was determined by a four-probe technique during the replacement of Na+ ions by Ag+-ions from the inside of a closed-end tube. Similary, the radial conductivity was measured during exchange starting from the outside. The conductivity-time curves display a minimum with a conductivity value below that of the Ag β″ alumina. The results also show that the change in conductivity with ion exchange time is steeper than that predicted by various models. The effects likely to produce additional resistances are presented.

An investigation of the interface between graphite and beta/beta″ alumina

Abstract The anodic degradation of beta/beta″ alumina of different compositions was investigated by voltammetric techniques in cells of the type Cu/Rodar alloy/carbon fibers/graphite layer/solid electrolyte/NaNO3/Pt/Cu at 350°C. The possibility was evaluated if the voltammetric results are useful as a diagnostic tool for the prediction of the failure of beta/beta″ alumina electrolytes in sodium sulfur cells. Single voltage sweeps were applied into the anodic and also the cathodic direction at relatively low sweep rates (0.1-100mV s−1), starting at the open circuirt voltage and extending to different voltages of reversal. The anodic current-voltage curves display a continuous rise of current with voltage up to 4.5 V, vs a Na reference electrode, followed by a gradual decrease afterwards. The anodic current is mainly attributed to the formation of sodium carbonate and carbon monoxide. The oxygen is supplied by the degradation of the solid electrolyte. The cathodic current-voltage curves reveal the intercalation of sodium into the graphite Structural studies of specimens from the solid electrolyte were carried out by electron microscopy before and after the electrochemical runs. They yielded information on how the anodic degradation proceeds with time.

Resistivity aging of beta-alumina ceramics in sodium-sulfur cells

Abstract The resistance of sodium-sulfur cells containing beta-alumina ceramic electrolyte changes with time. We show that this aging process can be divided into three separate stages reflecting different mechanisms and kinetics.

Interfacial behaviour of various liquid electrode/β″-alumina systems

Three different liquid electrode/sodiumβ″-alumina interfaces were investigated. The NaNO3/β″-alumina interface displays ohmic behaviour although the resistance is somewhat larger than expected. The sodium/β″-alumina interface exhibits asymmetric resistance with respect to d.c. current flow. Cyclic voltammetry results suggest that a Na2O film is being modified electrochemically during the sweeps. When a sodium amalgam electrode is utilized, the film, if it is present, has very little effect. The asymmetric resistance is not observed and there is good agreement between d.c. and a.c. resistances after only a small amount of sodium deposition.

Degradation of beta″-alumina electrolyte by calcium impurities

Abstract Calcium impurities, incorporated into the beta″-alumina electrolyte during its production, are known to cause a resistance rise of sodium/sulfur cells with time of cycling, decreasing the efficiency and leading to a short cell life. This paper deals with the effect of calcium impurities which are either contained in the sodium or introduced into the solid electrolyte by ion exchange. It is demonstrated that calcium impurities above about 50 ppm in the sodium are detrimental to the long term operation. The effect of calcium incorporated by ion exchange is even stronger than that of calcium impurities of 500 ppm in the sodium. The influence of calcium impurities in the sodium is due to a Ca-rich layer on the inner surface without Ca penetration into the bulk. In contrast, ion exchange does not only produce the Ca-rich layer, but also leads to a uniform distribution of calcium in the beta″ -alumina without preferential segregation in grain boundaries or the formation of second phase.

Impedance studies of the asymmetric resistance of beta″ alumina

Abstract A small ac signal was used to resolve the two components of the ac impedance of the sodium/beta″ alumina interface as a function of cell voltage. Both components were observed to change rapidly in the transition region between the high resistance and low resistance states. The occurrence of an electrochemical reaction within this potential range cannot be observed by impedance measurements. The two components reflect the properties of a thin interfacial film which is believed to be responsible for the asymmetric resistance.

Studies of the interface between indium and beta/beta″ alumina tubes

Abstract The anodic degradation of a beta/beta″ alumina electrolyte in contact with liquid indium was studied by voltammetric techniques at 35°C. The electrolyte tubes were found to crack when the anodic current reaches about 4 mA cm −2 , referred to the apparent surface area. Simultaneously, a passivation occurred which is attributed to film formation on indium in contact with the molten nitrate through the cracks. Transmission electron micrographs taken before and after the anodic cycle yielded information on how the degradation may proceed. At the cathodic side of the electroactive domain of the solid electrolyte, the process of sodium dissolution in the indium at lower voltages and the formation of indium-sodium compounds at larger voltages can be distinguished.