Vok 135, No. 3
ALUMINIZATION
OF
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1. R. Pichoir, in "Materials and Coatings to Resist High
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(1974).
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Coatings," M. Khobaib and R. Krutenat, Editors, p.
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6. D. C. Tu and L. L. Seigle, Thin Solid Films, 95, 47
(1982).
7. R. Sivakumar and L. L. Seigle, Met. Trans., 13A, 495
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8. B. K. Gupta, A. K. Sarkhel, and L. L. Seigle, Thin Solid
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17 (1981).
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Science & Engineering, State University of New
York, Stony Brook, New York (1983).
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G. Eriksson, Chem. Scr., 8, 100 (1975).
W. R. Daisak and N. Kandasamy, Work in progress in
the Dept. 'of Materials Science, State University of
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J. Eldridge and K. L. Komarek, Trans. Metall. Soc.
AIME, 230, 226 (1964).
J. I. Goldstein, D. E. Newbury, P. Echlin, D. C. Joy, C.
Fiori, and E. Lifshin, "Scanning Electron Microscopy and X-Ray Microanalysis," P l e n u m Press, New
York (1981).
G. Eriksson and E. Rosen, Chemica Scripta, 4, 193
(1973).
K. Schwerdtfeger and E. T. Turkdogan, in "Physicochemical Measurements in Metals Research," Vol.
IV, part 1, R.A. Rapp, Editor, p. 380, Interscience
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and Sons, Inc., New York (1951).
N. Wakao and J. M. Smith, Chem. Eng. Sci., 17, 825
(1962).
K. Nishida, T. Yamamoto, and T. Nagatta, Trans. Jpn.
Inst. Met., 12, 310 (1971).
F. Lihl and H. Ebel, Arch. Eisenhuttenw, 32, 483 (1961).
Chemical Stability of Sodium Beta"-Alumina Electrolyte in
Sulfur/Sodium Polysulfide Melts
Meilin Liu and Lutgard C. De Jonghe
Materials and Chemical Science Division, Lawrence Berkeley Laboratory and Department of Materials Science and
Mineral Engineering, University of California, Berkeley, California 94720
ABSTRACT
Immersion of sodium ~"-alumina electrolyte in sodium polysulfide and pure sulfur melts, at Na/S battery operation
temperatures, showed that the electrolyte was chemically attacked by the melts, and that the extent of degradation was
affected by a n u m b e r of factors, including surface morphology and chemistry of the electrolyte, melt composition, impurity contamination, etc. The corrosion reactions mostly initiated and concentrated on defected areas, and were catalyzed by
the presence of impurities such as water, moist air, oxygen, etc. The corrosion power of sodium polysulfide melts increased with the sulfur content in the range of Na2S2 to Na2Ss. The reaction products, formed at the interface, were believed to be Na2SO4, NaAI(SO4)2, A12(SO4).~,NaHSO4, Na2CO3, NaOH, etc. Precipitation of crystallized sulfates occurred on
preferred areas after saturation. Corrosion products of transition metals also deposited on the electrolyte surface. As a result, a partly insulating layer was formed on the electrolyte surface during immersion, which was a mixture of sulfates,
carbonates, and sulfides of transition metals.
Degradation of the ~"-electrolyte is one of the important
limitations which determine whether or not Na/S batteries
may be useful for energy storage and conversion systems.
There has been considerable effort in establishing the
mechanism of degradation of the electrolyte in contact
with the negative electrode (liquid sodium) (1). De Jonghe
et al. have reported the degradation of the electrolyte in
contact with the positive electrode (sulfur/sodium polysulfide melts) (2), but much remained u n k n o w n concerning
the nature of the electrolyte/polysulfide melt interface. Recently, Choudhury made an assessment of thermochemical stability of ~- and ~"-alumina electrolytes in pure sulfur with carbon (3). In the present paper, the degradation
of the electrolyte in various environments, in different
melt compositions (from Na2S2 to Na2S~ to pure sulfur),
with or without impurity contamination (water, moist air,
oxygen, cell case material, and carbon), are reported and
the reactions between the solid electrolyte and the melts
under these conditions were proposed
namic estimations.
based on thermody-
Experimental
Materials.--Polysulfides.--Four different polysulfides,
Na2S2, Na2S3, Na2S4, and Na2Ss, and pure sulfur were used.
The pure sulfur was obtained from the Lawrence Livermore Laboratory. The disulfide Na2S2 was prepared from
the monosulfide Na2S (from NAOH Chemicals) and from
pure sulfur in our laboratory, in a dry box (oxygen- and
water-free to less than 1 ppm). After the pure sulfur was
dried in the dry box for 3 days, the appropriate amounts of
Na2S and dry sulfur powders were mixed and put in a
quartz tube, sealed at about i0 2 torr, and then slowly
heated to 750~ to form Na2S2. The Na2S2 was cooled to
room temperature
and transferred to glass test tubes
within the dry box. The Na2S3 was obtained from BrownBoveri and Cie (BBC) and stored in vacuum. The Na2S4
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742
J. Electrochem. Soc.: S O L I D - S T A T E
SCIENCE
AND TECHNOLOGY
March 1988
Fig. 1. Scanning electron micragraphs of the polished surface of the electrolyte before and after five week immersion in sodium polysulfide
melts with different compositions. (a) As-polished before immersion; (b) immersed in Na2S2 (350~ (c) in Na2S3 (400~ (d) in Na2S4 (400~ (e)
in b~a2Ss(400~ (f) immersed in pure sulfur (350~
w a s p r e p a r e d b y D o w C h e m i c a l s C o m p a n y a n d s t o r e d in
t h e d r y box. Na2S~ w a s m a d e f r o m m i x t u r e s of e i t h e r Na2S~
or Na,_S4 w i t h p u r e sulfur.
~"-Alumina electroIyte.--The p o l y c r y s t a l l i n e e l e c t r o l y t e
c o n s i s t e d of b a r s w i t h a p p r o x i m a t e d i m e n s i o n s of 45 • 10
• 4 m m , a n d w a s p r e p a r e d b y C e r a m a t e c of Salt L a k e City,
U t a h (4). T h e c h e m i c a l c o m p o s i t i o n g i v e n b y t h e m a n u f a c t u r e r w a s 8.85 w e i g h t p e r c e n t (w/o) Na20, 0.7 w/o Li20, bala n c e A120.~.
Specimen preparation.--Two d i f f e r e n t s u r f a c e s - p o l i s h e d a n d a s - s i n t e r e d - - w e r e e x a m i n e d to d e t e r m i n e
h o w t h e s u r f a c e m o r p h o l o g y affects t h e s t a b i l i t y of t h e
electrolyte. S p e c i m e n s w e r e s e c t i o n e d w i t h a d i a m o n d
saw, g r o u n d w i t h a-A1203 a n d p o l i s h e d to a 1 ~ m d i a m o n d
f i n i s h (Fig. la). E v e n t h o u g h t h e p o l i s h e d s u r f a c e s w e r e
v e r y s m o o t h , s o m e i m p e r f e c t i o n s , s u c h as cavities a n d
l a r g e g r a i n s still r e m a i n e d . M e c h a n i c a l d a m a g e , i n c l u d i n g
m i c r o c r a c k s , m a y also b e p r o d u c e d d u r i n g p o l i s h i n g . Ass i n t e r e d s u r f a c e s are s h o w n i n Fig. 2a. S o m e w o r k e r s h a v e
reported that these surfaces might be covered by a thin,
g l a s s y film, w h i c h w o u l d b e f o r m e d d u r i n g s i n t e r i n g (5).
Specimens have different surface morphologies were
w a s h e d w i t h m e t h y l a l c o h o l in a n u l t r a s o n i c cleaner, a n d
t h e n h e a t e d u p to 800~ for 12h to d r i v e off a b s o r b e d water.
Moist air.--The d r i e d e l e c t r o l y t e was e x p o s e d to atmosphere (moist air) at room-temperature for 1 week (168h).
Fe-Ni-Cr and C.--Cell container materials (stainless steel)
and current collector material (graphite felt) were put in
with some polysulfide powders, before sealing the glass
capsules.
Procedures.--Because of t h e s e n s i t i v i t y of p o l y s u l f i d e s
to m o i s t u r e , t h e t e s t m u s t b e set u p i n a w a t e r a n d o x y g e n
free g l o v e box. S p e c i m e n s a n d p o l y s u l f i d e s w e r e p u t i n t o
glass t e s t t u b e s in t h e d r y b o x a n d s e a l e d u n d e r m e c h a n i cal v a c u u m . T h e s e a l e d t u b e s w e r e p l a c e d v e r t i c a l l y i n a
b o x f u r n a c e at t h e d e s i r e d t e m p e r a t u r e s (350 ~ or 400~
After the specimens had been kept in molten polysulfides
for t h e d e s i r e d p e r i o d (5 or 10 weeks), t h e glass t u b e s w e r e
c o o l e d to r o o m t e m p e r a t u r e a n d b r o k e n . T h e s p e c i m e n s
w e r e r e m o v e d , u l t r a s o n i c a l l y c l e a n e d i n m e t h y l alcohol,
a n d dried, b e f o r e c h a r a c t e r i z a t i o n a n d analysis. Specim e n s i m m e r s e d in s u l f u r w e r e c l e a n e d in CS2.
Results
Morphological characterization.--Effect of composition.--Figures 1 a n d 2 s h o w t h e c h a n g e s in s u r f a c e m o r -
p h o l o g y of e l e c t r o l y t e s b e f o r e a n d a f t e r 5 w e e k i m m e r s i o n
i n m e l t s w i t h c o m p o s i t i o n s of Na2S3, Na2S4, a n d Na2Ss, at
400~ a n d Na2S2 a n d p u r e s u l f u r at 350~
Impurity contamination.--Water.--Electrolyte speciT h e s u r f a c e s of p o l i s h e d s p e c i m e n s are s h o w n in Fig. 1.
m e n s w e r e e i t h e r e x p o s e d to 100% relative h u m i d i t y or imV e r y little h a s h a p p e n e d to t h e p o l i s h e d s u r f a c e i m m e r s e d
m e r s e d in distilled w a t e r at r o o m t e m p e r a t u r e for o n e
i n Na2Sz a n d Na2S3 melts. H o w e v e r , o n t h e p o l i s h e d surw e e k (168h) to p i c k u p water. T h e n , t h e s u r f a c e w a t e r w a s
face i n t h e Na2S4 melt, s o m e c o m p o u n d s f o r m e d o n a r e a s
r e m o v e d b y h e a t i n g t h e s p e c i m e n s at 100~ for 5 rain.
c o n t a i n i n g d e f e c t s s u c h as m i c r o c r a c k s , pores, etc., w h i c h
were introduced by mechanical polishing. On the polished
Oxygen.--After v a c u u m - d r y i n g a n d b e f o r e sealing, oxys u r f a c e i m m e r s e d in Na2S~, a n o n u n i f o r m a n d d i s c o n t i n u g e n gas w a s p u t i n t o t h e t e s t t u b e s to b u i l d u p a n o x y g e n
ous, t h i n s u r f a c e l a y e r w a s f o r m e d a n d m o r e c o r r o s i o n
a t m o s p h e r e of a b o u t 100 torr.
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Vol. 135, No. 3
SODIUM
BETA"-ALUMINA
ELECTROLYTE
743
Fig. 2. Sintered surfaces of the electrolyte (same experimental conditions and arrangement as in Fig. 1)
p r o d u c t s w e r e e v i d e n t at areas containing flaws or irregularities (Fig. le). The changes in surface m o r p h o l o g y w e r e
e v e n m o r e p r o n o u n c e d for the samples i m m e r s e d in p u r e
sulfur, w h e r e the surface was c o m p l e t e l y c o v e r e d by a corrosion layer (Fig. lf).
The surfaces of sintered electrolytes are s h o w n in Fig. 2.
After i m m e r s i o n , the surfaces w e r e c o v e r e d w i t h a thin
p r o d u c t layer, while s o m e c o m p o u n d s had a c c u m u l a t e d in
the surface pores. T h e e x t e n t of t h e reactions a p p a r e n t l y
i n c r e a s e d from Na2S2 to Na2S~ to p u r e sulfur.
Effect of impurity contamination.- F i g u r e s 3 and 4 s h o w
t h e c h a n g e s in m o r p h o l o g y of p o l i s h e d and sintered surfaces before and after i m m e r s i o n at 400~ for 5 w e e k s in
Na2S3 with the p r e s e n c e of oxygen, water, m o i s t air, or
stainless steel (Fe-Ni-Cr) plus graphite felt (C). An Na~S3
melt was chosen because, without impurities, this melt
was among the least reactive ones, as shown in Fig. 3b and
4b. However, in the presence of impurities, the change in
surface morphology and chemistry can be dramatic.
Steel + carbon (Fig. 3c and 4c).--The p r e s e n c e of transition
m e t a l s and g r a p h i t e felt c o n s i d e r a b l y affected the interaction b e t w e e n the Na2S3 m e l t and t h e electrolyte. A corrosion p r o d u c t layer c o v e r e d the entirety of the surface, and
s o m e octahedrally shaped crystals had formed.
Oxygen (Fig. 3d and 4d).--The effects of t h e p r e s e n c e of
b o u n d a r i e s are clearly r e v e a l e d and the altered surface top o l o g y indicates c h e m i c a l attack. S o m e corrosion products h a d b e e n d e p o s i t e d b e t w e e n grains.
Moist air (Fig. 3f and 4f).--The e x p o s u r e of t h e electrolyte
to m o i s t air (H20, Q , CO2, etc.) had the c o m b i n e d effects of
w a t e r and o x y g e n contamination.
Time dependence.--Figures 5 and 6 show the changes in
surface m o r p h o l o g y of electrolytes before and after imm e r s i o n for 5 and 10 w e e k s in m o l t e n Na2Ss, at 400~ This
c o m p o s i t i o n was c h o s e n in this case because Na2S~ is the
m o s t corrosive of the s o d i u m sulfides and h e n c e w o u l d
m a k e the differences m o s t obvious as i m m e r s i o n t i m e increased if the corrosion w e r e t i m e d e p e n d e n t .
Polished electrotytes (Fig. 5).--Figure 5a s h o w s the surface
of an as-polished electrolyte before i m m e r s i o n . F i v e w e e k s
later, s o m e corrosion c o m p o u n d s are n o n u n i f o r m l y distributed, and m o s t of t h e m are c o n c e n t r a t e d a r o u n d imperfections (Fig. 5b). After 10 weeks, a u n i f o r m c o m p o u n d
layer, c o v e r i n g the w h o l e surface, had f o r m e d and s o m e
p r e c i p i t a t i o n had occurred, leading to groups of plates h a p e d crystallites (Fig. 5c). The high magnification microg r a p h (Fig. 5d) shows that the layer is porous but not entirely uniform: more corrosion products had accumulated
in defect areas.
Sintered electrolytes (Fig. 6).---A c o m p a r i s o n w i t h virgin
materials clearly indicates that a c o m p o u n d layer, covering the entire surface, was well established after 5 w e e k s
i m m e r s i o n with s o m e crystals j u s t starting to grow (Fig.
6b). H o w e v e r , m a n y m o r e n e w crystals and c o m p o u n d s ,
f o r m e d at grain junctions, w e r e f o u n d on the surface after
10 w e e k s i m m e r s i o n (Fig. 6c and 6d). An e v e n h i g h e r magnification m i c r o g r a p h (Fig. 6e) s h o w s the detailed morWater (Fig. 3e and 4e).--In the case of a b s o r p t i o n of w a t e r
p h o l o g y of the flower-shaped crystal clusters and t h e thin
in electrolytes, there is an obvious e t c h i n g effect: the grain
layer. The thin layer looks p o r o u s and d e v e l o p e d d e e p e r in
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o x y g e n c o n t a m i n a t i o n on t h e N a z S J e l e e t r o l y t e s y s t e m are:
(i) creation of a corrosion layer on m o s t of the areas of the
p o l i s h e d surfaces (Fig. 3d); (ii) c o n s i d e r a b l e acceleration of
t h e reactions and increase of the d e p o s i t i o n of corrosion
products; and (iii) p r e c i p i t a t i o n of crystallized c o m p o u n d s
on the sintered surface in a d d i t i o n to the overall thin product layer that f o r m e d w i t h o u t o x y g e n present.
744
J. Electrochem. Soc.: S O L I D - S T A T E
t h e d e f e c t e d areas w h e r e p r e c i p i t a t i o n of crystallized corrosion products apparently had occurred.
Chemical characterization.--An A u g e r e l e c t r o n spect r u m o b t a i n e d f r o m a p o l i s h e d s u r f a c e of s a m p l e s imm e r s e d in t h e Na2Ss m e l t for 10 w e e k s is s h o w n i n Fig. 7a.
T h e e l e m e n t s p r e s e n t in t h e s p e c t r u m s h o u l d r e p r e s e n t
t h e a v e r a g e c o m p o s i t i o n of c o r r o s i o n c o m p o u n d s . F i g u r e
7b s h o w s t h e profile o f t h e i m p u r i t i e s in t h e d e g r a d e d layer
at d i f f e r e n t s p u t t e r i n g time. T h e q u a n t i t y of s u l f u r dec r e a s e s as s p u t t e r i n g t i m e i n c r e a s e s , clearly i n d i c a t i n g t h a t
a r e a c t i o n or c o r r o s i o n l a y e r h a d f o r m e d o n t h e e l e c t r o l y t e
surface. T h i s l a y e r w a s s p u t t e r e d off in 10 rain. T h e s u l f u r
p r o b a b l y w a s p r e s e n t as s u l f a t e s b e c a u s e a large q u a n t i t y
of o x y g e n w a s also d e t e c t e d at t h e s a m e time. A t t e m p t s to
i d e n t i f y t h e c o r r o s i o n c o m p o u n d s b y x-ray d i f f r a c t i o n
were unsuccessful.
Discussion
Experimental observations indicated that the electrolyte
r e a c t s c h e m i c a l l y w i t h Na2Sx melts. To i d e n t i f y p o s s i b l e
r e a c t i o n s at t h e e l e c t r o l y t e / m e l t interface, t h e r m o d y n a m i c
estimations were performed.
Thermodynamic considerations.--The ~"-alumina elect r o l y t e i n v e s t i g a t e d in t h e e x p e r i m e n t h a s a c h e m i c a l c o m p o s i t i o n of (0.86Na20 9 0.14Li20) 9 5.34A120~, w i t h s o m e
B-phase. As a r e a s o n a b l e a p p r o x i m a t i o n , t h e f o r m u l a
Na20 9 yA1203 is u s e d to r e p r e s e n t t h e m a j o r i t y B"-phase,
w i t h y = 5.34, a n d t h e m i n o r i y B-phase w i t h y = 8. T h e d a t a
o n t h e G i b b s free e n e r g y o f f o r m a t i o n of a n e l e c t r o l y t e
w i t h c o m p o s i t i o n of NasO 98A1203, Obtained b y W e b e r w i t h
e l e c t r o c h e m i c a l m e t h o d s a n d c a l c u l a t e d b y K u m m e r (6),
SCIENCE
AND TECHNOLOGY
March 1988
are u s e d for t h e B-phase. T h e s e d a t a w e r e e v a l u a t e d for
h i g h t e m p e r a t u r e s , a n d e x t r a p o l a t e d to t e m p e r a t u r e s
a r o u n d 300~176
T h e a p p r o x i m a t e G i b b s free e n e r g y of
f o r m a t i o n of t h e B"-phase w a s b a s e d o n t h e d a t a for t h e
B-phase a n d t h e Na20 activities i n B- a n d ~"-alumina, as rep o r t e d b y I t o h (7). T h e G i b b s free e n e r g i e s of f o r m a t i o n of
s o d i u m p o l y s u l f i d e s w e r e c a l c u l a t e d b y G u p t a (8) f r o m
o p e n - c i r c u i t cell v o l t a g e m e a s u r e m e n t s . O t h e r d a t a w e r e
o b t a i n e d f r o m t h e N B S t a b l e s of c h e m i c a l t h e r m o d y n a m i c
p r o p e r t i e s (9) a n d f r o m t h e J A N A F t h e r m o c h e m i c a l t a b l e s
(10). T h e AlzO~ r e l e a s e d in t h e r e a c t i o n s is a s s u m e d to b e
~-A1203, as p r o p o s e d b y C h o u d h u r y (3).
B a s e d o n t h e s e t h e r m o d y n a m i c d a t a ( T a b l e I) a n d t h e ass u m p t i o n s , t h e s t a n d a r d G i b b s free e n e r g i e s for t h e react i o n s b e t w e e n t h e solid e l e c t r o l y t e a n d s u l f u r / s o d i u m polysulfide m e l t s are e s t i m a t e d f r o m
ArGo = ~ (Pi 9h f~~ )(pFoduct)1
~
(Yl 9 Afv ~ )(reactant) [4.0]
The chemical reactions that could occur on the surfaces
o f t h e e l e c t r o l y t e are p o s t u l a t e d as follows
1. R e a c t i o n s b e t w e e n p u r e s u l f u r a n d t h e solid electrolyte
(3x + 1)
3
N a 2 0 - yA1203 + - S --~ - Na2S~
4
4
1
+ - Na2SO4 + yA12Oa
4
[4.1a]
Fig. 3. SEM micrographs of the polished surfaces of the electrolyte before and after five week immersion, at 400~ in Na2S3 melts, with or
without impurity contamination, (a) As prepared before immersion; (b) dry, uncontaminated samples immersed in pure Na2S3 (without impurity contamination); (c) dry samples immersed in Na2S3 with stainless steel (Fe-Ni-Cr) and graphite felt (C); (d) dry samples immersed in Na2S3 melt with an
oxygen atmosphere of about 100 torr; (e) wet samples immersed in Na2S3 melt; (f) samples exposed to moist air for one week before immersion.
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Vol. 135, No. 3
SODIUM
BETA"-ALUMINA
ELECTROLYTE
745
Fig. 4. Sintered surfaces of the electrolyte (same experimental conditions and arrangement as in Fig. 3)
(3x + 1)
N a 2 0 - yA1203 + - S --* Na2Sx
3
+ ~ A12(SO4)3 +
-
[4.1b]
(12x + 4)
2
N a 2 0 9 yA1203 + - S --~ - - NaAI(SO4)2
13
13
+ - - Na2S~ +
13
y -
A12Oa
[4.1c]
W h e n e x c e s s s u l f u r is available, as is t h e case w h e n t h e
e l e c t r o l y t e is i m m e r s e d in a s u l f u r melt, o n l y Na2Ss m a y
f o r m i n t h e a b o v e r e a c t i o n s ; h o w e v e r , Na2Sx w i t h x less
t h a n five m i g h t f o r m w h e n t h e a m o u n t of s u l f u r a v a i l a b l e
is l i m i t e d , as d u r i n g c h a r g i n g a Na/S b a t t e r y .
The chemical reactions between the electrolytes and
p u r e s u l f u r p r o p o s e d b y C h o u d h u r y (3) p r o d u c e c o r r o s i o n
p r o d u c t s Na2SO3, Na2SO4, S Q , a n d 02. O u r e s t i m a t i o n indicates, h o w e v e r , t h a t t h e t h e r m o d y n a m i c a l l y s t a b l e corr o s i o n p r o d u c t s are Na2SO4, AI_~(SO4)~, NaAI(SO4)2, etc., bec a u s e S O j S O 3 a n d 02 c a n r e a c t f u r t h e r to p r o d u c e sulfates.
T h e f o r m a t i o n of SO2 a n d NaSO3 is p o s s i b l e , b u t t h e form a r i o n of SO3 a n d Na2SO4 a p p e a r s m o r e f a v o r a b l e t h e r modynamically.
2. R e a c t i o n s b e t w e e n Na2S~ a n d t h e solid e l e c t r o l y t e
N a 2 0 . yA1203 +
(3x - 2)
1
4
Na2Sx -o ~ Na2SO4
Fig. 5. SEM micrographs of the polished surfaces of the electrolyte
immersed in pure Na2Ss, at 400~ for different periods of time. a, Virgin material as prepared before immersion; b, five week immersion; c,
(3x + 1)
ten week immersion; d, a high magnification micrograph showing the
+ - Na2S(~_I) + yAI203
[4.2a]
4
detailed surface morphology after ten week immersion.
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J. Electrochem. Soc.: S O L I D - S T A T E S C I E N C E A N D T E C H N O L O G Y
746
March 1988
7
A
6
5
i'
Z
2
0
I
0
0.2
0.4
Kinetic
0.6
0.8
Energy, k e y
6 i .../.. ~i.............
i Sputtered
i ~...........
i i100
..............
{.............i...........i............ii.............i::.............':............
angstroms/mlnute
5
........ !
re ative to Ta 205
..................
:
i:.............. T...........
3-
s
!
1"
0
2
4
Sputter
8
6
Time
10
(min.)
Fig. 7. (A) Auger electron spectrum of a polished surface of the electrolyte after ten week immersion in pure Na2Ss at 400~ The corresponding morphology is shown in Fig. 5 (c) and (d). (B) Profile of the impurities in the degraded (corrosion) layer formed on the electrolyte
surface during ten week immersion in pure Na2S5 at 400~ (concentration of impurities v s . sputtering time).
(xNaXNa (E) + e' + - -
1)
Na2Sx (M)
2
X
[4.3b]
--> - Na2S(x I>(M) + VNa' (E)
2
or
Fig. 6. Sintered surfaces of the electrolyte (same experimental conditions and arrangement as in Fig. 5). Fig. 6e is a high magnification
SEM microgroph showing the detailed morphology of the flower-shaped
crystal clusters and the degraded layer after 10 week immersion.
N a 2 0 9 yA1203 +
3
Na2S(~,-1) +
y -
A1203
4(3x
+
13
4 ( 3 x + 1)
2)
2
Na2Sx ~ - - NaAI(SO4)2
13
NazS(~ u +
( 1 )
y - ~
A1203
[4.2c]
13
The reactions with negative Gibbs free energies, see
T a b l e II, a r e t h e o n e s t h e r m o d y n a m i c a l l y
p o s s i b l e , alt h o u g h s o m e m a y n o t b e f a s t e n o u g h to b e s i g n i f i c a n t .
T h e r e a c t i o n m e c h a n i s m c o u l d i n v o l v e at f i r s t a p a r t i a l
Na + depletion of the electrolyte, described by
Na~N~ (E) = N a + (M) + VN~' (E)
Compound
350oc
400~
Source
Na2S~
Na2S4
Na2Sa
Na~S2
~-A12Oa
(y = 8)
~"-AlzOa
(y = 5.34)
Na2S
~-A1203
H20
CO~
NaOH
SO~
H2S
Na2SO4
Na2CO3
-95.8
-95.1
-92.7
-88.4
-2,963.9
-95.7
-95.1
-92.2
87.2
-2,930.4
-2,018.9
1,995.8
Itoh
-80.2
-353.9
-50.9
-94,5
-78,4
-80,2
-10,3
- 272.1
-228.4
-79.6
350.1
-50.3
-94.5
-77.0
-76.5
-10.4
267.4
-225.1
JANAF
A12(SO4)3
NaAl(SO4h
NaHSO4
-700.6
486.4
225.5
669.9
-474.2
-201.7
[4.2b]
3
N a 2 0 9 yA1203 -~
1
Table I. The extrapolated standard Gibbs free energies of formation
of the compounds involved in the reactions between the electrolyte
and sulfur/sodium polysulfide melts
(kcal/mol)
( 3 x - 2)
1
- - Na2Sx ---> A12(SO4)3
+-
x
Na~Na (g) + e' + ~ S (M) - ~ ~ NazS= (M) + VN~' (E)
[4.3a]
Gupta
Kummer
a
w h e r e E a n d M r e f e r to t h e s o l i d e l e c t r o l y t e a n d to t h e
a E s t i m a t e d from the relevant data in N B S and JANAF.
melt, respectively
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SODIUM
V o k 135, No. 3
BETA"-ALUMINA
Table II. The estimated standard free energies of the reactions
between the electrolyte and sulfur/Na2S~(M)(without impurity
contamination)
(kcal/mol)
(ArG~
(ArG~
:C
Equation no.
Eq. [4.1a]
Eq. [4.1b]
Eq. [4.1c]
Eq. [4.2a]
Eq. [4.2b]
Eq. [4.2c]
as in
Na2S~.
13
y=8
13"
y = 5.34
~
y=8
B"
y = 5.34
5
4
3
2
1
5
4
3
2
1
5
4
3
2
1
5
4
3
2
5
4
3
2
5
4
3
2
-6.8
-6.3
-4.6
-1.3
4.9
-1.3
-0.6
1.8
6.1
14.3
-3.0
-2.4
-0.2
3.8
11.4
-4.2
1.4
6.3
13.0
2.2
9.7
16.3
25.2
9.2
7.1
13.2
21.5
-10.5
-i0.1
-8.3
-5.0
1.1
-5.0
-4.3
-2.0
2.4
10.6
-6.7
-6.1
-3.9
0.1
7.6
-8.0
-2.4
2.6
9.3
- 1.6
5.9
12.5
21.5
-3.6
3.4
9.5
17.7
-9.1
-6.7
-6.5
-2.8
2.9
-1.8
-1.2
1.7
6.7
14.3
-4.9
-4.3
-1.7
3.0
10.0
-6.8
0.7
6.0
10.5
1.4
11.4
18.4
24.5
-2.0
7.3
13.8
19.3
-12.4
-11.9
-9.8
-6.0
-0.3
-5.0
-4.4
-1.5
3.5
11.1
-8.1
-7.6
-4.9
-0.3
6.7
-10.0
-2.5
2.8
7.3
- 1.9
8.2
15.2
21.2
-5.2
4.0
10.5
16.1
E l e c t r o n e u t r a t i t y r e q u i r e s t h a t t h e c r y s t a l r e l e a s e t h e exc e s s o x y g e n (Eq. [4.3c]). T h i s o x y g e n c a n r e a c t w i t h Na2Sx
(M) to give SO3 (Eq. [4.3d])
1
0 % (E) -~ Vo (E) + - 02 (M) + 2e'
2
ELECTROLYTE
747
s u r f a c e c o m p a r e d to t h e p o l i s h e d s u r f a c e f r o m w h i c h s u c h
films h a v e b e e n r e m o v e d .
Both experimental observations and thermodynamic
calculations, s u m m a r i z e d in Table II, i n d i c a t e t h a t t h e corr o s i o n p o w e r o f s o d i u m p o l y s u l f i d e m e l t s i n c r e a s e s as t h e
c o n t e n t o f s u l f u r i n c r e a s e s f r o m Na2S2 to Na2S~, t h e n o r m a l
c o m p o s i t i o n r a n g e o f a practical cell operation.
Influence of impurity contamination.--Oxygen.--With
t h e p a r t i c i p a t i o n o f o x y g e n , t h e c h e m i c a l r e a c t i o n s can b e
w r i t t e n as follows
2 ( x - 1)
2
02 + - Na20 - yA1203 +
Na2Sr
(3x + 1)
(3x + 1)
2x
--->
2 y ( x - 1)
Na2SO4 + - A1203
(3x + 1)
(3x + 1)
(x - 1)
02 + m
2(3x + 1)
Na20 9yA1203 +
2
Na~S~.
(3x + 1)
( x - 1)
2
--~ (3x + 1) NaAI(SO4)~ + (3x + 1) Na2SO4
(x+
2
2
2
02 (M) + ~ Na2S~ (M) --* -3 Na2S(x-~ (M) + ~ SOs (M)
1 ) ( y - 1)
A1203
2(3x + 1)
[4.4b]
[(3x - 2)(x - 1) + 12y]
O2 ~
Na2Sx
6(3x + 1)y
2 ( x - 1)
3(3x + 1)y
Na20 - yAI~O3
2 ( x - 1)
[4.3c]
[4.4a]
3(3x + 1)
(xA12(SO4)3 +
+
6y
[(x - 1) +
1)
Na2S(x-1)
12y]. Na2SO4
6(3x + 1)y
[4.4c]
[4.3d]
T h e SO3 c a n r e a c t f u r t h e r to p r o d u c e c o r r o s i o n p r o d u c t s
Na2SO~, NaAI(SO4h, A12(SO4)3, etc., t h r o u g h t h e r e a c t i o n s
SO3 + Na20 - yA1203 ~ Na2SO4 + yA1203
[4.3e]
1
1
(y - 1)
SO3 + ~ N a 2 0 . yAI_,O3 ~ - NaAI(SO4h + - A1203
2
4
1
y
SOa + - Na20 - yA1203 ~
A12(SO4)a
(1 + 3y)
(1 + 3y)
1
+- Na2SO4
(1 + 3y)
Obviously, t h e s e r e a c t i o n s are m u c h m o r e f a v o r a b l e
t h e r m o d y n a m i c a l l y in t h e p r e s e n c e o f o x y g e n , s i n c e t h e y
h a v e a large, n e g a t i v e hrG ~ as s h o w n in T a b l e III. E x p e r i m e n t a l r e s u l t s a g r e e d v e r y well w i t h t h i s calculation. A sign i f i c a n t reaction, e v i d e n c e d b y t h e d e p o s i t i o n o f c o r r o s i o n
p r o d u c t s , a n d p r e c i p i t a t i o n o f crystallized p r o d u c t s w e r e
f o u n d o n t h e e l e c t r o l y t e s u r f a c e s w h e n s o m e free o x y g e n
h a d b e e n i n t r o d u c e d into test t u b e s b e f o r e s e a l i n g (see
Fig. 3d a n d 4d). This f u r t h e r s u p p o r t s t h e a s s e r t i o n t h a t t h e
02 r e l e a s e d d u r i n g d e g r a d a t i o n is an i n t e r m e d i a t e p r o d u c t
a n d will r e a c t f u r t h e r to p r o d u c e sulfates.
Water.--When w a t e r h a s i n t e r c a l a t e d in t h e electrolyte, t h e
reactions b e t w e e n polysulfides and the electrolyte
changed and the corrosion products expected here were
NaHSO4 a n d NaOH, w h i c h c o u l d c a u s e c o n t i n u e d d i s s o l u tion of the electrolyte
T h e c o m b i n a t i o n o f t h e s e r e a c t i o n s gives t h e over-all rea c t i o n s in Eq. [4.1] a n d [4.2].
x(3x - 2)
(x - 1)
I n brief, (i) c h e m i c a l r e a c t i o n s initiate at s u r f a c e i m p e r Na20 9yAt2Oa +
. Na2Sx +
H~O
f e c t i o n s ; (ii) s i n c e t h e d e g r a d e d layer is p o r o u s , t h e reac( 4 x - 1)
( 4 x - 1)
t i o n will c o n t i n u e to d e v e l o p into t h e solid electrolyte,
p r e f e r e n t i a l l y at t h e s e areas o f i m p e r f e c t i o n ; (iii) o n l y t h e
(3x + 1)x
x
s u l f a t e s are t h e r m o d y n a m i c a l l y s t a b l e c o r r o s i o n p r o d u c t s ,
Na2S(~_~ +
NaHSO4
( 4 x - 1)
( 4 x - 1)
w h i l e t h e 02 a n d SO3 are i n t e r m e d i a t e c o m p o u n d s d u r i n g
degradation.
[3- or [Y'-alumina e l e c t r o l y t e s , u s e d in t h e a s - s i n t e r e d
(x - 2)
+ N a O H + yA1203
[4.5a]
state, m a y b e c o v e r e d b y a v e r y thin, soda-rich, g l a s s y sur( 4 x - I)
face film (5). S u c h a film w o u l d n o t b e as s t a b l e as t h e crystalline electrolyte. T h e r e f o r e , t h e r e a c t i o n s [4.1] a n d [4.2],
are e x p e c t e d to b e m o r e f a v o r a b l e initially o n t h e s i n t e r e d
If water and oxygen coexist, the reaction should be
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J. E t e c t r o c h e m . Soc.: S O L I D - S T A T E
748
Table III. The estimated standard free energies of the reactions
between the electrolyte and Na2S,(M) with impurity contaminations
(3x + 1)
Na2Sx + N a 2 0 9 yAI203 +
$
y=8
IB"
y = 5.34
$
y =8
~"
y = 5.34
Eq.[4.4a]
5
4
3
2
-91.6
-91.4
-91.5
-92.2
-93.4
-93.1
-93.0
-93.3
-90.4
90.1
-90.2
-90.9
-92.0
91.6
-91.5
-91.8
Eq.[4.4b]
5
4
3
2
-82.8
-83.3
-84.5
-87.2
-83.2
-83.7
-84.8
-87.5
-80.0
80.6
-81.9
-85.0
-80.4
-81.0
-82.2
-85.2
Eq.[4.4c]
5
4
-579.3
-445.8
-406.9
-319.6
-574.3
-438.8
-401.9
-313.4
3
2
-314.1
-190.6
233.9 - 3 0 6 . 5
-154.2 -186.3
(2-x+3Y)
A1203
3
4
--[Na2S~
(x + 2)
(x - 2)
+ 2
Eq. [4.5a]
5
4
3
2
-4.2
23.7
52.9
115.3
-22.0
5.0
32.4
89.2
8.3
47.2
81.5
136.4
-7.1
31.0
63.7
113.7
Eq. [4.5b]
5
4
3
2
-166.1
-181.7
-213.7
-311.8
168.9
-184.2
-215.6
-311.8
-138.6
-152.0
-179.9
-265.9
-141.0
-154.1
-181.6
-265.9
(3x + 1)
N a 2 0 9 yA1203 + - O~
2(x - 1)
+
1
1
[Na2Sz + H 2 0 + CO2] ~
Na2CO3
(x - 1)
(x - 1)
2
(x - 2)
+
NaHSO4 +
(x-
x
Eq.[4.6a]
5
4
3
-318.4
-238.8
-161.0
2
5
4
3
2
-85.1
-652.0
-537.1
-423.9
-312.6
-695.7
-540.8
-427.6
-316.4
-80.0
-578.3
-472.3
-368.6
-267.0
-581.5
-475.5
-371.8
-270.2
5
4
3
2
-669.7
-548.9
-429.8
-312.6
-676.2
-554.5
-434.4
-316.4
-610.0
-493.4
-379.2
-267.0
-615.7
-498.3
-383.2
-270.2
5
4
3
2
-722.4
-584.0
-447.3
-312.6
-737.4
-595.2
-454.8
-316.4
-672.2
-534.9
-399.9
-267.0
-685.1
-544.6
-406.4
-270.2
Eq.[4.6d]
$"
-301.0
-225.1
-151.5
1)
(x-
Na2SO4 + yA1203
1)"
H20 +
(x-
1)
1
N a 2 0 " yA1202 +
(x-
1)
[4.6d]
C o u p l e d w i t h o x y g e n or e v e n c a r b o n d i o x i d e , w h i c h
c o u l d c o m e f r o m t h e a b s o r p t i o n o f m o i s t air o n t h e e l e c t r o l y t e s u r f a c e d u r i n g cell c o n s t r u c t i o n , w a t e r d o e s m a k e t h e
p o l y s u l f i d e s m o r e s u s c e p t i b l e to c o r r o s i o n . T h e p o s s i b l e
reaction p r o d u c t s n o w include NaHSO4 and NaOH, w h o s e
m e l t i n g p o i n t s are 320 ~ a n d 317~ r e s p e c t i v e l y . T h u s , t h e
p r e s e n c e o f w a t e r c a n l e a d to a n i n c r e a s e d r e a c t i o n of t h e
e l e c t r o l y t e in t h e p o l y s u l f i d e m e l t s at a b o u t 350~ (see Fig.
3e a n d 4e).
Fe-Ni-Cr and C . - - A n i m p o r t a n t a s p e c t o f s u l f u r sided e g r a d a t i o n i n p r a c t i c a l Na/S cells is d e p o s i t i o n of m e t a l
sulfides originating from container corrosion. The deposits
f o r m e d o n t h e e l e c t r o l y t e s u r f a c e , as s h o w n in Fig. 3c a n d
4c, h a v e b e e n i d e n t i f i e d as FeS2, Cr2S3, Ni3S2, NiS2,
(Ni,Fe)S, a n d NaCrS2, etc., a n d are s i m i l a r to t h o s e rep o r t e d b y P a r k (11) a n d B a t t l e s (12). T h e p o s s i b l e r e a c t i o n s
c a n b e w r i t t e n as f o l l o w s
F e + 2Na2Sx-~ 2Na2S(x-l~ + FeS2
(x - 2)
[4.6c]
$"
Equation no.
Eq. [4.6c]
NaAl(SO4)a
(x + 2)
x
Eq.[4.6b]
[4.6b]
8
[x(y - 1) + 2(y + 1)]
- NaHSO4 +
A1203
( x + 2)
(x + 2)
-227.4
-150.2
Equation no.
(kcal/mol CO2 or H20)
(A~G~
(a~G~
$
$"
~
A12(SO4)3
4
CO2]-~--Na2CO3
(x + 2)
+H20+
(ArG~
~
(x-2)
+3
2 ( 3 x + 1)
N a 2 0 9 yA1203 + - 02
(x + 2)
(kcal/mol H20)
13"
02
2
+ H 2 0 + CO2 ~ Na2CO3 + 2NaHSO4
37
as in
Na2S.~
(ArG~
~
M a r c h 1988
AND TECHNOLOGY
(kcaYmol 02 )
(GG~
(&-G~
Equation no.
SCIENCE
[4.7a]
7Fe + 8Na2Sx-~ 8Na2S(z-l~ + Fe7S8
Na2S~
3FeS2 + 4SO3 + 7Na2Sx--~ 3FeSO4 + 7Na2S(x+l~
(3x + 1)(x - 2)
(x - 1)(2x - 4)
+
x
Oz -->
Ni + Na2Sx ~-~ NaaS(x-l~ + N i S
NaHSO4
(x - 1)
(x - 2)
(x - 2)y
-- NaOH + - A1203
(x
1)
( x - 1)
3Ni + 2Na2S~-~ 2Na2S~_~ + Ni3S2
2Cr + 3Na2Sx-+ 3Na2S(x-l~ + Cr2S3
[4.5b]
T h e a d d i t i o n a l p r e s e n c e of o x y g e n m a k e s t h e r e a c t i o n s
p o s s i b l e , a s it is e v i d e n t i n T a b l e III.
Moist a i r . - - I f s o m e 02, CO2, a n d H 2 0 are p r e s e n t b y abs o r p t i o n f r o m m o i s t air, w h i c h is a l m o s t u n a v o i d a b l e , t h e
f o l l o w i n g initial r e a c t i o n s a r e q u i t e f a v o r a b l e , as f o l l o w s
f r o m a n e x a m i n a t i o n o f t h e i r A~G~ v a l u e s l i s t e d in T a b l e III
3 ( x - l)
Na2Sx + H~O + CO2 + - O2
2
--->Na2CO3 + H2S + (x - 1) SO3
[4.7b]
[4.6a]
[4.7c]
3Cr + 2C ~ Cr3C2
7Cr + 3C --+ Cr~C3
The reactions between the electrolyte and carbon have
a l s o b e e n s t u d i e d r e c e n t l y b y C h o u d h u r y (3). T h e prop o s e d r e a c t i o n p r o d u c t s i n c l u d e Na~CO3 a n d CO.
Dissolution/precipitation.--As s h o w n in Fig. 5 a n d 6,
a f t e r five w e e k s i m m e r s i o n , t h e s u l f a t e s p r o d u c e d b y t h e
r e a c t i o n s d e s c r i b e d a b o v e , h a d n o t y e t s a t u r a t e d t h e Na2Sx
m e l t s , a n d n o t m u c h p r e c i p i t a t i o n w a s f o u n d at t h a t t i m e .
A f t e r 10 w e e k s i m m e r s i o n , h o w e v e r , t h e m e l t s w e r e s a t u r a t e d w i t h t h e s u l f a t e s , a n d a lot o f p r e c i p i t a t i o n of c r y s t a l lized p r o d u c t s w a s f o u n d o n all t h e s u r f a c e s o f t h e electrolyte. T h e flower-like c r y s t a l s , a s c l e a r l y s e e n in Fig. 6, are
b e l i e v e d to b e s o d i u m s u l f a t e s .
T h e H2S a n d SO3 c a n r e a c t f u r t h e r w i t h t h e e l e c t r o l y t e to
f o r m s u l f a t e s : NaHSO4, A12(SO4)3, NaAl(SO4)2, etc.
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SODIUM BETA"-ALUMINA ELECTROLYTE
VoL. 135, No. 3
It appears, see Fig. 1 and 2, that more chemical reactions,
deposition and or precipitation of corrosion products occurred in sulfur-rich melts, i.e., Na~S~ and pure sulfur, and
that more dissolution of corrosion products occurred in
sulfur-poor melt, i.e., Na2S2. This is perhaps due to the
changes of the solubility of the sulfates in the sulfide melts
as the composition of the sulfides changes.
749
Since the corrosive power of the polysulfide melts increases at a composition corresponding to the fully
charged state and is enhanced for free sulfur, it would
seem advisable to avoid heterogeneous electrode reactions
that expose the electrolyte surface for unnecessarily long
periods to the most corrosive sulfide melts that may occur
at the end of charge.
Conclusions
Acknowledgments
Sodium [3"-alumina electrolytes are chemically attacked
by Na2Sx melts and pure sulfur during immersion at high
temperatures. The likely reactions can be written as described in Eq. [4.1]-[4.7], and occur preferentially on surface
imperfections of the solid electrolyte.
The thermodynamically stable corrosion products are
Na2SO4, NaAl(SO4)2, A12(SO4)a, NaHSO4, Na2CO3, and
NaOH. Most of these have high melting point and deposit
on the electrolyte surface. Some have a low melting point
and can be dissolved in the polysulfide melts, leading to
dissolution or etching of the electrolyte surface.
There is a limited solubility of sodium sulfates in sodium
sulfide melts. Precipitation of the crystallized sulfates occurred after prolonged exposure to sulfide melts and occurred preferentially at imperfections such as boundaries
of large grains, or surface steps on the electorlyte surface.
The degree of corrosion increased as the content of sulfur increased in the range of Na2S2 to Na2S5 to sulfur.
I n addition to the reactions between the electrolyte and
Na~S~ or sulfur, the corrosion products of transition metals, and current collector material (graphite), also deposit
on the electrolyte surface.
All of the corrosion reactions were greatly accelerated by
contaminations of impurities, such as water and oxygen,
and transition metals.
I n a practical cell, the environment that the electrolyte
will be exposed to is expected to be a combination of the
above situations to some degree. The degraded layer
formed on electrolyte surfaces in an actual Na/S cell would
be expected to be a mixture of (i) sulfates and carbonates,
produced by reactions [4.1] through [4.6], and (it) corrosion
products of cell parts, by reactions [4.7]. This layer may
block sodium-ion transport, cause inhomogeneous current
distribution, and could spall off when new compounds
form underneath it, leading to a progressive degradation of
positive electrode contact interface of the electrolytes.
The authors wish to thank Dr. S. Visco for valuable discussions and suggestions. This work was supported by the
U.S. Department of Energy under contract no. DE-AC0376SF00098.
Manuscript submitted July 7, 1986; revised manuscript
received Aug. 12, 1987.
Lawrence Berkeley Laboratory assisted in meeting the
publication costs of this article.
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Development of Low Chromium Substitute Alloys for High
Temperature Applications
Je M. Oh*
United States Department of the Interior, Bureau of Mines, Albany Research Center, Albany, Oregon 97321
ABSTRACT
The oxidation of a base-line Fe-8Cr-16Ni (weight percent) composition was compared to that of alloys with small additions of A1, St, Mo, and Mn. Oxidation was done in air over the temperature range of 600~176
for up to 1000h. Alloys that
contained A1 and Si showed excellent oxidation resistance because of the formation of a layer containing Si at the scalemetal interface. Small additions of A1 seemed to form oxide nuclei during a transient oxidation period, aided the formation of a layer containing Si and/or a chromium oxide, and provided better oxide adherence. Molybdenum and manganese
additions stabilized a fully austenitic microstructure at room temperature. The oxidation behavior of these alloys was
compared to that of 304 stainless steel and some commercial superalloys. Reaction kinetics, oxide morphologies, and oxidation mechanisms of the substitute alloys are described.
One of the Bureau of Mines research goals is to minimize
the requirement for domestically scarce minerals through
substitution and conservation. Chromium, in particular, is
the element which provides the characteristics of oxidation and corrosion resistance to stainless steels (SS) by
forming a protective chromium oxide scale. Because the
~Electrochemical Society Active Member.
United States has no commercial chromium deposits,
chromium is considered to be a particularly vulnerable
strategic metal. Therefore, there is interest in developing
low chromium alloys that could substitute for stainless
steels in some applications. A significant fraction of the
stainless steel used in the United States is used in applications where resistance to high temperature oxidation is
iml3ortant.
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