sodium polysulphide melts

Corrosion Science, Vol. 33, No. 4, pp. 605~15, 1992

0010-938X/92 $5.00 + 0.00 © 1992PergamonPressplc

Printed in Great Britain.

THE

CHEMICAL STABILITY OF BETA/BETA"-ALUMINA IN SULPHUR/SODIUM POLYSULPHIDE MELTS R. S. GORDON,* S. N.

HEAVENS,t~ A. V. VIRKAR§ and N. WEBERI[

*Virginia Polytechnic Institute and State University, Blacksburg, Virginia, U.S.A. t Chloride Silent Power Limited, Runcorn, Cheshire, U.K. §University of Utah, Salt Lake City, Utah, U.S.A. ]]Beta Power Inc., Salt Lake City, Utah, U.S.A. Abstract--Experiments in which fl"-aluminaceramic is exposed to sulphur or sodium polysulphide can indicate the formation of a reaction layer. Analysis of thermodynamic data and kinetic studies indicates that the reaction layer is not a corrosion but a surface reaction caused either by traces of second-phase sodium aluminate in the ceramic or by equilibration of Na20 activitybetween the ceramicsurface and the sulphur melt. The absence of corrosion is supported by the evidence of long-term testing of fl"-alumina ceramics as electrolytes in Na/S cells. INTRODUCTION BETA ALt3MXNAis the generic name given to a family of ceramic compounds formed from alkali metal oxides and alumina and whose crystallographic structure is identified as fl-alumina (hexagonal) and/or fl"-alumina (rhombohedral).l The commonest form encountered is based on sodium oxide, in which the fl/fl"-alumina phase is essentially a non-stoichiometric sodium aluminate possessing unusually high Na + conductivity. In synthesis of the ceramic both fl and/3" phases are formed; in the final product the fi" phase is favoured owing to its higher ionic conductivity. In one of its principal uses the ceramic functions as a separator electrolyte in sodium-sulphur battery cells. The long-term stability of fl"-alumina in the Na/S cell environment is therefore of interest to battery developers. Beta"-alumina is usually considered to be one of the more stable of the electrochemically active materials, l especially in regard to its unusually low electronic conductivity and apparent lack of oxygen pressure dependence below temperatures of 1025 K. The stability offl/fl"-alumina especially with reference to sodium metal has been investigated by several authors and has recently been reviewed. 2 In contrast, the stability of fl/fl"-alumina to sulphur has received relatively little attention. In general, studies ~5 have indicated an absence of evidence for reactivity between/5"-alumina and sulphur or sodium sulphides. Recently Liu and D e Jonghe 6 presented evidence that suggested that fl"-alumina could be chemically unstable in sulphur/sodium polysulphide melts. The purpose of this p a p e r is to assess, on the basis of recent and past thermodynamic and kinetic studies, whether fl"-alumina is inherently unstable to sulphur or sodium sulphides, or whether it could become unstable in certain conditions. To address this question, it is necessary to: (1) assess the thermodynamic data in the literature concerning possible reactions between phases in the N a 2 0 - A 1 2 0 3 system and those in the N a - S system; (2) examine the Manuscript received 24 June 1991. Author to whom correspondence should be addressed. 605

606

R.S. GORDONet al.

results of attempts to measure directly the kinetics of such reactions; and (3) review the performance of Na/S cells over several years of continuous operation. THERMODYNAMIC STABILITY For oxides and aluminates in the NaEO-AI203 system a general equation can be written describing reaction with sulphur: Na20-yAl203 + 4z S--~ (1 - z)Na20, yA1203 -6 3d4Na2S5 -6 z/4Na2SO4

(1)

in which the reaction product may be fl-alumina, fl"-alumina, NaAIO2 or some form of alumina depending on the values o f y and z. In this section the free energy change of reaction (1) is estimated for the three cases in which the reactant is (a) fl-alumina (y -- 8); (b) fl"-alumina (y ~ 5); (c) NaAIO2 (y -- 1). The stoichiometry of fl-alumina is usually represented by y = 9-11, but a soda-rich material has been assumed. In fl/fl"-alumina ceramics the fl and fl" phases coexist as polytypes of the same composition; in any event the precise stoichiometry turns out to be of limited significance. One of the difficulties in establishing whether reaction (1) can act as a corrosion mechanism is that to calculate the reaction free energy change AG r some information on the reaction products is required, and in almost no case have reaction products been identified. While other reaction products such as NaESx (x = 2-5), A12(SO4) 3 and NaAI(SO4)2 are possible, reaction (1) represents the thermodynamically most favourable case. If, however, oxygen is present and participates in the reaction, the most favoured reaction product is, as is shown later, likely to be AlE(SO4) 3. (a) Beta alumina Beta alumina is a compound of limited stoichiometry which can coexist with aalumina in a Na20-depleted composition or with sodium aluminate (NaAIO2) in a Na20-rich composition. The corrosion reaction of primary concern is: Na20- 8A1203 + 4S ~ 8A1203 + 3/4Na2S5 + 1/4Na2SO4. (r-alumina)

(2)

This reaction describes the oxidation of sulphur and the acidic attack by sulphur on the basic Na20 component of r-alumina. As is evident from the binary phase diagram between Na20 and A1203,7 loss of Na20 from r-alumina involves first the removal of NazO until the alumina-rich phase boundary is reached, at which point a-alumina begins to form as a second phase. This sequence can proceed rapidly at temperatures greater than 1800 K. At 700 K, significant bulk reaction is doubtful since the mobility of oxygen and aluminum ions in the r-alumina structure is extremely limited. Surface reactions, which are kinetically more favourable, can be a factor at lower temperatures. If a-alumina is proposed as a reaction product, the r-alumina phase in reaction (2) should be that phase which is in equilibrium with a-alumina, i.e. alumina-rich ralumina. Thermodynamic data are available only for binary r-alumina in equilibrium with a-alumina. The standard free energy of formation A G f of r-alumina can be calculated from the activity of Na20 over the a-alumina/r-alumina coexistence

fl/fl"-Alumina in sulphur/sodiumpolysulphidemelts

607

T (°C)

600 500 ! !

-8.0C

400 !

=,E

~ ~ ~ !

on - 1 6 . 0 0

~o

-28.00 0.60 1.

~ i Choudhuw Dubreuil et al. Ito et al. Rbg et al. Brisley and Fray

Jacob Fray

- 20.00 %%

- 24.00

FIG.

i

Dewing

Z

o

200

A Elrefaie and Smeltzer

~ + ~ ~ DA

-12.00

300 i

,

I 1.00

,

J , 1.40 1000/T k

= 1.80

,

J 2.20

Na20 activity in the fl-alumina/a-aluminacoexistence: - - - represents the stability/instabilityboundaryof r-alumina in liquidsulphur.

equilibrium. Several determinations of NaaO activity have been made in EMF measurements of galvanic cells as reviewed by Itoh et al. ,s and a summary of all the Na20 activity data 8-16 is given in Fig. 1. It should be pointed out that equilibrium in these cells at 900-1100 K is in general reached only after a long period (>40 h), and all values at lower temperatures of around 600 K pertinent to Na/S cell operation require extrapolation which can lead to significant uncertainty in thermodynamic calculations. The free energy of reaction (2) thus depends on: (1) The assumed value for AGf of the r-alumina phase in equilibrium with a-alumina, calculated from the equation AGf(fl-alumina) = R T In a(Na20 ) + 8AGf(AI203) + AGf(Na20) in which a(Na20) is the Na20 activity in the fl-alumina/a-alumina two phase coexistence, AGf(AIzO3) is the free energy of formation of a-alumina, etc. (2) AGf for Na2SO4 and Na2S5. (3) The assumption that a-alumina is a product of the sulphur corrosion reaction. This is experimentally unverified at low temperatures. The free energies of formation of NaESO4 and Na2S5 are known (Table 1); the values of Gupta and Tischer 17 agree with those of Morachevskii et al. 18 In reaction (2) the Na2S5 product is in equilibrium with sulphur and will tend to be sulphur-rich. The literature values of AGf for sodium polysulphides are, however, insensitive to stoichiometry. The values of AGe for a-alumina are also known and will cancel out of reaction (2) since they are used to calculate AGf offl-alumina. The cancellation is not valid if the A120 3 reaction product is not the a phase. For the same reason the stoichiometry of the fl phase in reaction (2) is significant only if the reaction product is not the a phase.

608

R . S. GORDON et al.

TABLE 1. Species Na2SO 4 Alpha-alumina Gamma-alumina NazS5 Na20-8AI203 Na2-8A1203 NaAIO2 Na20 Na2SO4 A12(SO4)3 SO2 ( g a s )

STANDARD FREE ENERGIES OF FORMATION (kJ tool - 1 ) Source

573 K

JANAF JANAF JANAF G u p t a a n d T i s c h e r 17 Elrefaie and Smeltzer 9 I t o h et al. 8 NBSIR-81-2343 JANAF JANAF JANAF JANAF

-1157.7 - 1496.1 - 1479.1 -401.8 - 12584.7 - 12546.2 - 1012.6 -340.5 -1157.7 -3083.8 -300.7

623 K -1137.7 - 1480.5 - 1463.5 -401.1 - 12448.1 - 12414.8 - 1001.6 -333.3 -1137.7 -2932.7 -300.3

673 K -1117.9 - 1464.9 - 1448.2 -400.6 - 12313.1 - 12281.6 -990.4 -326.1 -1117.9 -2804.2 -299.8

Less certain is the value of the activity of Na20 in the f l - a l u m i n a / a - a l u m i n a coexistence. Table 2 shows that, depending on what value is taken for the Na20 activity, the free energy AGr of reaction (2) can be either positive or negative (reaction (2) here corresponds to Liu and De Jonghe's 6 equation (4.1a) with x = 5 and y = 8). The activities of Na20 used by Liu and De Jonghe 6 were measured by Weber and reported by Kummer. 7 Those values represent an average activity over the narrow homogeneity range of fl-alumina between a-alumina and NaAIO2 and are inappropriate for a calculation involving a-alumina as a reaction product since the Na20 activity will be overestimated. The values of AGr quoted by Liu and De Jonghe are therefore likely to be too negative. The data for a(Na20 ) in Refs 8 and 9 compare reasonably well at temperatures around 1000 K. Owing to the difference in the observed temperature dependence of the measured EMFs, however, different conclusions are drawn in extrapolating the two sets of results down to 600 K. In Fig. 1 the data of Itoh et al. 8 predict a positive AGr for reaction (2) at temperatures below 500 K. Using the Na20 activity data of Elrefaire and Smeltzer, 9 in the middle of the range of data shown by Itoh, a positive AGr is predicted up to about 800 K. Extrapolations from most of the eight measurements (Fig. 1) in the literature of the Na20 activity in alumina-rich fl-alumina will predict stability in liquid sulphur at temperatures below 800 K. For reference and assistance in making extrapolations, a Na20 activity boundary between stability and instability of fl-alumina in liquid sulphur is shown as the dotted line in Fig. 1. Activities of Na20 above this line will lead to negative values of AGr (thermoTABLE 2.

N a 2 0 ACTIVITIES IN THE ALPHA/BETA ALUMINA COEXISTENCE AND FREE ENERGIES (kJ m o l - i ) OF REACTION (2) Temperature:

Source Liu and De Jonghe 6 I t o h et al. s Elrefaie and Smeltzer 9

623 K

673 K

log a ( N a 2 0 )

AG

log a ( N a 2 0 )

AG

-18.7 -19.9 -22.7

-28.5 -14.7 + 19.3

-16.7 -18.4 -20.8

-38.1 -16.3 + 13.8

fl/fl"-Alumina in sulphur/sodium polysulphide melts

609

dynamic instability to sulphur) and vice versa. Depending on the particular measurement of the Na20 activity which is selected, one can generate slightly negative or slightly positive values for the free energy of reaction (2). Barsoum 2 attempted to obtain a consensus from the spread of thermodynamic data in the literature on the Na20-A1203 system, but use of his 'best-fit' values does not alter the conclusions. Because of the long temperature extrapolations and the fact that the predictions for AG~ of reaction (2) are so close to zero, stability or instability cannot be reliably predicted on thermodynamic grounds, and recourse must be made to direct experimental evidence. It is not necessary to consider in detail alternative possibilities for the reaction product in reaction (2). For example the reaction N a 2 0 - 8A1203 + 16/3S--~ 71/9A1203 + Na2S 5 + 1/9AI,(SO4) 3

(3)

(fl-alumina)

for which AGr = + 41.6 kJ m o l - I at 623 K, assuming the data of Ref. 9 for fl-alumina, is less favourable thermodynamically than reaction (2). (b) Beta" alumina Beta" alumina is a ternary compound with a very narrow range of stoichiometry. On the sodium-rich side of its coexistence, fl"-alumina is probably in equilibrium with NaAIO2 at a relatively high Na20 activity. The generic reaction between sulphur and Na20 at any activity can be written (Na20) + 4S ~ 3/4Na2S~ + 1/4Na2SO4.

(4)

At a(Na20) = 1, the reaction free energy AGr = - 2 5 2 . 3 kJ mo1-1 at 623 K. Thus at sufficiently high activities of Na20 , AG~ of reaction (4) can be significantly negative, and Na20-rich compositions of fl"-alumina can be predicted on thermodynamic grounds to react with sulphur to form fl"-alumina phases with lower Na20 activity: Na20 • yA1203 + 4z S--+ (1 - z ) N a 2 0 , yA1203 + 3z/4Na2Ss + z]4NazSO4. (5) (/f'-alumina)

(//"-alumina)

Owing to a lack of information on the reaction product, AG~ is difficult to estimate but is likely to be slightly negative, depending on the exact stoichiometry y and z. (c) Sodium aluminate Possible reactions of sodium aluminate with sulphur are 2NaAIO2 + 7/2S --~ l/8Na20 • 8A1203 + 21/32Na2Ss + 7/32Na2SO4

(6)

(fl-alumina)

2NaA102 + 16/5S--o 1/5Na20 • 8A1203 + 3/5Na2S5 + 1/5Na2SO4

(7)

(//"-alumina)

2NaAIO2 + 4S --o A1203 + 3/4Na2S5 + 1/4NazSO4.

(8)

Using the data of Ref. 9 to calculate AGf of fl-alumina (Table 1), for reaction (6) ±Gr = - 6 4 . 9 kJ m o l - l at 623 K. In reaction (7) AGf offf'-alumina is less certain; the data 19of Choudhury yield a value of - 7 9 5 0 kJ mol- 1 at 623 K but this is suspected to be insufficiently negative. 2 It can at least be said that for reaction (7) AGr <~ - 5 5 kJ tool -1 at 623 K. In reaction (8) AGr will depend on the alumina phase in the reaction product; if a-A120 3, AG r = - 6 2 . 4 kJ tool -1 at 623 K; this will be somewhat less negative if a metastable alumina is formed.

610

R.S. GORDONet al.

All these reactions indicate that NaAIO2 is potentially less stable in sulphur than is fl/fl"-alumina. It is therefore far more probable for NaAIO2 to react with sulphur to form fl or fl"-alumina than it is for fl or fl"-alumina to react with sulphur to form a-alumina. Since there may be a small amount of dispersed NaAIO2 in sintered fl"-alumina, it is possible that this trace might be susceptible to reaction with sulphur.

The nature of the reaction product According to the phase equilibrium study of the Na20-MgO-A1203 ternary, 2° loss of Na20 from fl"-alumina involves a sequence of phase changes starting with reaction (5) and leading ultimately (at 2000 K) to the appearance of a-alumina from a fl (solid solution)/fl'"-alumina mixture. The reaction sequence in lithia-stabilized fl"alumina is still more complex but also ends with the a-alumina phase appearing from a fl (not fl")-alumina phase. At 600 K it is unlikely that the final product of any de-alkalization of flor fl"-alumina would be a-alumina. Weber 21 showed that in the system Na20-AI203-MoO 3 the alumina phase precipitating from liquid at 900-1100 K is the mullite-type phase m-Al203. The only alumina phase (other than a) for which thermodynamic data are available is gamma alumina, which is less stable than a by 17 kJ mo1-1 in the range 573-673 K (Table 1). For the highest realistic value of a(Na20 ), AGr of reaction (2) is only slightly negative (Table 2); should any form of alumina less stable than a be the reaction product--e.g, as a result of the fl--~ m transition, which requires less structural rearrangement, being kinetically more favourable--AGr would become positive. From the thermodynamic data it can be concluded that, depending on the activity of Na20 in the fl/a-alumina two phase coexistence and the nature of the alumina phase in the reaction product, fl-alumina is probably stable, or possibly marginally unstable, to molten sulphur at 573-673 K. For fl"-alumina compositions on the Na20-rich side of the phase field, the possibility of corrosion by sulphur (and Na2Ss) is greater due to the higher Na20 activity. In any event, depletion of Na20 from the fl" phase would, on the basis of phase equilibrium studies, be expected to lead first to the formation of the fl phase before the appearance of any alumina phase. KINETIC STABILITY Assuming for the present that AGr of reaction (2) or (5) is negative, are the kinetics at 600 K sufficiently rapid to cause significant corrosion? Reactions between fl- or fl"-alumina and molten sulphur have been investigated directly by various techniques. 4--6The most reliable test for reactivity is to disperse a fine powder of fl- or fl"-alumina in a sulphur melt and search for direct evidence of reaction. Any solid material capable of releasing Na20 should undergo reaction (4) to some extent, and the reaction can be followed by detecting the formation of either sodium sulphate or sodium polysulphide. Formation of sodium polysulphide can be detected using an in-capsule differential scanning calorimetry (DSC) technique. Janz and Rogers 22'23 showed how sulphur and a range of sodium polysulphides can be thermally characterized by melting weighed quantities of sodium and sulphur together and observing their melting points by DSC. Ingram eta/. 5'24 mixed powdered sulphur with powdered glass, fl/fl"-alumina or NaA102 in sealed aluminum pans and searched for melting

fl/fl"-Alumina in sulphur/sodiumpolysulphidemelts TABLE 3,

611

THE OXYGEN DIFFUSION COEFFICIENT IN fl AND fl"-ALUMINA

/3-alumina /3"-alumina

573 K

673 K

2.5 x 10 19 3.0 ),( l0 33

6.9 x 10-18 7.4 x 10 30

endotherms to indicate the formation of polysulphides. With NaAIO2 and Na + conducting glasses, well-defined endotherms were identified by DSC. In contrast, none of the fl- or fl "-alumina powders showed any evidence of reaction, either from DSC endotherms or from reaction products, after being held in contact with molten sulphur at 573 or 623 K for periods of several months. Molten sulphur was found to extract Na20 only from the surface of NaA102 and the conductive silicate and borosilicate glasses, the reaction being complete after only a few days. Kummer and Weber 3 also conducted corrosion studies by immersing powdered NaA102 in molten sulphur at 623 K. It was found from the formation of sodium sulphate that the reaction proceeded to only 2% completion in 7 days. Smaga and Battles 4 used weight change measurements and SEM observations of sintered fl"-alumina exposed to molten sodium and sodium polysulphides for 167 days at 623 K. Two types of fl"-alumina from different sources were studied, Li20 and MgO stabilized. Small weight gains were recorded and attributed to potassium pick-up by ion exchange from the sodium or polysulphide melts. Once the potassium weight gains were subtracted, on average no net weight change occurred in the fl"-alumina. The test samples showed no discoloration nor change in surface appearance. Liu and De Jonghe 6 used SEM and Auger spectroscopy to detect the formation of reaction products such as Na2SO4, NaAI(SO4)2, A12(SO4)3, NaHSO4, Na2CO3, NaOH etc. following exposure of dense fl"-alumina ceramics, polished and assintered, to liquid sulphur or sodium polysulphide. The presence of reaction products was inferred from thin surface reaction layers in which sulphur, aluminum and oxygen were detected (Figs 5-7 in their paper6). It is possible to deduce the reaction kinetics from their measurements. In their Auger spectrum (Fig. 7b 6) almost all the sulphur is sputtered off within 5 min, so given the sputtering rate of 100/~ min-1 (0.17 nm s-l), the layer must have been 0.05 ktm deep, which is consistent with the SEM observations. Liu and De Jonghe reported that this reaction layer had formed during 10 weeks exposure to NazS5 at 673 K, indicating a corrosion rate of 0.25/~m per year, if it is linear. The kinetics of the reaction are therefore very slow. The slow reaction kinetics at 600 K are not unexpected. Once a small quantity of Na20 has been extracted from the surface of fl"-alumina according to reaction (5), additional reaction requires diffusion of both sodium and oxygen through the reaction layer. While sodium diffusion is rapid, the mobility of oxygen is very low; the diffusivity of oxygen at low temperatures in most oxides is less than 10-2o m 2 s-i .25 For fl and fl" alumina the diffusion coefficient of oxygen as a function of temperature has been measured by isotopic exchange by McHale et al. 26'27 Using their best fit data gives the values shown in Table 3 for the diffusion coefficient (mZs -1) at 573 and 673 K.

612

R.S. GORDONet al.

De Jonghe et al. 28 also estimated an upper bound on the diffusion coefficient of oxygen in fl-alumina of 5 x 10 - 1 8 m 2 s -1, but observed an anomalously high bleaching rate of discoloured fl-alumina at 523 K, which was interpreted in terms of an oxygen interstitial defect transport mechanism. The oxygen diffusion mechanism was, however, rejected by Barsoum 2 on thermodynamic grounds as an explanation for the discoloration. Such a mechanism is in any case not available in the relatively close-packed conduction planes of fl"-alumina, in which the diffusivity is orders of magnitude lower. Liu and De Jonghe's reaction rate suggests a diffusion coefficient of the order of 4 x 10 .22 m 2 S-1. A further aspect that needs to be considered in assessing experimental evidence of corrosion is whether the fl"-alumina ceramic used in the tests was fully stabilized and well equilibrated. The ceramic tested by Liu and De Jonghe 6 appeared from their (Fig. 7b 6) Auger spectrum to have been Mg-stabilized, which in contrast to Li-stabilized fl"-alumina, tends to require high sintering temperatures during fabrication and in which it is relatively difficult to reach equilibrium, especially if a fast-firing regime had been used. 29 This increases the likelihood of obtaining a significant proportion of residual NaAIO2 in the ceramic, which would be responsible for observable reactivity with sulphur. Finally, in Liu and De Jonghe's tests 6 there was a large excess of S/Na2Sx reactant compared to the surface area of the ceramic. In considering reactions such as reaction (2) it is customarily assumed that the sulphur melt is saturated with A1203, Na2S 5 and Na2SO 4. It is possible that some minor transient corrosion reaction could proceed in approaching this equilibrium state to account for the thin sulphur and oxygen containing films observed by Liu and De Jonghe on fl"-alumina. THE INFLUENCE OF OXYGEN ON THE BETA"-ALUMINA/SULPHUR REACTION The possibility of enhanced reactivity in the presence of oxygen was raised by Liu and De Jonghe 6 (reaction 4.4a in their paper): 1/2Na20 • 8A1203 + 1/8Na2S 5 + 02 --~ 4A1203 + 5/8Na2SO4.

(9)

The free energy of reaction (9) was reported to be -383.4 kJ mo1-1 at 623 K. However, it cannot be concluded from this that fl-alumina is very unstable in the presence of oxygen, because sulphur and sodium polysulphides will burn in oxygen (to form SO2) even when fl-alumina is not present. The reaction should be rewritten: Na20.8A1203 + 3/2SO2 --~ 8A1203 + Na2SO4 + 1/2S.

(10)

Using the data of Ref. 9, for reaction (10) AGr = -82.9 kJ tool -1 at 623 K, which is substantially less negative than in reaction (9) owing to the large AGr (-337.8 kJmo1-1 at 623 K) of the oxidation of molten sulphur/sodium polysulphide: 1/8Na2S5 + 1/4S + 02 ~ 1/8Na2SO4 + 3/4SO2.

(11)

In fact when oxygen is present the most favoured reaction is likely to be Na20.8A1203 + 75/2SO2 ~ 8A12(SO4)3 + Na2SO4 + 25/2S

(12)

fl/fl"-Alumina in sulphur/sodium polysulphidemelts

613

for which AGr = - 8 9 0 kJ mo1-1 at 623 K. From this one might expect oxygen to cause a dramatic enhancement of the corrosion reaction; but it does not, for the following reason. When liquid sodium polysulphides are present, gaseous SO2 will react with them in competition with reactions (10) and (12) to form the more benign species Na2SO 4 and S according to the reaction: Na2S 5 + 2SO 2 ~ Na2SO 4 + 6S.

(13)

For reaction (13) at 600 K, AGr = - 1 4 2 . 3 kJ mol - l , and being a liquid/gas reaction the kinetics will be rapid compared to those of reactions (10) and (12) which involve the corrosion of a solid. Consequently the potential for oxygen to enhance the corrosion of fl"-alumina in a Na/S cell should exist only in the fully-charged state. Whenever Na2S 5 is present it acts as an effective oxygen getter via reaction (13). In a conventional Na/S cell the sulphur electrode consists of a matrix of sulphur/sodium polysulphide and carbon fibres for enhancing electronic conductivity. Since the carbon can itself act as an oxygen getter, oxygen-enhanced corrosion offl"-alumina is never likely to occur, not even in cells in the fully charged state. Liu and De Jonghe 6 also considered contamination by water and CO2, but all exothermic reactions 4.5 and 4.6 in their paper also involve oxygen. Reaction between sodium aluminates and water, CO~ or H2S in the absence of oxygen is favourable only at relatively high Na20 activity. The effect of these reactions in practice is therefore minimal; nonetheless it is desirable to assemble Na/S cells in dry conditions. In a hermetically sealed cell the oxygen supply is limited and any such corrosion reactions will terminate automatically. THE STABILITY OF BETA"-ALUMINA IN Na/S CELLS The test of predictions of stability or instability on the basis of thermodynamics or kinetic studies ultimately lies in the results of long-term in-cell testing. Two decades of development have led to the production of fl"-alumina electrolytes which exhibit Na/S cycle lives approaching 6000 cycles, or 8500 cycles in accelerated tests. 3~3-~ Numerous cells exhibiting the required capacity retention and internal resistance stability have operated continuously for over 5 years and the longest-lived cells have been cycled for over 8 years. Tests have also been conducted in which cells were operated for 2-5 years in the overvoltage or fully-charged state, in which the cathode composition was in the S/Na2S5 two phase region with the highest sulphur activity. No evidence for corrosion of the fl"-alumina electrolytes was detected. Extensive post mortem examination 3°'32'36 of Na/S cells cycled for periods up to 4 years showed no corrosion or discoloration of the fl"-alumina ceramic by sulphur or sodium sulphide. These observations indicate the absence of any significant corrosion reaction. Furthermore, if a surface reaction at the fl"-alumina/sodium polysulphide interface resulted in the formation of a passivating layer, some resistance instability would occur. The internal resistance of long-lived Na/S cells is stable to within 0.01% per cycle and capacity loss less than 3% per year of operation (Fig. 2). Incidences of resistance instability have been shown to be caused by a number of factors such as seal leakage, container corrosion, impurity contamination, or poor wetting, but never corrosion of the ceramic electrolyte.

R.S. GORDONet al.

614

Cell 5987 261241221A 20 I ~

~16 I

g •12 t'~ @ lom~ 8tm

Capacity '~.+~

i

+~

+ ~ +~+.

+.+.+o++

~n_m_m/m.m-m'm'Im -m-m-m~m Resistance

6141210

Cycles/1000 FzG.2. Resistance and capacityof a Na/S cell over a period of 6 years.

In this paper only the possibility of static chemical corrosion by sulphur and sodium polysulphides has been investigated. The possibility of electrochemical corrosion enhanced by the passage of Na + has not been studied and will be the subject of a future paper. However, the results of long-term cell testing at battery operating temperatures do not suggest substantial differences in behaviour according to whether or not cells have been cycled. SUMMARY In regard to its chemical stability fl/fl"-alumina is unique as a solid electrolyte. It is reasonable to expect that a fl/fl" alumina ceramic prepared somewhere in the homogeneity range will equilibrate rapidly in sodium by absorbing Na20. In molten sulphur, Na20 is abstracted from the conduction planes so that the Na20 activity comes into equilibrium with the melt, without the fl/fl"-alumina structure being affected. Thermodynamic data suggest that r-alumina is stable, or at most marginally unstable, in sulphur and that NaEO-rich fl"-alumina may be unstable to some degree. In the latter event, depletion of Na20 from fl"-alumina would result in a surface layer resistant to further corrosion. In the unlikely event that the surface reaction observed by Liu and De Jonghe 6 is a genuine corrosion reaction, its kinetics are evidently very slow. A significant corrosion reaction does occur between sulphur and NaA102. Concentrations of up to 1% NaAIO2 can occur in sintered fl/fl"-alumina ceramics and it is probably advisable to minimize this by ensuring complete conversion to fl"alumina during sintering and annealing. CONCLUSIONS On the basis of the review of existing thermodynamic and kinetic data and of extensive use of fl/ff'-alumina in Na/S cells, there is no evidence for any significant corrosion reaction between fl/fl"-alumina and sulphur or sodium polysulphide. There is some evidence of a limited surface reaction that may be either the equilibration of Na20 activity or a slow reaction between NaA1Oa and sulphur.

fl/ff'-Alumina in sulphur/sodium polysulphide melts

615

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