Effects of Calcium, Potassium, and Iron Ions on Degradation of β″‐Alumina

Abstract

The influence of calcium, potassium, and iron impurities in sodium-sulfur cells on ~"-alumina electrolytes were studied. Sections of the ceramic were analyzed with the electron microprobe after cyclic electrolysis of the cells. No degradation by iron was detected. The distribution of potassium in the ~"-alumina became more uniform as electrolysis proceeded, and the cell impedance increased by factors of two or three. Calcium was the most damaging impurity; even in small concentrations it caused large increases in cell impedance and calcium concentrated in the grain boundaries of the ~"-alumina.Impurities can degrade the performance and life of the ~"-alumina electrolyte in a sodium-sulfur cell (1-6). In the absence of an electric field potassium exchanges with sodium in large grains in ~-alumina in contact with a sodium source, and cracks result (6). In this work the influence of potassium, calcium, and iron impurities on degradation of ~-alumina after cyclic electrolysis was studied with the electron microprobe and the scanning electron microscope. Calcium was found to be the most damaging impurity; even in small concentrations it causes large increases in the resistance of p"-alumina, and can also cause it to crack.Experimental Two different cells were used, as shown in Fig. I. In one there was sodium inside the ~"-alumina tube and sulfur-polysulfide outside; in the other there was Na2S4 in both chambers. Impurities were added to the outer chamber as sulfides.Two different ~"-aluminas were used, one with 9.0% Na20 and 0.8% Li20 (9.0-0.8) and the other with 8.85% Na~O and 0.75% Li20 (8.85-0.75).The cells were baked at 850~ for 12 hr at 0.02 mm Hg before filling. Sulfur (sublimed) was dried under a reduced pressure at about 70~176 and sodium was filtered through a glass frit at about 150~ Commercial Na2S4 (Alfa) was washed with toluene and then dried in vacuum at 130~ for 48 hr. All reactants were placed into the cell under a dry atmosphere and then melted into the cell. The cell was sealed with dry argon at a third of atmospheric pressure. For the Na/S cells 15g of Na and 30g of S were used.The Na/S cells were first discharged until the composition of the outer chamber reached about Na2S4, and then charged and discharged in a 4 hr cycle. The current density was 25-30 mA/cm 2 for both charging and discharging, and this value was low enough to exclude the possibility of degradation due to the high current density mechanism. The composition of the outer chamber was near Na2S4, owing to the short " Electrochemical Society Active Member.cycle time and the low current density. Simulation cells were also operated in 4 hr cycles with a current density of 25-30 mA/cm 2. wir "ex 32 dun lit ~lun,. ,a ~11 elt Fig. I. Structures of Na-S cells and simulation cells 1a2S4 100'/ ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 169.233.248.124 Downloaded on 2015-03-31 to IP

Bismuth Islands for Low-Temperature Sodium-Beta Alumina Batteries

“…Taken together, these data confirm that the surface of β″-Al 2 O 3 must be clean of moisture and oxygen to prevent oxide formation in the experimental environment, such as in a glovebox, when a sessile drop technique is often used. Because the surface impurities of β″-Al 2 O 3 cause the contact edges of liquid Na to become pinned by the solid oxide layer, a further reduction in contact angle by completely isolating the surface from moisture- and oxygen-rich environments is advisable and possible.…”

“…To realize low-temperature NBBs, β″-Al 2 O 3 must have a low resistance and sufficient ionic conductivity (0.2–0.4 S/cm at 300 °C) at low operating temperatures . These can be attained by thinning β″-Al 2 O 3 and producing high-quality β″-Al 2 O 3 with fewer bulk impurities such as silicon and calcium that form glassy sodium aluminosilicate phases and intergranular calcium aluminate phases, respectively. − In addition, high ionic conductivity in electrolytes can be achieved by increasing the volume fraction of the β″-phase in two-phase mixtures (β-Al 2 O 3 /β″-Al 2 O 3 ) via doping with divalent cations such as Mg 2+ , Ni 2+ , Zn 2+ , and Cu 2+ . , Ionic conductivity is also affected by the microstructure, for example, larger average grains lead to higher conductivity . Another approach for reducing operating temperatures is to replace β″-Al 2 O 3 with other sodium ion conductors such as glass, glass-ceramic electrolytes (e.g., Na 3 PS 4 glass-ceramic electrolyte), and a Na super ionic conductor (NASICON)-type crystal with the general chemical formula of Na 1+ x Zr 2 Si x P 3– x O 12 (0 < x < 3) …”

The primate appendix: A reassessment

“…Viswanathan and his coworkers, who pretreated the ceramics in dry or humid environment, demonstrated that the presence of moisture played a role in the poor wetting 68. Moreover, it has been reported by many researchers that the impurity calcium, introduced in the cell by the ceramic powders during synthesizing the β ″‐alumina electrolyte, caused obvious increase in cell resistance 69–72. Mobility and accumulation of calcium on the Na/ β ″‐alumina interface were confirmed by Demott et al in 1981, who achieved the results through tracking radioactive Ca‐45 73.…”

Toward Commercialization of the Beta-Alumina Family of Ionic Conductors

“…24,25 In preparing a powder suitable for isostatic pressing, the characteristics required are good flow properties for fast moldfilling and high green strength to allow One important consideration is the need to control silica and calcia levels in boehmite below ϳ200 ppm.…”