Thermodynamic investigations of ternary lithium-transition metal-oxygen cathode materials

Mat. R e s . B u l l . , Vol. 15, p p . 561-570, 1980. P r i n t e d in t h e USA. 0025-5408/80/050561-10502.00/0 C o p y r i g h t (c) 1980 P e r g a m o n P r e s s Ltd.

THERMODYNAMIC INVESTIGATIONS OF TERNARY LITHIUM-TRANSITION METAL-OXYGEN CATHODE MATERIALS

N. A. Godshall, I . D. R a i s t r i c k , and R. A. Huggins Department of Materials Science and Engineering Stanford U n i v e r s i t y , Stanford, C a l i f o r n i a 94305

(Received February

1, 1980; R e f e r e e d )

ABSTRACT The reaction of l i t h i u m with three t r a n s i t i o n metal oxides (MnO, LiFeOz, and LiCoOz) has been investigated by an e q u i l i b r i u m e l e c t r o chemical technique in c e l l s of the type:

(-) AI,Lio.gAl/LiCl-KCl(1)/[LixMOy] (+) Each system e x h i b i t s long constant voltage plateaus characterized by three-phase e q u i l i b r i a . The compositional range of reaction of l i t h i u m with MnO is 2.0 equivalents, whereas3.0 equivalents may be reacted with the compounds LiFeOz and LiCoOz. The ternary ircn and cobalt oxide systems have been found to be k i n e t i c a l l y fast (I0-15 mA/cmz) and r e v e r s i b l e at 400°C. The free energies of formation ~Gf ° of LisFeO~, LiFeOz, and LiCoOz w e r e calculated and f o u n d to be -399.88, -154.18, and -131.62 kcal/mole, r e s p e c t i v e l y . Replacementof s u l f i d e with oxide cathode m a t e r i a l s might reduce the high temperature l i t h i u m battery corrosion problems c u r r e n t l y associated with s u l f u r containing c e i l s .

Introduction Recent work on cathode materials capable of reversibly reacting with significant amounts of a l k a l i metal ( L i , Na) f o r use in high-temperature b a t t e r ies has focused on the s u l f i d e s of a number of t r a n s i t i o n metals such as iron and t i t a n i u m , eg. FeS, FeSz, and TiSz [ 1 , 2 ] . However, the use of such s u l fides at high temperatures, t y p i c a l l y 400-450Oc, causes severe material corrosion problems. Several ternary t r a n s i t i o n metal oxide systems [LixMnO], [LixFeOz], and [LixCoOz] h a v e b e e n investigated because of the reduced corrosive nature of oxides compared with t h e i r s u l f i d e counterparts. Thesemetal oxides have the further advantage of r e q u i r i n g no special handling or preparation techniques, since they can be prepared in a i r , unlike the s u l f u r - c o n t a i n i n g compounds.

561

562

N.A.

GODSHALL, et al.

Vol. 15, No. 5

The s t r u c t u r e of most ternary Li-M-O compounds (where M i s a f i r s t - r o w t r a n s i t i o n metal) has been found to depend l a r g e l y on the metal-to-oxygen ratio. For example, nearly a l l compounds with 57.1 at.X oxygen e x i s t with the spinel s t r u c t u r e - - r e g a r d l e s s of the l i t h i u m - t o - t r a n s i t i o n metal r a t i o . Likewise, nearly a l l compounds with 50 at.X oxygen e x i s t with a NaCl-related s t r u c t u r e . Previous work in the Li-Ti-O ternary system indicated rapid l i t h i um-ion transport in the NaCl-related s t r u c t u r e s , although t h e i r voltages in l i t h i u m c e l l s were quite low [ 3 ] . For t h i s reason, i t was desired to study NaCl-related s t r u c t u r e s in other t r a n s i t i o n metal systems. The electrochemical reduction of MnO, LiFeOz, and LiCo0z by reaction with l i t h i u m was s t u d i e d . i n c e l l s of the type: ( - ) A I , L i o ~ A I / L i C I - K C I ( I ) / [ L i x M O y ] (+) M = Mn, Fe, Co The cathode material ( p o s i t i v e electrode) was a ternary compound composed of l i t h i u m , oxygen, and a f i r s t row t r a n s i t i o n metal, M, such as Mn, Fe, or Co. lhe choice of t r a n s i t i o n metal was r e s t r i c t e d to the f i r s t row of the t r a n s i t i o n metal series, in order to preserve a high s p e c i f i c energy. Theseionic oxides have been found not to be soluble in the LiOI-KCI e l e c t r o l y t e . Rectangular brackets have been used throughout t h i s work to connote an o v e r a l l composition of the p o s i t i v e electrode. That i s , [LixFe0z] does not imply single-phase behavior over the e n t i r e compositional range of " x " . Rather, t h i s method of describing the o v e r a l l electrode composition i s very general, and has the advantage that i t requires no p r i o r knowledge of which phases, or even how many phases, are present in the electrode.

Experimental Procedure The e l e c t r o l y t e used in a l l experiments was LiOI-KCl molten s a l t of euteotio composition (42 moleX KCl). Experiments were performed at 400°C inside a h e l i u m - f i l l e d glove box from which Hz0, 0z, and Nz had been removed to less than

1 ppm.

L i t h i u m i o n s were passed t h r o u g h t h e l i q u i d e l e c t r o l y t e from a n e g a t i v e AI,Lio3A! counter electrode. In a d d i t i o n , t h e c e l l v o l t a g e was measured b e t ween t h e positive working e l e c t r o d e and a r e f e r e n c e electrode of t h e same A1,Lio~AI two-phase composition, w h ic h was a l s o in c o n t a c t w i t h the LiC1-KCl electrolyte. At 400°C, the voltage of t h i s two-phase A I , L i o p A l mixture l i e s 300 mV more p o s i t i v e than pure l i t h i u m of u n i t a c t i v i t y [ 4 , 5 ] . I t was necessary to use an a l l o y of l i t h i u m in the LiCI-KCI melt because exposure of the melt to u n i t a c t i v i t y of l i t h i u m causes e l e c t r o n i c leakage [6] and p a r t i a l

decomposition [7]. An experimentally applied current was imposed across the eel1, and the c e l l voltage was continuously monitored as a f u n c t i o n of time. The i n i t i a l c o n d i t i o n , with no current flow, was that of a constant open c i r c u i t c e l l p o t e n t i a l Co. A constant c u r r e n t was t h e n a p p l i e d f o r a f i x e d t i m e , after which t h e c e l l was once a g a i n open c i r c u i t e d .

Vol. 15, No. 5

CATHODE MATERIALS

563

The c e l l p o t e n t i a l responded f i r s t with an immediate IR drop across the e l e c t r o l y t e , followed by a d i f f u s i o n l i m i t e d , or phase-change l i m i t e d time-dependent process. That i s , the p o t e n t i a l of the cathode changed because the a c t i v i t y of Li changed at the surface, as the Li concentration increased w i t h i n the bulk o f the sample. T h e n , a f t e r the current was stopped, the conc e n t r a t i o n of Li again became constant throughout the bulk, and a new e q u i l i brium voltage CI was observed [ 8 ] . This process was repeated many times, so that the o v e r a l l composition of a s i n g l e sample was changed c o u l o m e t r i c a l l y . This was a much f a s t e r and more accurate method than t r y i n g to measure many d i f f e r e n t i n d i v i d u a l l y - p r e p a r e d samples. The path over which the o v e r a l l composition of a h y p o t h e t i c a l cathode material MOy i s changed i s shown schematically by the dashed l i n e of Figure 1(a). The e q u i l a t e r a l t r i a n g l e represents an isothermal section of a ternary diagram. The three elements ( L i , M, and O) l i e at the corners of the diagram. The darkened areas represent single-phase regions, and the l i n e d areas represent two-phase regions, in which the l i n e s themselves represent t i e l i n e s , the same as are found in two-phase regions in binary phase diagrams. The open areas represent regions of three phases in e q u i l i b r i u m , or t i e t r i a n g l e s . I t may be noted t h a t , in general, as l i t h i u m i s e l e c t r o c h e m i c a l l y reacted with an i n i t i a l phase MOy, i t i s possible f o r the electrode composition to pass through one, two, and three-phase regions. A corresponding e q u i l i b r i u m p o t e n t i a l curve i s i l l u s t r a t e d in Figure 1(b), where tile e q u i l i b r i u m voltage in one and two-phase regions i s shown to vary with the amount of l i t h i u m present in the sample. In three-phase regions, however, i t i s known from the Gibbs phase r u l e for a ternary system that a l l i n t e n s i v e v a r i a b l e s are f i x e d , so that the a c t i v i t y of Li ( a n d hence the e q u i l i b r i u m p o t e n t i a l of L i ) would be independent of the amount of Li present.

O~gen

. . . . . . . .

~

. . . .

1 . . . .

~A

I

(~)

I

. . . .

I

. . . .

"Tn-B

I

....

~ m [~o~

(b) FIG.

I:

One(1), t w o ( I I ) , and t h r e e ( I I I ) - p h a s e regions i l l u s t r a t e d in ( a ) a h y p o thetical ternary isotherm and (b) an e q u i l i b r i u m p o t e n t i a l curve.

I.

564

N.A.

GODSHALL, et al.

Vol.

15, No. 5

Experimental Results Reaction of l i t h i u m with the binary oxide MnO resulted in the e q u i l i b r i u m voltage curve shown in Figure 2, w h e r e the open c i r c u i t p o t e n t i a l is p l o t t e d on the ordinate, and tile number of Li equivalents reacted per t r a n s i t i o n metal is p l o t t e d on tile abscissa. The f i r s t few Li atoms passed through the c e l l were injected i n t o a single phase region of small compositional width. F r o m 0.05 l i t h i u m to 2.0 lithiums, i t was found that three e q u i l i b r i u m phases coincided to give a constant voltage of +0.92 v o l t s versus pure Li. As 2.0 l i t h i u m s were approached, tile voltage was found to drop towards zero, since the average valence of the manganese ion also approached zero at t h i s l i m i t .

I B.O

'

' ' '

I ' '

''

I ' ' ' '

i ' ' ' '

-

I

'~

I

,

T=4~O°C n - o - n IN

r-1

X.,X..X

Ft

O~

REGION

V~

11

~

1.0

O.B

o.o

y.,

~

r~,'~-"~'

....

. . . . . . . . .

w

-

I

0

,

,

,

,

I

0.5

,

,

,

,

I

,

,

1

,

,

I

,

,

,

1.5

,

B

z in [T.Lu-O] Equilibrium

FIG. 2: P o t e n t i a l Curve f o r

X-ray powder p a t t e r n results showed t h a t i n c l u d e d the o r i g i n a l MnO, e l e m e n t a l Mn, and LizO.

[LixMnO]

the three phases p r e s e n t The proposed c e l l r e a c t i o n

is simply: 2 Li + MnO ~ LizO + Mn

(I)

It was found that this system oould be reversed, as illustrated by the crosses in Figure 2. Withdrawal of Li, however, was not complete, probably due to the dispersion of some lithium oxide into the surrounding electrolyte.

Vol. 15, No. 5

CATHODE MATERIALS

565

Several ternary iron oxides were i n v e s t i g a t e d in search of higher c e l l voltage reactions. Results o f l i t h i u m reaction with the ternary compound LiFeOz, where one l i t h i u m equivalent was already present in the s t r u c t u r e at the i n i t i a t i o n of the experiment, i s shown in Figure 3. Once again a narrow region of r a p i d l y decreasing voltage was found, followed by a long three-phase plateau. This plateau yielded a voltage of 1.323 v o l t s versus Li (or 1.023 V vs. A I , L i o ~ A I ) , but existed over a compositional range of only 1.5 l i t h i u m equivalents. I t was immediately followed by another three-phase plateau at 1.297 V versus Li (0.997 V vs. A I , L i o ~ A I ) which also covered a range of 1.5 lithium equivalents. That i s , 3.0 l i t h i u m s could be reacted with the i n i t i a l compound LiFe0z, but two d i s t i n c t plateaus (which d i f f e r e d by only 26 mV) were observed. The l o c a t i o n of t h i s change in voltage (x=2.5) corresponded to the p o s i t i o n of the t i e l i n e between LisFe0~ and Fe. The proposed reaction for the complete reduction of LiFeOz i s : 3 Li + LiFeOz ~ 2 Liz0 + Fe The LiFe0z c e l l was r e v e r s i b l e , tional limits.

I

(2) so long as

. . . .

I

it

. . . .

was not cycled to i t s composi-

I

. . . .

S.5

I

'-

T=400oC

~.o

1.5

-

~

.

~

1.o

~1

0.5

o.o

.

.

.

.

'nO0

0

-

I

,

,

,

,

1

I

,

,

,

,

2

1

,

,

,

,

3

x in [U~e02] FIG. 3: E q u i l i b r i u m P o t e n t i a l Curve f o r [LixFe0z]

Reaction of Li with LiCo0z yielded s i m i l a r behavior to that found with LiFeOz--except that the three-phase plateau existed at a higher p o t e n t i a l , 1.636 v o l t s versus Li (or 1.336 V vs. AI,LioogAI) with no noticeable change in the e q u i l i b r i u m p o t e n t i a l over the e n t i r e r a n g e of 3.0 l i t h i u m equivalents, Figure 4. Once again, three l i t h i u m s could be reacted before the voltage decreased to zero. 3 Li + LiCo0z ~ 2 Liz0 + Co

(3)

566

N.A.

GODSHALL,

et al.

Vol.

15, N o .

5

The LiCo0z c e l l e x h i b i t e d the same r e v e r s i b l e behavior as found with LiFe0z. The currents which could be passed through c e l l s containing such ternary compounds (with NaCl-related s t r u c t u r e s ) have been found to be quite favorable. A current density of 10 to 12 mA/cmz could be passed during both charge and discharge of pressed p e l l e t samples of LiFeOz. The LiCoOz compound was s l i g h t l y f a s t e r s t i l l , y i e l d i n g 12 to 15 mA/cmz. In comparison, the Mn0 samples reached high o v e r p o t e n t i a l s (300-400 mV) at even 2 to 4 mA/cmz. Similarly, e q u i l i b r a t i o n times f o r the reaction of l i t h i u m with LiFe0z and LiCo0z samples were on the order of seconds, whereas e q u i l i b r a t i o n times f o r the MnO samples were on the order of minutes and hours.

I 2.5

-

2.0

-

. . . .

1

,..2,.'2.

1.5

I

. . . .

I

T=400Oc

k

.~.

. . . .

-

a~

~

-

~

-

THREE PHASE REGION

1.o

-

O

~

0.5

o.o

-

l 1

,

,

,

,

]

,

,

,

2

,

I

,

,

3

x in

,

,

I 4

[L~CoO,]

FIG. 4: E q u i l i b r i u m P o t e n t i a l Curve for [LixCoOzI

Discussion Much recent work on cathode materials has focused on solid-solution electrode materials, as exemplified by many transition metal dichalcogenides like LixTiSz, LixVSz, and LixNbSez [9]. These materials involve only a single phase, whereas displacement reaction m a t e r i a l s , s u c h as i l l u s t r a t e d in equat i o n s I through 3, involve multi-phase reactions. The often c i t e d advantage of s o l i d - s o l u t i o n reactions is that of easy r e v e r s i b i l i t y , since no phase change i s involved [10,11]. Two disadvantages are that t h i s leads to a composition-dependent voltage and, in common cases, a rather l i m i t e d homogeneity range of less than one Li e q u i v a l e n t . Displacement reactions, on the other hand, lead to a constant e q u i l i b r i u m voltage over t h e i r useful compositional range. In a d d i t i o n , tile t h e o r e t i c a l

Vo]. 15, No. 5

CATHODE MATERIALS

567

s p e c i f i c energy of displacement reaction electrodes i s g e n e r a l l y higher than for s o l i d - s o l u t i o n electrodes, since t h e i r range of composition can be as high as 3 or 4 l i t h i u m e q u i v a l e n t s . The psuedo-ternary isotherm f o r the Li-Fe-O system is shown in Figure 5 to illustrate several essential features. The t h r e e ternary compounds, LiFesOe, L i Fe O z , and LisFeO~, a l l l i e on t h e t i e l i n e between LizO and Fez03, w h i c h a l s o r e p r e s e n t s a l i n e o f c o n s t a n t Fe v a l e n c e o f 3+. In f a c t , it is a g e n e r a l phenomenon t h a t l i n e s e m a n a t i n g from LizO r e p r e s e n t l o c i of c o n s t a n t transition metal valence. I t may a l s o be o b s e r v e d t h a t t h e t i e l i n e between LizO and e l e m e n t a l Fe r e p r e s e n t s a l i n e o f z e r o Fe v a l e n c e . L i t h i u m c o u l d be reacted w i t h the s t a r t i n g m a t e r i a l u n t i l the o v e r a l l c o m p o s i t i o n reached t h i s l i n e o f c o m p l e t e r e d u c t i o n t o LizO and t r a n s i t i o n m e t a l M: (2y-x)

Li

+ LixMOy ~ y LizO + M

(4)

Titration o f Li i n t o a g i v e n s t a r t i n g m a t e r i a l , l i k e L iFe O z , f o l l o w s the p a t h o f t h e dashed l i n e i n t h i s f i g u r e . T h e r e f o r e , by u s i n g d i f f e r e n t starting m a t e r i a l s , i m p o r t a n t phase d i a g r a m i n f o r m a t i o n can be d e t e r m i n e d , since tile transition from one v o l t a g e p l a t e a u t o a n o t h e r on an e q u i l i b r i u m p o t e n t i a l d i a g r a m i n d i c a t e s t h e c r o s s i n g o f a t i e l i n e i n t h i s t e r n a r y i s o t h e r m [ 1 2 ] , and t h e r e f o r e a l s o i n d i c a t e s a change i n t h e e l e c t r o d e r e a c t i o n s .

Oxysen

~

FezOs

;, L i ~ F e FIG.5: Lithium-lron-Oxygen

T e r n a r y System a t

400°C

Such i s t h e case i n t h e i r o n s y s t e m , where t h e p r e s e n c e o f t he m a r g i n a l l y s t a b l e compound LisFeO~ s p l i t s the r e a c t i o n p a t h i n t o two c o n s t a n t - v o l t a g e tie t r i a n g l e s which d i f f e r slightly i n v o l t a g e (26mY). That i s , reaction 2 is more p r o p e r l y e x p r e s s e d as t h e c o m b i n a t i o n o f two r e a c t i o n s = 1.5 Li

+ LiFeOz ~ 0 . 5 LisFeO~ + 0 . 5 Fe

8 o = 1. 023 V

(5)

568

N.A.

GODSHALL, et al.

Vol. 15, No. 5

And 1.5 Li + 0.5 LisFeO~ + 0.5 Fe -~ 2 LizO + Fe

C° = 0.997 V

(6)

This technique also permits a determination of the thermodynamic propert i e s of these intermediate ternary compounds where l i t t l e or no l i t e r a t u r e data existed thus far. The i n t e g r a l f r e e e n e r g y of f o r m a t i o n (AGf ° ) of each initial compound was c a l c u l a t e d by i n t e g r a t i o n of its respective equilibrium potential c u r v e [ 5 ] a c c o r d i n g to the e q u a t i o n :

2y AGfO(LixMOy) = zaFICOdx + y A G f ° ( L i z O )

(7)

X

The f r e e e n e r g i e s of f o r m a t i o n of LisFe0~, LiFeOz and LiCo0z were c a l c u l a t e d and f o u n d to be - 3 9 9 . 8 8 , - 1 5 4 . 1 8 , and - 1 3 1 . 6 2 k c a l / m o l e , respectively. The reliability of t h e s e v a l u e s i s t h o u g h t to be g o o d , s i n c e t h e v a l u e f o r manganese ( I I ) o x i d e o b t a i n e d here ( - 8 0 . 3 4 kcal/mole) a g r e e s q u i t e w e l l w i t h the literature value (-80.14 kcal/mole) [13]. This difference c o r r e s p o n d s to a d i f f e r e n c e of o n l y 3 mV i n t h e v o l t a g e p l a t e a u . S t a r t i n g compounds lying closer to t h e oxygen c o r n e r in this diagram yield higher cell voltages, because t h e s e h i g h e r o x i d e s r e p r e s e n t l e s s s t a b l e compounds. They t h e r e f o r e y i e l d more e n e r g y when reduced to s t a b l e compounds during the e l e c t r o c h e m i c a l reactions which occur within the c e l l . This effect, unfortunately, i s somewhat l i m i t e d by t h e f a c t t h a t t h e h i g h e s t o x i d e s reduce to lower oxides when placed in the molten s a l t e l e c t r o l y t e . T h a t is, t h e i r e q u i l i b r i u m oxygen p a r t i a l pressure is too high when in contact with the LiCI-KCI s a l t . For instance, HnOz and LiMnzOw y i e l d very high t h e o r e t i c a l voltages versus l i t h i u m , but were found to reduce when placed in the molten salt.

Conclusions

These r e s u l t s are summarized i n T a b l e 1. I t was f o u n d t h a t the LiFeOz and LiCo0z compounds p e r f o r m q u i t e f a v o r a b l y as c a t h o d e m a t e r i a l s in hightemperature molten salt cells, yielding 3 equivalents of Li per mole at attractive v o l t a g e s : a p p r o x i m a t e l y 1.0 v o l t v e r s u s A 1 , L i o ~ A I f o r t h e i r o n s y s tem, and 1.34 v o l t s v e r s u s A l , L i o p A 1 f o r t h e c o b a l t system a t 400°C. Both the k i n e t i c s and v o l t a g e of t h e manganous o x i d e appear t o be too low for practical use i n m o l t e n s a l t b a t t e r i e s . However, the k i n e t i c s of the i r o n and c o b a l t systems appear to be q u i t e fast. I t was p o s s i b l e to u t i l i z e curr e n t densities for sintered pellets on the order of I0 to 15 mA/cm z at low overpotentials, and with rapid equilibration. Powdered materials should yield considerably higher apparent current densities,

Vol. 15, No. 5

CATHODE MATERIALS

569

TABLE I Results of Lithium Reaction with MnO, LiFeOz, and LiCoOz at 400°C.

RANGE COMPOUND

MnO

CELL

AGf°

OF x

POTENTIAL

(kcal/mole)

(Ax)

(V vs. A I , L i o ~ A I )

CURRENT

SPECIFIC

DENSITY

ENERGY

(mA/cmz)

(Wh/Kg)

-80.34

2.0

0.620

2-4

237

LisFeO~

-399.88

1.5

0.997

10-12

313

LiFeOz

-154.18

3.0

1.023

10-12

413

LiCoOz

-131.62

3.0

1.336

12-15

539

The t h e o r e t i c a l s p e c i f i c energies of the iron and cobalt systems make them quite a t t r a c t i v e , y i e l d i n g 900 and 1110 Wh/Kg with respect to pure lithium, respectively. Tile corresponding values, using an AI,Lio~AI negative electrode, are 413 Hh/Kg f o r the i r o n , and 539 Hh/Kg for the cobalt compound. These values may be compared with the AI,Lio.gAI/FeS c e l l which has a t h e o r e t i cal s p e c i f i c energy of 458 Hh/Kg. The p o t e n t i a l advantages of these oxide systems over t h e i r s u l f i d e count e r p a r t s are t w o - f o l d . The ease of preparation of such oxide materials compared with t h e i r s u l f u r analogs might g r e a t l y reduce t h e i r production costs, since these compounds are t y p i c a l l y prepared in a i r . Tile most important advantage, however, would l i k e l y be the markedly reduced corrosion problems associated wi'th these oxides, in comparison with tile s u l f i d e s used c u r r e n t l y . I n v e s t i g a t i o n of other t r a n s i t i o n metal oxide systems i s c o n t i n u i n g , w i l l be reported l a t e r .

and

Acknowledqement This work was supported by the United States Department of Energy under contract number EC-77-S-02-4506 and LBL subcontract number 4503110.

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Properties

of