Degradation of beta″-alumina electrolyte by calcium impurities

Solid State Ionics 14 (1984) 225-230 North-Holland, Amsterdam

DEGRADATION OF BETA"-ALUMINA ELECTROLYTE BY CALCIUM IMPURITIES M.W. BREITER *, N.S. CHOUDHURY and E.L. HALL General Electric Company, Corporate Research and Development Center, P.O. Box 8, Schenectady, N Y 12301, USA Received 12 June 1984

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.

1. Introduction It is well established [1,2] that calcium, introduced as an impurity into beta"-alumina during the fabrication process, is one of the main causes o f the cell resistance rise with time of cycling of sodium/sulfur cells. The appearance of asymmetry [ 1 - 5 ] o f the resistance of beta".alumina tubes with respect to the direction of the flow o f d c current was also attributed [1,2] to calcium impurities. Usually the dc resistance is larger for discharge (sodium dissolution) than charge (sodium deposition). Reducing the level o f calcium impurity in the starting A1203 powders (Baikowski type AS2 ~: powder for instance) led [1,2] to beta"-alumina tubes which did not display asymmetry after extended cycling. Ion exchange experiments were carried out on single crystals [6] and polycrystalline samples [7] of beta"-alumina at higher temperatures. Complete conversion from Na- to Ca-beta"-alumina was reported in both cases. Thus ion exchange, although at a lower rate, appears also feasible from calcium impurities in * Formerly with GE/CRD, now with Institut fiir Technische Elektrochemie, Technisehe Unlversit~t Wien, 9 Getreidemarkt, A-1060 Wien, Austria. * Balkowski International Corp., Charlotte, NC 28210, USA 0 167-2738/84/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

the sodium or the sulfur [8] at the operating temperature range ( 3 0 0 - 3 5 0 ° C ) o f sodium/sulfur cells. This paper deals with two aspects of the influence of calcium impurities on the cycling behavior o f sodium/sulfur cells: (a) Effect of calcium impurities in the sodium; (b) behavior of beta"-alumina tubes which had been partially ion-exchanged with Ca 2+. The two approaches are complimentary. Beta"-alumina tubes o f the composition 8.7% Na20, 0.75% Li20, produced with Baikowski type AS2 powder, were used in these investigations.

2. Experimental section The first step in the study of the effect of calcium impurities in the sodium consisted in the construction o f special half cells for electrolyte filling with sodium. Shiny granules of calcium metal were weighed and added to the inside of the beta"-alumina tube before the evacuation of the half cell and the pinch-weld of the filling tube. The granule was either dropped into the bottom o f the tube or placed on a stainless steel. wire grid which was held above the top of the ceramic tube. Calcium dissolved in the sodium during f'filing.

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M.W. Breiter et al./Degradation of 3"-AI electrolyte by Ca impurities

The half cells were filled with sodium in a bath of molten sodium nitrate. The resistance was determined for charging and discharging to check for asymmetry. After cleaning, the tubes were visually inspected for pitting which occurred at the larger calcium contents. Tubes which were heavily pitted could not be incorporated into sodium/sulfur cells. Two sodium/sulfur cells which contained increasing amounts of calcium in the sodium were tested in our automatic cycling device. The changes o f the cell resistance and cell capacity with time of cycling were monitored until the cell failed. After disassembly the cause o f failure was established. Sodium was removed by washing with methanol from the half cells which contained pitted tubes after the electrolyte filling. The inner and outer surface of these tubes were studied by scanning electron microscopy (SEM) and by energy dispersive X-ray spectroscopy (EDX) in the SEM. The effects o f partial Ca 2+ ion exchange on the behavior o f the beta"-alumina tubes were investigated. The ion-exchange experiments were performed in a bath o f Ca(NO3) 2 and NaNO 3 contained in an alumina crucible and held at 330°C. The starting mixture for the bath was 50 w/o Ca(NO3) 2-4H20 (Fisher) and 50 w/o NaNO 3 (Baker) crystals. During the initial warm-up some amount of frothing was observed, presumably due to the dehydration of the Ca(NO 3)2 "4H20 crystals. Subsequently a clear liquid was obtained. All ion-exchange experiments were done in the clear bath; the time o f exchange generally varied from 15 min to 2 h. Prolonged exchange resulted in the disintegration o f the tubes in the bath. Following the ion-exchange, the tubes were thoroughly washed in methanol to re-

move any nitrate sticking to the tube surface. Samples from some of the Ca 2+ ion-exchanged tubes were examined by transmission and scanning transmission electron microscopy. The samples were cut from the tubes by sectioning normal to the tube axis using a diamond wafering saw. The wafers were mechanically polished to a thickness of 50 to 100/~, then ion milled using a 6 keV beam of argon ions. The resulting samples had an electron-transparent area across the tube from the inner to the outer surfaces. These specimens were examined in an analytical electron microscope operating at 100 keV which was equipped with an energy dispersive X-ray spectrometer.

3. Experimental results Table 1 summarizes the results obtained by the electrolytic Idling of half cells with sodium in the presence o f different amounts of Ca. When the discharge resistance was considerably larger than the charge resistance, Rdi s is quoted one minute after application of the current of 1 A since Rdi s decreases [5] with time. Sodium/sulfur cells could only be manufactured from the half cells when the calcium content of the sodium was less than or equal to 200 ppm. The cycling results of these two cells are compiled in table 2. The heavy pitting at the bottom of the tube of half cell 769 is illustrated in fig. 1 and the preferential pitting at the upper part o f the tube in cell 779 is shown in fig. 2. The fiber-like structure inside a pitted area is displayed at a magnification of 1000 in

Table 1 Effect of calcium impurity in the sodium during the electrolyte filling. Cell no.

C765 C789 C769 C773 C779

Amount of Ca (ppm) 50 200 500 500 500

Failure during filling No No No Yes No

Serious pitting

Filling resistances, mohms At the end of filling Rch Rdi s

After about 9.5 Ah Rch Rdis No No Yes . No

79 81 .

. 139

96 102 . 211

79 77 81 .

Ah

78 87 174

18.7 16.1 20

105

19.7

. 105

M.W. Breiter et al./Degradation of f -Al electrolyte by Ca impurities

227

Table 2 Effect of calcium impurity in the sodium on cell resistance of sodium/sulfur cells. Cell no.

Early discharge resistance (mohms)

Later discharge resistance (mohms)

Life at failure

Cause of failure

C765 C789

59 85

66 110

341 Ah/cm 2 302 Ah/cm 2

Crack in seal area Crack in seal area

fig. 3 taken b y SEM. The pieces with pronounced pitting were cut from the tube o f cell 769 for the SEM studies. A careful check revealed no appreciable degradation on the inner surface below a pit on the outer surface. An attempt was made to detect Ca or CaO by EDX. However, calcium signals were not found when focussing a narrow beam on numerous spots o f the outer and inner surface or integrating over larger areas. A visual search for CaO crystals which are easily recognized by their shape did not

yield any positive results for the inner surface either. A beta"-alumina tube which had been ion-exchanged with Ca for 15 rain was electrolytically filled to 8 Ah. The charge resistance Rch was determined to be 230 mohm while Rdi s was 721 mohm after 1 min o f elec-

Fig. 1. Preferential pitting at the bottom of the beta"-alumina tube in half ceil 769.

Fig. 2. Preferential pitting at the top of the beta"-alumina tube of half cell 779.

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M.W. Breiter et al./Degradation of #"-Al electrolyte by Ca impurities

Fig. 5. Energy dispersive X-ray spectrum of Ca rich pocket in area shown in fig. 4.

Fig. 3. SEM of pitted area at 1000X. trolysis at 1 A. Filling was stopped at this point to avoid the cracking o f the tube inside the NaNO 3 bath. Another tube which had been exchanged for 2 h cracked at the b o t t o m during the sodium filling. It

Fig. 4. TEM of sample after 15 min of ion exchange. Arrows mark the Ca rich pockets.

showed severe pitting at the b o t t o m area, comparable to that in fig. 1. Transmission electron micrographs and energy dispersive spectra were taken from samples o f tubes which had been ion exchanged for 15 min or 2 h respectively. In the samples with 15 min o f exchange some Ca-rich pockets were discovered, with an example shown in fig. 4. The pockets are indicated by arrows. A typical EDX spectrum o f such a pocket is given in fig. 5. Small Ar peaks which are due to the ion milling gas are sometimes seen as in fig. 5. A typical microstructure for samples which had

Fig. 6. TEM of sample after 2 h of ion exchange.

M. I¢. Breiter et al./Degradation of #"-Al electrolyte by Ca impurities

229

is fairly uniformly distributed in the matrix grains, with variations in Ca distribution in fig. 8 due to differences in sample thickness alone. It is not localized at grain boundaries or second phases.

4. Discussion

4.1. Effect o f calcium impurity in the sodium

Fig. 7. Energy dispersiveX-ray spectrum close to the inner surface of sample after 2 h of ion exchange. been exchanged for 2 h is shown in fig. 6. A large number of cracks along grain boundaries are visible. Otherwise the microstructure is uniform. There is no indication that second-phase regions are present. An EDX spectrum taken from the beta"-alumina matrix close to the inner surface is presented in fig. 7. Significant calcium is seen. The ratio of the height of the sodium peak to the height of the Ca peak remained practically the same across the sample when moved from the inner to the outer surface. An X-ray map using the Ca signal is given in fig. 8. The calcium

Fig. 8. X-ray map, using the Ca signal, of sample after ion exchange for 2 h.

Table 1 reveals that the trend to asymmetric resistive behavior of the beta"-alumina tube increases with the calcium content in the sodium. It is the largest for cell 769 when the Ca granule had been directly added to the bottom of the tube. The calcium content is large initially and gradually declines with time when more sodium is electrolyzed into the tube. Since the electrolysis starts at the bottom of the tube where a spool of Free nickel wire touches the beta"-alumina surface, degradation by Ca occurs predominantly there. This is reflected by the preferential pitting close to the bottom of the tube in fig. 1. The effect of 500 ppm Ca was less drastic when the Ca granule was added on the stainless steel wire grid at the top of the tube (cell 779). Calcium can only dissolve in the sodium after the sodium level reaches the granule on the grid. Preferential pitting is located near the top of the tube (see fig. 2). The cross section of the lower part of the tube in fig. 1 demonstrates that the formation of the pits starts at the outer surface. This is confirmed by the SEM result that degradation of the beta"-aiumina was not detected on the inner surface below the pits. It is suggested that the pitting is due to current channeling. Most of the inner surface of the beta"-alumina tube becomes covered by a deposit rich in calcium, probably CaO. The current flows mainly through those areas which are free of this deposit. The local current density is large. Local heating leads to a reduced resistance of the ceramic at those spots. In turn, this increases the local current density which finally grows large enough to produce the pits. The exact mechanism of pit formation in molten NaNO 3 is not yet known. The Ca-rich layer is located directly at the inner surface. It does not penetrate to an appreciable degree into the ceramic. The layer is removed by washing in methanol. This is the reason that Ca signals were not detected on the inner surface by EDX. The conclusion

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M.W. Breiter et al./Degradation of fl"-Al electrolyte by Ca impurities

on the removability of the Ca-rich layer by the methanol wash is supported by earlier evidence [4]. Removing the sodium from sodium/sulfur cells with the internal sulfur electrode brought the cell resistance which had risen during cycling back to the original value. If the cell had become asymmetric, the Na removal restored the symmetric resistive behavior, at least for several tens of cycles. Although the half cell 789 was slightly asymmetric at the end of the electrolytic filling, this behavior was not seen in the cycling results of the respective sodium/sulfur cells. It is suggested that a difference of about 10 mohm between Rch and Rdi s is masked by the slightly larger contribution of the sulfur electrode to the total resistance during charge. The cell resistance, determined during discharge by the automated device, increased from 85 mohm to 110 mohm at the end of life for cell 789. The resistance rise of cell 789 is attributed to the Ca impurity (200 ppm). Cell failures occurred after 341 Ah/cm 2 for cell 765 and after 302 Ah/cm 2 for cell 789. Cracks in the seal area were the cause. It is not known at present if cracking in the area of the glass seal might be affected by the Ca impurity in the sodium. In contrast, there is no doubt that half cell 773 failed during the electrolytic charging because of the influence of Ca. Probably, one or several pits were produced in the vicinity of a natural flaw. This led to cracking.

4.2. Effect o f partial replacement o f Na by Ca through ion exchange The asymmetric resistive behavior of the beta"-alumina tube after partial exchange of Na by Ca was pronounced more strongly than that of the tubes in table 1. Asymmetry was attributed to Ca-rich deposit on the inner surface of the latter tubes. This layer has also to be present on the inner surface of the ion-exchange tubes. It has a larger impact because the film is either thicker or more uniform in this case. The removal of the Ca-rich layer will not lead to a permanent cure against resistance rise or asymmetry because Ca ions can move to the inner surface from the bulk and reform the layer. This interpretation is in agreement with the results of ref. [4].

Once the Ca ions penetrate the ceramic to an appreciable extent as in the sample, characterized by figs. 6 through 8, the mechanical stability of the beta"-alumina tube decreases owing to the large number of cracks along grain boundaries (see fig. 6). While the existence of Ca-rich pockets was discovered in the sample which had been exchanged for 15 rain, such pockets are virtually absent after 2 h of exchange. The absence of second phase regions and the uniformity of the Ca distribution suggest that only the process of ion exchange took place under the described experimental conditions, however, the Ca 2+ ion exchange may have occurred at preferred locations during the early stages. A strong asymmetric behavior similar to that of the ion-exchanged tube was observed with tubes of different compositions [3]. Impurities which are incorporated during the manufacturing process have a similar impact as ion-exchange, i.e., Ca ions appear to be incorporated in the grains.

Acknowledgement This work was partially supported by the New York State Energy Research and Development Authority.

References [ 1] D.S. Demott, Extended abstracts of the Minneapolis meeting (Electrochem. Soc., Princeton, N.J. 1981) p. 1161. [2] G. Lebnert and B. Hartmann, Extended abstracts of the Detroit meeting (Electrochem. Soc., Princeton, N.J. (1982) p. 539. [3] M.W. Breiter, B. Dunn and R.W. Powers, Electrochim. Acta 25 (1980) 613. [41 D.S. Demott, J. Electrochem. Soc. 127 (1980) 2312. [5] M.W. Breiter and B. Dunn, Electrochim. Acta 26 (1981) 1247. [6] B. Dunn and G.C. Farrington, Mat. Res. Bull. 15 (1980) 1773. [7] E.E. HeUstrom,Extended abstracts of the Detroit meeting (Electrochem. Soc., Princeton, N.J., 1982) p. 482. [8] I. Yasui and R.H. Doremus, J. Electtochem. Soc. 125 (1978) 1007.