Thermodynamic and electrochemical investigations of the Nasicon solid solution system

Solid State Ionics 18 & 19 (1986)969-973 North-Holland, Amsterdam

969

THERMODYNAMICAND ELECTROCHEMICALINVESTIGATIONS OF THE NASICON SOLID SOLUTION SYSTEM

Joachim MAIER, Udo WARHUS, and Eberhard GMELIN M a x - P l a n c k - l n s t i t u t f u r Festk~rperforschung, D-7000 Stuttgart-80

The thermodynamic properties of the Nasicon s o l i d s o l u t i o n system (Na1+vZrgSi P: 01~, O
1. INTRODUCTION The Nasicon s o l i d s o l u t i o n system is expected to o f f e r good prospects for a h i g h l y conduct i v e and a p r e t t y cheap s o l i d sodium e l e c t r o l y t e f o r the sodium s u l f u r c e l l . I Numerous in-

SAMPLES 2.1 Experimental 6 As the r e s u l t of a v a r i e t y of experiments ( a l l s o l i d state reactions, l i q u i d phase sin-

vestigations have been performed concerning

t e r i n g , sol-gel prpcess, hydrothermal synthesis)

the preparation conditions and the e l e c t r i c a l

and of educts the two f o l l o w i n g optimized pre-

conduction properties. 2 But in spite of the

paration procedures were used: ( i ) a dry oxi~c

extreme importance of i n v e s t i g a t i n g the i n t r i n -

process (800°C-1200°C) using ZrSi04, Na2C03,

sic thermodynamic s t a b i l i t y

ZrP207 and Na2SiO3 as s t a r t i n g materials f o r

of the s o l i d solu-

tions (Nas(x))- e.g. against a decay i n t o the

Nas(l.5~x~3) and ( i i )

end-members or against a p r e c i p i t a t i o n of ZrO2

ing from aqueous solutions of Na2SiO3,NH4H2P04,

- as well as the thermodynamic s t a b i l i t y

ZrOCI 2 • 8H20 and Na2CO3 for Nas(O~x~l.8). The d e t a i l s and the advantages of these methods

against

the active masses in the sodium s u l f u r c e l l ,

a sol-gel process s t a r t -

are discussed elsewhere. 6

no q u a n t i t a t i v e data besides one enthalpy value 3 f o r Na4Zr2Si3012 has been found in the l i t e rature. Nonetheless, such experiments are pre-

were annealed in t i g h t l y closedPt-capsules.

suppositions f o r a f u r t h e r discussion of the

The samples were characterized by DSC measure-

u s a b i l i t y of the materials and possible optimi-

ments, optical methods ( p o l a r i z a t i o n microscope,

zations. The i n t r i n s i c s t a b i l i t y

SEM), X-ray i n v e s t i g a t i o n s ,

of Hong's pha-

ses have been questioned by several authors 4'5

For long-time s t a b i l i t y

tests the compounds

l y s i s (ICP), by density and

by chemical anaweight control.

because the reaction of the stoichiometric a-

2.2 Results and Discussion

mounts of the educt phases very e a s i l y r e s u l t

By the help of the optimized procedures sin-

in product mixtures with s l i g h t amounts of ZrO 2 as a second phase.

gle phase Nasicon of the ideal composition could be prepared (15 d i f f e r e n t compositions6). Long-time d u r a b i l i t y tests (3-5d) indicate that the compositions are stable upto the

0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

970

~ Maier et al. / The Nasicon solid solution system

solidus temperatures. The v a r i a t i o n of the preparation methods shows that the presence of a

I

> OZ

CaCO31CaO

J/"

ZrO2-phase originates from imperfect conditions, be i t that ZrO2 does not completely react as a poorly reactive educt or be i t that i t is formed during the preparation process. The l a t t e r

i

""

86o

o5t

~ooo

"%'~T- ' ~

i

is possible e i t h e r by being formed as an i n e r t intermediate or being produced by v o l a t i l i z a t i m

/

of Na- or P-compounds in open systems, or by LL°

exceeding the solidus temperatures of Nas(x). Moreover, the p o l a r i z a t i o n microscope revealed

--

s

t 900

700 151

t I100

CaCOs/CoO

i

in accordance with the l i t e r a t u r e 7'8 always glassy phases (beneath Hong's Nasicon) in those materials which have been prepared according to ref. 4 but in t i g h t l y closed Pt-capsules. Our IR spectra did not show the c h a r a c t e r i s t i c sig-

>1o 700

900 T/K

1100 --

nals of condensed s i l i c a t e or phosphate groups being postulated as structure elements in r e f . 5 . 3. ELECTROCHEMICALAND CALORIMETRICMEASUREMENTS 3.1 Experimental For the electrochemical determination of the thermodynamic data, the cell

FIGURE 1 Cell voltage vs. temperature f o r d i f f e r e n t compositions. The l i n e a r branches correspond to d i f f e r e n t P(CO~)-regimes. (The a i r branch for Nas(2) correspbnds - in contrast to Table i to a mixture of Na~ZrSi~O7, ZrP?O7 and ZrO? which i~ k i n e t i c a l T y stable at To@er tempe= ratures±~ ) (v(0,3) = 1/2 and v(O~x<3) = (1+x)/2), CaCO3/CaO

-Au,CO2/Na2CO3/Na-B-alumina/Nas(x), ZECE(X)/Au+ has been used, where SEeE denot~ those coexist-

buffer p e l l e t s were a d d i t i o n a l l y brought into

ing phases which are formed i f small amounts of

the cell tube. The cell could be i s o b a r i c a l l y

Na20 were removed from Nas(x) (see Table I ) .

closed and heated in a resistance furnace. The

In order to f i x the CO2 p a r t i a l pressure which

02 p a r t i a l pressure is determined by the ambient

affects the overall reaction (cf. Table I)

atmosphere (normally a i r ) . elsewhere 6,9,10.

v(x) Na2CO3+ZEVE(X)¢E(X)~Nas(x)+v(x)C02 Table I : Coexisting phases x 0 0.5 2.0 2" 5 3

x 0 0.5 2.0 2.5 3

#E(X) ZrPgO,; ZrO~ ZrP~07/;"ZrO~ ; ZrSiO a ZrP~O'; SiOZ ZrSiO~ L I , 4 . ZrP,,O~; SIO~;- ZrSlO~4 ~ / ZrO2; Na2Zr~1207

~E(X) 1.5; 0,5 1.25; 0.25; 0.5 0.5; 0.5; 1.5 0.25; 0.75; 1.75 0.5; 1.5

(i)

Details can be found

At very low temperatures the s p e c i f i c heats (I.5K - lOOK) were measured by f u l l y automated adiabatic calorimetry (ADC)11. The data above lOOK were obtained by DSC measurements. (i00 550K : Perkin Elmer DSC2, 400 - 810K : Du Pont thermoanalyser type 990). 3.2 Results, Evaluatio~and Discussion For three examples the reversible parts of the e.m.f. (E) vs. T curves are shown in Fig. i . One recognizes several l i n e a r branches. Typicall y the low temperature branch corresponds to the regime where P(C02) is f i x e d by the CO2 p a r t i a l

J. Maier et al. / The Nasicon solid solution system

971

were also calculated f o r Tm, whereby the react i o n entropy has to be corrected by the term RInP(C02). The data obtained by d i f f e r e n t branches agree f a i r l y w e l l , a fact which increases /111~ j1111~ 0

the r e l i a b i l i t y

and proves the correct P(C02)

dependence. Starting from Tm the S~(x) and AfH~(x) values are computed by using our Cp re-

mJm ~ m c ~ , : 2 i V, V ~ ~'

V

'

~

J

K

NASICON (x: 3 }

s u l t s and also l i t e r a t u r e data for the elements

to.550K."D~ (perkinKlrner] (opensymbols) 400Kto 8IOK:DSC (DuPont) T/K 50O

60O

700

Na, Zr, P, Si, and 02 . A l l Cp-data above room temperature can be represented by the polynomial 80O

FIGURE 2 The s p e c i f i c heat as a function of temperature f o r Nas(O,l,2,3). pressure of the s t a r t i n g atmosphere (normally

Cp(X,T) = A(x) + B(x)T + C(x)T -2 + D(x)T -0"5 + E(x)T 2

(2)

Hence, the standard entropy and the standard formation enthalpy f o r each composition and each temperature has the form

a i r : 30 Pa C02). The branch at moderate tempe-

S°(x,T) = K~(x) + A(x)InT + B(x)T - ~C(x)T -2 - 2D(x)T -0"5 + ~E(x)T 2 (3)

ratures corresponds to the regime where the CaCO3/CaO b u f f e r begins to work. Depending on the amount of material a t h i r d branch may arise

and

at higher temperatures, where the buffer is used

AfH°(x,T) + A~A(x)T + ~ f B ( x ) T 2 a f C ( x )~- + = KH(x) 2AfD(x) /T + ~ f E ( x ) T 3(4)

up and a constant p a r t i a l pressure, which equals to P(C02, buffer) f o r the point of i n t e r s e c t i o n tained. This point of i n t e r s e c t i o n agrees with

respectively. The operator Af refers to the f o r mation of the elements. The r e s u l t i n g c o e f f i -

the calculated value. The point of i n t e r s e c t i o n

cients are l i s t e d in Table I I , where the r e l e -

of the low-temperature branch and the buffer

vant phase t r a n s i t i o n s are included. The com-

of the two high temperature branches, is main-

branch y i e l d s quite accurately the p a r t i a l pres-

plete procedure w i l l be given in more d e t a i l

sure of CO2 in a i r .

elsewhere. I0 The obtained thermodynamic data

The s p e c i f i c heats are shown in Fig. 2 f o r d i f f e r e n t compositions. One recognizes phase t r a n s i t i o n s for Nas(2) 12'13 as well as f o r Nas (3)12,14" To ensure a maximum precision, the evaluatim has been performed as follows: from the several l i n e a r branches the standard enthalpy and the standard entropy of the reaction was calculated f o r a mean temperature Tm where the l i n e a r E-T r e l a t i o n s h i p is c e r t a i n l y f u l f i l l e d . From l i t e rature data 15,16 for the thermodynamic values of the compounds involved in eq. I , the standard formation enthalpy and the standard entropy

Table I I : Coefficients o f the therm, functions x 0 0.5 2.0 2.5 3 x

10-3A

B

J/molK

J ~

0.3882 -4.5203 0.8059 -0.6819 3.8754

0.1013 4.8522 -0.6685 1.1106 -2.8147

IO-3&A J/molK

&B

10-7C JK /mol -0.4665 -5.5903 -1.5617 -1.9364 3.5091 IO-7AC

J/mo-~IK JK/mol

(el.chem.)

I0-4D

103E

J/molK "5

J/molK j

-0.0974 7.2820 -0.2120 1.6671 -5.4749

0.0000 -2.2061 0.4866 -0.4300 1.1809

IO-4AD

I03AE

j/~.5

~

10-4K~ J/molK -0.2016 3.2905 -0.4334 0.5817 -2.7013 IO-6KH J/mol

0 -0.0467 0.1275 -0.5604 0.1091 -0.0583 -4.9220 0.5 -4.9720 4.8797 -5.6871 7.4895 -2.2606 -6.6040 2.0 0.3042 -0.6371 -1.6672 -0.0015 0.4436 -5.9290 2.5 -1.2004 1.1433 -2.0449 1.8786 -0.4692 -6.4441 3 3.3402 -2.7807 3.3977 -5.2624 1 . 1 4 5 5 -5.2299 (&:formation data, r e f e r to Na(1),Zr(s),Si(s),P(red).O2(g ) )

972

,L

Maier et al. / The Nasicon solid solution system

Table I I I : Thermodynamicstandard data for 298 K &H°,e.m,f. S°,e.m.f. S°,cal. &HO,]it. x "~'T~-~ ~7~ITJT~ 0 -4877 371 365 --0.5 -5112 375 379 --2.0 -5813 406 414 --2.5 -6044 420 421 --3 -6278 429 425 (-6357) 3 (A:formation data,refer to Na(s),Si(s),P(red),O2(g))

Tabie IV: Thermodynamic stability against Na x products of the degradation ~degG/kJmoi-I reaction (deg) 298 K 600 K 0 3.0Na3P+2.0Na2ZrO3+6.0Na~O -1120 -930 0.5 2.SNa3P+2.0Na2ZrO3+O.5Na~SiO4+4.0NagO -!070 -910 2.0 1.0Na3P+2.0Na2ZrO3+I.5Na~Si04+O.SSi~ -720 -640 2,5 0.SNaIP+2.0Na~ZrO~+I.5Na~SiO.+I -450 -380 Z J ~ .~ "OSi 3 2.0Na2Zr03+1.5Na4S~04+1.5SI -170 -130

r e s u l t is t h a t the i n t e r a c t i o n parameter WGWhich contains the whole thermodynamic i n f o r m a t i o n , f o r 2 9 8 K a r e l i s t e d in Table I I I .

reads approximately

I n t e g r a t i n g our Cp-data (see Fig. 2) and s e t t i n g S~(x) to zero leads to independent values f o r S~(x). The comparison of the S°(x) va-

Wc(x'T)

_+ 7 = 1+9x+.007[~ l - .O09x[~]

(7)

[ kJ mo1-1]

lues obtained c a l o r i m e t r i c a l l y with those ob-

The negative values f o r AmixG, which may be ob-

tained by e.m.f, measurements (cf. Table I I I )

tained from Table I I ,

i n d i c a t e the i n t r i n s i c

are in very good agreement, which again increa-

stability

ses the r e l i a b i l i t y

end-members. This is supported by the long-time

of our e l e c t r o c h e m i c a l l y

of Nas(x) concerning a decay i n t o the

measured AfH°-data (computed from the same cur-

s i n t e r t e s t s (see above). A m i s c i b i l i t y

ve!) The only a v a i l a b l e l i t e r a t u r e

expected at lower temperatures f o r a subregular

value f o r

AfH°, v i z . f o r Nas(3), which has been determined

model I I

calorimetrically

could not be found e x p e r i m e n t a l l y , c e r t a i n l y

by Shibanov et a l . 3 is 80 kJ

smaller than our value (see Table I I I ) .

By

temperature ~ (600 ~ IO0)K)

because of the low r e a c t i o n rates.

repeating the e v a l u a t i o n made by these authors, but using up-to-date l i t e r a t u r e

(critical

gap being

data, a corrected

The s i n t e r tests also show the s t a b i l i t y against a Z r O 2 - p r e c i p i t a t i o n ( f o r a thermodynamic approach see r e f .

value is obtained which is greater than the o r i g i n a l one I0.

11).

On the basis of the thermodynamic data the

The thermodynamics of the s o l i d s o l u t i o n system can be described by a mixture model. For

thermodynamic s t a b i l i t i e s

against sodium and

s u l f u r are also c a l c u l a t e d . The assumed degra-

t h i s purpose, the excess f r e e enthalpy of Nas(x),

dation reactions (deg) and the corresponding

GeX(x), which equals the increase of G by the

°G-values (the data f o r the products are ta-

mixing process (AmixG(X))

ken from r e f s . 9, 10, 15, 16) are l i s t e d in

( 3 - x ) / 3 Nas(O) + x/3 N a s ( 3 ) ~ N a s ( x )

(5)

Table IV. One recognizes t h a t Nas(x) is unstable against sodium, a r e s u l t which had also been

in comparison to an ideal mixture of Nas(O) and

confirmed e x p e r i m e n t a l l y , with the exception of

Nas(3), is w r i t t e n as

Nas(3). 14 This composition seems to be stable 14 f o r k i n e t i c reasons. As Table IV shows, the

GeX(x,T) = 1/9 x (3-x) WG(X, T)

(6)

Si end-member reveals the lowest a f f i n i t y WG is zero f o r an ideal m i x t u r e , constant con-

values.

These r e s u l t s loosened i n t e n s i v e experiments to

cerning x f o r a r e g u l a r , and l i n e a r f o r a sub-

improve the conduction p r o p e r t i e s of the poor

r e g u l a r mixture. Our i n v e s t i g a t i o n s in t h i s re-

conductive Nas(3) phase. 14 At any r a t e the usa-

gard are not y e t completed, but the p r e l i m i n a r y

bility

of the well conductive Nas(x) phases in

,L Maier et al. / The Nasicon solid solution s y s t e m

the Na-S-cell is only imaginable under conditions with very low reaction rates (optimized micros t r u c t u r e ; room-temperature battery).

Progr. Solid E l e c t r o l y t e s , eds. T.A. Wheat, A. Ahmad and A.K. Kuriakose (C#NMET, Ottawa, 1983) pp. 91 - 127. 3. E.V. Shibanov and V.G. Chuklantsev, Russ. J. Phys. Chem. 48 (1974) 133.

4. SUMMARY For the f i r s t

973

time consistent thermodynamic

data f o r the Nasicon s o l i d solution system (obtained by electrochemical and c a l o r i m e t r i c experiments) are given, Hong's composition range is shown - experimentally and thermodynamically to be i n t r i n s i c a l l y

stable, but thermodynamical-

l y unstable against sodium. Thus, the use of Nasicon in the Na-S-cell implies a minimization of the reaction rate of the h i g h l y conductive materials by optimizing the microstructure or lowering the operating temperatures. ACKNOWLEDGEMENTS We should l i k e to thank B. Reichert, L.Viczian

4. U. yon Alpen, M.F.BelI, and H.H. H~fer, Solid State lonics 3/4 (1981) 214. 5. A. C l e a r f i e l d , M.A. Subramanian, W. Wang, and P. Jerus, Solid State lonics 9/10 (1983) 895. 6. U. Warhus, Dissertation, S t u t t g a r t (1985). 7. J. Engell, S. Mortenson, and L. Moller, Sol i d State lonics 9/10 (1983) 877. 8. Ph. Colomban, Densification and microstructure of NASICON ceramics as a function of sol-gel-process, in: Proc. lOth I n t . Symp. R e a c t i v i t y Solids, eds. P. Barret and L.-C. Dufour ( E l s e v i e r , Amsterdam, 1985), in press. 9. J. Maier and U. Warhus, J. Chem. Thermodynamics, in press.

and K. Graf for t h e i r technical help, O.Buresch

lOoJ. Maier and U. Warhus, in preparation.

for the chemical analysis, and K.D. Kreuer, A.

11. E. Gmelin, Thermochimica Acta 29 (1979) 1.

Rabenau, and W. Weppner for helpful discussions. One of us (U.W.) is indebted to the A. Krupp von Bohlen and Halbach S t i f t u n g f o r f i n a n c i a l

12.H. Kohler and H. Schulz, Solid State lonics 9/10 (1983) 795.

support.

13,U. von Alpen, M.F. B e l l , and W. Wichelhaus, Mater. Res. B u l l . 14 (1979) 1317.

REFERENCES

14.K.D. Kreuer, H. Kohler, U. Warhus, and H. Schulz, in preparation.

1. G.R. M i l l e r , B.J. McEntire, T.D. Hadnagy, J.R. Rasmussen, R.S. Gordon and A.V. Virkar, Processing and properties of Na-~"-alumina and NASICON e l e c t r o l y t e s , in: Proc. I n t . Conf. Fast lon Transp. Solids, eds. P. Vashishta, J.N. Mundy and G.K. Shenoy ( E l s e v i e r , Amsterdam, 1979) pp. 83 - 86. 2. J.B. B o i l o t , G. C o l l i n , and Ph. Colomban, in:

15 .R.A. Robie, B.S. Hemingway, and J.R. Fisher, Thermodynamics of Minerals and re~ated Substances at 298.15 K and 1bar ( i 0 ~ Pascals) Pressure and at higher temperatures (Geological Survey B u l l e t i n , Washington, 1979). 16 . I . Barin, O. Knacke, and O. Kubaschewski, Thermochemical Properties of Inorganic Substances (Springer Verlag, B e r l i n , 1975).