Phase Equilibria in Lithium-Chalcogen Systems
II. lithium-Sulfur
P. T. Cunningham, S. A. Johnson, and E. J. Cairns*
Argonne Nationa~ Laboratory, Chemical Engineering Division, Argonne, Illinois 60439
ABSTRACT
The e q u i l i b r i u m phase diagram of the l i t h i u m - s u l f u r system has been investigated. A miscibility gap extends from 0.2 to 37.0 atom per cent (a/o) lithi u m above a monotectic t e m p e r a t u r e of 364.8~ the critical t e m p e r a t u r e is
greater t h a n 600~ The only intermediate phase found is Li2S, which melts
at 1372~
Recent research and development work on h i g h - s p e cific-power, high-specific-energy secondary cells based
on alkali m e t a l / c h a l c o g e n couples (1) has focused attention on the need for more complete phase equilibr i u m data for these systems. The work reported here
was u n d e r t a k e n as part of a broad p r o g r a m to determine the l i t h i u m - c h a l c o g e n phase diagrams. Results
for l i t h i u m - s e l e n i u m have been published (2).
Previous work on the l i t h i u m - s u l f u r system has not
been extensive. 1 Bergstrom (3) reported the p r e p a r a tion of l i t h i u m sulfide and polysulfides in liquid a m monia. Pearson and Robinson (4) have reported their
preparation in aqueous and alcoholic solution and by
direct combination of the elements u n d e r boiling
napthalene. Pearson and Robinson also reported a
portion of the Li-S phase diagram between Li2S and
Li2S2, indicating that Li2S was stable up to its observed
m e l t i n g point (900~176
and that Li2S2 was slightly
decomposed at its reported m e l t i n g point of 369.5~
The compound Li2S prepared in liquid a m m o n i a has
been reported to have a n antifluorite structure (5)
with a lattice p a r a m e t e r of 5.708A.
Experimental
Materials.--The sulfur used was a special h i g h - p u r i t y
(99.999+%) grade obtained from A m e r i c a n Smelting
and Refining Company, South Plainfield, New Jersey.
The lithium, a h i g h - p u r i t y reactor grade (99.98%), and
l i t h i u m sulfide ( 9 7 + % ) were obtained from the Foote
M i n e r a l Company, Philadelphia, P e n n s y l v a n i a . These
materials were used w i t h o u t f u r t h e r purification. Li2S
was mixed with the appropriate t e r m i n a l phase in a
p u r i f i e d - h e l i u m - a t m o s p h e r e glove box (6) to obtain the
desired composition for the various experiments. The
samples were contained in borosilicate glass, quartz,
a l u m i n u m , vitreous graphite, or n i o b i u m d e p e n d i n g on
the t e m p e r a t u r e range covered and the sample composition.
Apparatus and procedures.--The e x p e r i m e n t a l techniques used included t h e r m a l analysis (heating and
cooling curves), differential t h e r m a l analysis (DTA),
h i g h - t e m p e r a t u r e centrifugation, chemical analysis,
and x - r a y diffraction.
Heating and cooling curves were obtained on samples weighing from 1O to 20g using a f u r n a c e - w e l l
attached to the floor of the h e l i u m - a t m o s p h e r e glove
box (7). S u l f u r - r i c h samples for t h e r m a l analysis were
contained in borosilicate glass, quartz, or a l u m i n u m
vessels, which were u s u a l l y sealed to p r e v e n t rapid
sulfur loss. The necessity of sealing the sample cont a i n e r made cooling curves almost useless because of
extreme undercooling of samples that were not vigorously stirred. Some samples were r u n in open containers with stirring b u t the compositions of these
* Electrochemical Society Active Member.
K e y w o r d s : l i t h i u m , c h a l c o g e n , s u l f u r , sulfide.
1 D u r i n g t h e p r e p a r a t i o n of t h i s p a p e r , i t c a m e to o u r a t t e n t i o n
t h a t R. S h a r m a h a d also s t u d i e d t h i s s y s t e m c o n c u r r e n t l y a n d h a d
also p r e p a r e d a m a n u s c r i p t .
samples are uncertain. T e m p e r a t u r e s were measured
with p l a t i n u m vs. p l a t i n u m - l O t r h o d i u m t h e r m o couples fabricated from wire which had been calibrated against NBS pure tin (mp 231.88~
zinc (mp
419.58~
and a l u m i n u m (rap 660.0~
DTA samples, which weighed about 80 mg, were
contained in a l u m i n u m or n i o b i u m capsules w i t h pressfit caps. Data were obtained with a du Pont Model 900
Differential T h e r m a l A n a l y z e r using a 5 ~
rate
of t e m p e r a t u r e change with a powdered a l u m i n a reference m a t e r i a l and a d y n a m i c helium atmosphere. The
thermocouples used were described above a n d the
over-all accuracy of the apparatus was d e t e r m i n e d to
be .+_I~ using NBS s t a n d a r d quartz a n d KNOa. 2
The m e l t i n g point of Li2S was d e t e r m i n e d using a
m o l y b d e n u m - w o u n d resistance furnace. The samples
were placed in n i o b i u m or vitreous graphite crucibles
and heated in an argon atmosphere. T e m p e r a t u r e was
measured with a calibrated optical p y r o m e t e r and the
occurrence of fusion was visually determined.
Chemical analysis was used to d e t e r m i n e the compositions of materials t a k e n from samples in which
phase separation occurred. Phase separation u n d e r n o r mal gravity was usually slow, and in m a n y cases a
h i g h - t e m p e r a t u r e centrifuge was used to increase the
r a t e of phase separation. A m i x t u r e of Li2S and sulfur
having the over-all composition of interest was placed
in a quartz ampoule, which was evacuated and sealed.
The ampoules were placed in a rocking furnace and
mixed for several hours (in some cases several days)
at a t e m p e r a t u r e about 25~ above the p l a n n e d centrifugation t e m p e r a t u r e prior to placing them in the
centrifuge. After centrifugation for several hours at
speeds corresponding to centrifugal forces about 250
times n o r m a l gravity, the ampoules were quenched in
liquid nitrogen and placed in a h e l i u m - a t m o s p h e r e
glove box where they were b r o k e n open and the zones
of the samples were carefully separated mechanically.
The same q u e n c h i n g a n d separation procedure was
used for samples in which phase separation was
achieved at n o r m a l g r a v i t y after 7 to 10 days at t e m perature. Chemical analysis was carried out on the
separated materials using flame photometry to analyze
for lithium. The sulfur content was calculated b y
difference. The accuracy of this procedure (--+5% of
the l i t h i u m weight) was confirmed by analyzing several samples for sulfur as well, using s t a n d a r d g r a v i metric techniques.
X - r a y diffraction powder patterns were obtained for
a large n u m b e r of samples having a wide v a r i e t y of
compositions and t h e r m a l histories, using a 114.6 m m
D e b y e - S c h e r r e r camera with CuK~ radiation.
Results and Discussion
All of the e x p e r i m e n t a l results are s u m m a r i z e d in
Fig. 1. The t h e r m a l analysis and DTA results, which
T h e q u a r t z a n d K N O s are S R M 755 a n d S R M 750, r e s p e c t i v e l y ,
a v a i l a b l e f r o m t h e Office of S t a n d a r d R e f e r e n c e M a t e r i a l s , :National
B u r e a u of S t a n d a r d s , W a s h i n g t o n , D.C. 20234.
1448
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Vol. 119, No. 11
PHASE EQUILIBRIA: Li-S SYSTEM
14oo~ , I
1300~--6 0 0 --
\
\L;
L~
LI+L
2
5 0 0 -(D
o
Li 2 S(s) + L 2
Jnf--
400
(3E
hJ
13hJ
t--
Li(I)
{37.o)
300
Li(I) + Li2S(s )
LizS(s) + L I
200 -
r
180.5
/Li2S(s)+
S(mono)
Li(s)+ LiaS(s)
10( --
9 i
I00
Li
t
I
80
i
I
L i 2 S (s) + S (ortho)
i
60
40
COMPOSITION, o t % Li
f
I
20
0
S
Fig. 1. Partial phase diagram of the lithium-sulfur system. 0 ,
Thermal analysis; ~ , DTA; I-1, chemical analysis.
a r e p r es en t ed in Table I, show that five t h e r m a l effects
w e r e observed. The few m e a s u r e m e n t s m a d e on the
l i t h i u m - r i c h side of Li2S indicate that Li2S does not
affect the m e l t i n g point of pure l i t h i u m (180.5~
within the accuracy of these m e a s u r e m e n t s ( _ I ~
and that there are no other t h e r m a l invariants present
in this composition r a n g e b e lo w 860~
The melting
4- 10
point of Li2S was observed to be 1372~
~ in the
--
5
.
resistance f u rn ace for samples contained in both niobium and vitreous graphite crucibles. This v al u e is
considerably h i g h e r than that reported by Pearson and
1449
Robinson (4), w h o indicated that t h e i r e x p e r i m e n t s
w e r e v i t i a t e d by r ap i d reaction of t h e samples w i t h
t h ei r glass and porcelain containers, a p r o b l e m w hi c h
we also encountered.
On the s u l f u r - r i c h side of Li2S, t h e r m a l effects w e r e
observed at 104.0 ~ • 3.7~ 115.5 ~ -- 1.0~ and 364.8 ~ +_
4.0~
These t e m p e r a t u r e s are w e i g h t e d a v e r a g e s
w h e r e the w e i g h t i n g is a d m i t t e d l y s o m e w h a t subjective. The indicated uncertainties show the v a r i a n c e of
the data. The effects observed at 104 ~ and 115.5~ are
associated with the t r a n s f o r m a t i o n f r o m a ( o r t h o rhombic) sulfur to # ( m o n o cl i n i c) sulfur an d the m e l t ing of # sulfur, respectively. The accepted v a l u e for
the = --> # transition is 95.5~ (8) but in none of our
experiments, even w i t h pure sulfur, was the effect
observed at this temperature. The fact that the ~-#
transition is observed about 10 ~ above t h e e q u i l i b r i u m
t e m p e r a t u r e is t y p i cal and in good a g r e e m e n t w i t h the
t e m p e r a t u r e for t h e appearance of the ~ f o r m in thin
films of = sulfur (9). P u r e sulfur was observed to melt
at llS.O~ in our experiments, in fair a g r e e m e n t with
the accepted v a l u e of 118.9~ (8). W i t h l i t h i u m present, this transition was observed at 115.5~
w hi c h is
significantly l o w er than 118.9~ and is p r e s u m a b l y the
t e m p e r a t u r e of a eutectic reaction [L1 --> Li2S(s) +
sulfur].
The t h e r m a l effect at 364.8~ is a t t r i b u t e d to a m o n o tectic reaction [L2 -~ Li2S(s) + L1]. The e x t e n t of the
miscibility gap (see Fig. 1) was d e t e r m i n e d b y c h e m ical analysis of m a t e r i a l t ak en f r o m two easily distinguishable zones, w h i c h w e r e observed in samples
q u e n c h e d f r o m t e m p e r a t u r e s above 375~ and of o v e r all composition f r o m 15 to 30 a / o lithium. The upper
zone was v e r y similar to p u r e sulfur w h i c h had the
same t h e r m a l history, w h e r e a s the l o w e r zone was a
red, g l a s s y - a p p e a r i n g m a t e r i a l w h i c h appeared to be
homogeneous but w h i ch gave a r o o m - t e m p e r a t u r e
x - r a y p o w d e r p a t t e r n cl ear l y showing Li2S and ~ sulfur. These zones are associated w i t h the separated
liquid phases LI and L2, r e s p e c t i v e l y (see Fig. 1).
Separation of the liquid phases u n d e r n o r m a l g r a v i t y
was slow; about 6 days w e r e r e q u i r e d to ach i ev e clear
separation. With c e n t r i f u g a t i o n at 250g, separation was
achieved in 6 hr. Chemical analyses, the results of
w h i ch are shown in Table II, w e r e p e r f o r m e d on m a t e rial that was m e c h a n i c a l l y separated. Considerable
care was t a k e n to insure that the samples w e r e homogenous and cross-contamination, which would lead
to an a p p a r e n t n a r r o w i n g of the miscibility gap, was
Tabie L Summary of thermal data on the lithium-sulfur system
Over-all
composition
( a / o IA i n S)
M e t h o d (~)
Container
material
100 ( L i )
87.1
66.7 (Li2S}
66.7 (Li2S)
66.7 (Li2S)
66.7 ( L i ~ )
63.3
60.0
52.2
47.5
41,9
40.5
37:3
36.4
34.3
30.0
28.2
23.7
23.1
14.8
13.0
9.3
9.0
8.6
5.0
4.7
0.0 (S)
DTA
DTA
DTA
TA
Visual
Visual
DTA
TA
DTA
DTA
DTA
DTA
DTA
DTA
DTA
DTA
DTA
DTA
TA
TA
TA
TA
DTA
DTA
TA
DTA
DTA
Nb
Nb
A1
Quartz
Nb
Vitreous graphite
A1
A1
A1
A1
Nb
Nb
A1
A1
A1
A1
A1
A1
AI
Glass
Glass
Glass
A1
A1
Glass
A1
A1
Maximum
temperature
(~
475
860
400
1080
1425
1400
400
395
400
410
405
405
420
405
405
400
405
405
390
400
410
500
400
390
400
400
395
Weight average values
Temperature
of t h e r m a l e f f e c t s o b s e r v e d (~
130.3
160.7
No effect observed
No effect observed
1372
1372
109.3
-102.6
102.9
106.3
105.6
105.6
105.0
104.9
105,0
105,2
105.0
-101.9
101.0
102.7
102.6
102.6
-102.8
102.6
104.0
114.6
-114.5
115.5
116.4
116.4
118.1
116.4
115.4
116.3
115.8
--
359.1
367.0
365.0
366.8
369.2
364.1
366.0
365.5
367.1
360.7
367.1
364.1
365.9
359.5
365.0
366.0
363.4
365.0
3,61.0
363.0
11"4.7
115.0
114.6
114.5
114.9
1~.9
118.0
115.5
(a) M e t h o d s u s e d w e r e h e a t i n g a n d c o o l i n g c u r v e s ( T A ) w h e r e t h e r e p o r t e d t e m p e r a t u r e s
t h e r m a l a n a l y s e s (DTA) w h e r e r e p o r t e d t e m p e r a t u r e s a r e those of the i n t e r s e c t i o n .
180.5
a r e t h o s e of t h e r m a l
364.8
arrests,
1372
a n d differential
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1450
J. Electrochem. Soc.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y
Table II. Summaryof chemicalanalysis
C o m p o n e n t a n a l y z e d (see Fig. 1)
Temperature
prior to
quenching
(~
L~
L~-
Sample
weight
(g)
Lithium
weight
(mg)
Atom
per cent
lithium
Sample
weight
(g)
456
417
417
398
396
0.0995
0.1340
0.4032
0.3087
0.1771
0.3421
0.1735
0.1448
---
0.054
0.084
0.15
0.099
0.082
0.15
0.012
0.0097
--
0.25
0.29
0.17
0.15
0.21
0.20
0.032
0.031
--
0.0961
0.1573
0.2248
0.1923
0.1615
0.1143
0.1319
0.1910
0.4123
0.1717
10.9
17.2
25.4
21.4
19.3
12.8
14.8
20.7
43.4
16.8
37.3
36.2
37.1
36.7
38.5
36.8
36.8
36.0
35.2
33.4
375
375
0.1558
0.2777
0.21
0.48
0.1523
0.1981
17.0
22.0
36.7
36.6
602
602
508
508
456
377
0.3580
0.~
0.~
0.62
0.79
0.1684
Lithium Atom
weight per cent
(mg)
lithium
19.0
37.0
avoided. Scatter in the data for the l i t h i u m - r i c h side
of the miscibility gap is generally w i t h i n the experim e n t a l u n c e r t a i n t y of the analysis ( •
of the l i t h i u m
weight p r e s e n t ) , b u t there is greater scatter on the
s u l f u r - r i c h side. Based on these results, the region of
immiscibility is t a k e n to be from about 0.2 to 37.0 a/o
l i t h i u m at the monotectic temperature.
T h e r m a l effects associated with liquidus crossings
were not observed. A t t e m p t s to establish the location
of the upper liquidus b y d e t e r m i n a t i o n of the equilibr i u m sulfur vapor pressure as a function of t e m p e r a ture for compositions greater t h a n 37 a / o l i t h i u m gave
very scattered results. The time necessary for equilibration between the vapor and the condensed phase
was at least several days. It is significant that the
vapor pressure of samples with less t h a n 34 a/o l i t h i u m
was not significantly different from that for pure sulfur
w i t h i n the e x p e r i m e n t a l u n c e r t a i n t y ( • 10 % ).
X - r a y powder patterns were t a k e n on a large n u m ber of samples of various compositions and t h e r m a l
histories. I n all cases, the observed lines were accounted for b y the appropriate t e r m i n a l phase and
Li2S.
N o v e m b e r 1972
The phase d i a g r a m which we have found for the
l i t h i u m - s u l f u r system is v e r y m u c h like that for the
l i t h i u m - s e l e n i u m system, and previous comments (2)
regarding that system and the implications of the phase
equilibria in terms of the performance and design of
electrochemical cells will not be repeated. Additional
investigation of the l i t h i u m - s u l f u r system is needed
in the region b e t w e e n Li2S and l i t h i u m and at t e m peratures greater t h a n 600~
Acknowledgments
The authors are indebted to A r g o n n e ' s Analytical
Support Group for assistance in x - r a y and chemical
analyses and to Dr. R. C. Vogel and Dr. A. D. Teveb a u g h for their e n c o u r a g e m e n t a n d support. This work
was performed u n d e r the auspices of the U n i t e d States
Atomic Energy Commission.
Manuscript submitted April 13, 1972.
A n y discussion of this paper will appear i n a Discussion Section to be published in the J u n e 1973 JOURNAL.
REFERENCES
1. E. J. Cairns and H. Shimotake, Science, 164, 1347
(1969); E. J. Cairns, R. K. Steunenberg, and H.
Shimotake in "Encyclopedia of Chemical Technology" S u p p l e m e n t Volume, 2nd ed, p. 120, J o h n
Wiley a n d Sons, Inc., New York (1971). These
articles review work on l i t h i u m chalcogen cells,
as well as other high t e m p e r a t u r e systems and
contain additional references.
2. P. T. C u n n i n g h a m , S. A. Johnson, and E. J. Cairns,
This Journal, 118, 1941 (1971).
3. F. W. Bergstrom, J. Am. Chem. Soc., 48, 146 (1926).
4. T. G. P e a r s o n and P. L. Robinson, J. Chem. Soc.,
1930, 413.
5. E. Zintl, A. Harder, and B. Dauth, Z. Elektrochem.,
40, 588 (1934).
6. C. E. Johnson, M. S. Foster, and M. L. Kyle, Nucl.
Applications, 3, 563 (1967).
7. C. E. Johnson, S. E. Wood, and C. E. Crouthamel,
J. Inorg. Chem., 3, 1487 (1964).
8. "Handbook of Chemistry and Physics", 52nd ed., The
Chemical R u b b e r Co., Cleveland, Ohio (1971).
9. "Elemental Sulfur," B. Meyer, Editor, Interscience,
New York (1965).
Electrochemical Behavior of Titanium
Effect of Ti(lll) and Ti(IV)
N. T. Thomas* and Ken Nobe*
Schoo~ ol Engineering and Applied Science, University of Calilornia, Los Angeles, California 90024
ABSTRACT
Anodic polarization of t i t a n i u m i n 1N H2SO4 containing T i ( I V ) showed
that T i ( I V ) inhibited dissolution a n d facilitated passivation. Polarization of
t i t a n i u m in IN H2SO4 c o n t a i n i n g both T i ( I I I ) and Ti(1V) indicated that
T i ( I I I ) accelerated the anodic dissolution of t i t a n i u m in both the active and
passive region. A n empirical expression for anodic dissolution of t i t a n i u m i n
1N H2SO4 was obtained
ia : ka[H+] ~
exp (0.43 F~/RT)
A dissolution m e c h a n i s m based on the b a r r i e r layer model and T e m k i n adsorption of an adsorbed i n t e r m e d i a t e is shown to be consistent with the exp e r i m e n t a l data. A similar m e c h a n i s m is used to i n t e r p r e t the accelerated dissolution of t i t a n i u m with Ti (III).
Not m a n y investigations on the effect of t r i v a l e n t
a n d t e t r a v a l e n t t i t a n i u m on the polarization of t i t a n i u m
electrodes have been reported. A n d r e e v a and K a z a r i n
(1) and W e i m a n (2) reported that t e t r a v a l e n t t i t a n i u m
had an i n h i b i t i n g effect on the corrosion of t i t a n i u m in
* Electrochemical Society A c t i v e M e m b e r .
Key words: anodic dissolution, polarization, corrosion.
sulfuric acid a n d in boiling 5% HC1, respectively.
Tsvetnova et al. (3) found that the presence of t r i v a l e n t
t i t a n i u m had a depassivation effect on passive titanium.
I n this study, the effect of t e t r a v a l e n t and t r i v a l e n t
t i t a n i u m on the electrochemical behavior of t i t a n i u m
is examined.
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