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Compositional dependence of the electrochemical and structural parameters in the Nasicon system (Na1+xSixZr2P3−xO12)
Solid State Ionics 3/4 (1981) 215-218 North-Holland Publishing Company
C O M P O S I T I O N A L D E P E N D E N C E OF T H E E L E C T R O C H E M I C A L AND S T R U C T U R A L P A R A M E T E R S IN T H E N A S I C O N S Y S T E M (Nal+xSi~Zr2P3-xOl2) U. V O N A L P E N V A R T A Batterie AG, D-6233 Kelkheim, FRG
and M.F. B E L L and H . H . H ( ~ F E R Max-Planck-Institut [iir Festk6rperforschung, D-7000 Stuttgart 80, F R G
Mixed crystals of the Nasicon composition Nai+xZr2SixP3-xOz2 0.4 ~
I. Introduction
It has recently been shown that materials with the composition Nal+xZr2SixP3-xO12 (0.4 ~< x <~ 2.8) belong to the best fast sodium ion conductors available today [1,2]. The ionic conductivity of the compositions exhibit a maximum for 1.8 ~< x ~< 2.2. Na3ZrzSi2POI: is the material with the highest conductivity, which is, at a t e m p e r a t u r e of 300°C, comparable to that of Na-/3 alumina [3,4]. This material is named Nasicon for sodium superionic conductor [5]. In fig. 1 the phase diagram of Nasicon is shown with the end compounds SiO2, NaeZrO3 and ZrP207. Nasicon solid solutions exist on the line which is defined by the end compounds N a 4 Z r 2 S i 3 O l 2 ( x = 3) and NaZr2P3Ol2 (x = 0). C o m p o u n d s in the composition range of 1.8 ~< x ~< 2 exhibit a monoclinic crystal symmetry while outside of these composition limits a r h o m b o h e d r a l symmetry is observed. From both conductivity m e a s u r e m e n t s and X-ray diffraction powder pattern analysis a second-order phase transition has been proposed for all the compounds with a monoclinic structure [6,7]. The change from the monoclinic into the rhombo-
hedral phase at the transition point between 150 and 200°C is accompanied by an anomaly in the specific heat [6] and the dilatomic curve [7]. This thermal anomaly may cause serious restrictions for the use of Nasicon tubes in Na/S cells because thermal shocks will cause a cracking of the separator tubes. The stability of Nasicon with molten sodium of 350°C is still the subject of intensive chemical and crystallographical investigations. Na2ZrO 3
No~,Zr2 Si3012
ZrP 2 07 Fig. 1. Phase diagram of Nasicon.
0167-2738/81/0000-0000/$02.50 O North-Holland Publishing C o m p a n y
216
U. yon Alpen et al. / Electrochemical and structural parameters of Nasicon
2. Preparation of Nasicon The preparation of Nasicon from the compounds Na2CO3, NI-I4H2PO4, ZrO2 and SiO2 as proposed by Hong [1] never leads to a pure monophase material. Nasicon prepared in this way contains considerable amounts of free ZrO2, the amount of which depends on the stoichiometry of the material and the temperature of the calcination process. It is well known that ZrO2 badly dissolves in silicate-containing compounds [8]. Another preparation method for Nasicon has been studied by starting with highly reactive gels from aqueous solutions consisting of elements of the final solution [7]. This method did not lead to ZrO2-free Nasicon, too. We have prepared Nasicon compounds including the end phases Na4ZrzSi3012 (x = 3) and NaZr2P3Ol2 (x = 0). Due to the low solubility of ZrO2 the end compounds x = 3 could be prepared in form of a pure monophase only using Zr(CsH702)4 instead of ZrO2 as proposed by Tranqui et al. [9]. Both end compounds did not reveal any considerable ionic conductivity, even at a temperature of 300°C. The preparation of mixed crystals of Nasicon in the composition range 1.8 ~< x ~< 2.4 mixing the end compounds in the stoichiometric composition did not result in ZrO2-free Nasicon, too. This implies that ZrO2 segregates even from pure mixtures. There may be three possible reasons to explain the fact that Nasicon can in no way be prepared as a pure phase: (i) Nasicon is characterized by an extremely small range of stoichiometry. (ii) Nasicon exists only as a high-temperature phase, then Nasicon is a metastable phase at ambient temperatures. (iii) Nasicon does not exist as a solid solution in the phase diagram on the line which is defined by the two end compounds Na4Zr2Si30~2 and NaZr2P3012. The validity of the third possible explanation has been proven by the discovery of the new solid solution of the composition Nal+xZr2 x/3SixP3 1012 21/3 for 0 ~< x ~< 3.
3. Structural and electrochemical parameters of the solid solution compounds of Nal+xZr2-x/~ixPs-xO12-z~/s including the end compounds NaZr~PsOI2 and NaIZrSisOI0 The new solid solution compounds have been investigated including the same end compound NaZrzP3012 for x = 0 as in the Nasicon system while on the sodium-rich side the corresponding end compound with less ZrO2, Na4ZrSi3Ol(~ has been chosen. It was found that in the composition range of 0 ~
ZrO 2
Na4Z~kSi3O12 Na4ZrSi3Ol°
NaZr2P3012
StO 2
Fig. 2. Location of Nasicon and the new mixed crystals in the quaternary system P2Os, SiO2, Na20 and ZrO2.
U. yon Alpen et al. / Electrochemical and structural parameters of Nasicon
217
1.111o II
'~
1. I l l e 11
:~.
! "-e.
i r
i. III~ I? b
= --
f -.
O
L' <
1. l l l e 13
I. l l l e - I 4
.
t
2
--
< £
EC£YL"
~
Fig. 3. A r r h e n i u s p l o t o f t h e p r o d u c t o f t h e i o n i c c o n d u c t i v i t y a n d t h e a b s o l u t e t e m p e r a t u r e as f u n c t i o n of t h e i n v e r s e a b s o l u t e t e m p e r a t u r e f o r Na4ZrSi3Om.
are shown in the phase diagram with the end compounds P205, SiO2, Na20 and ZrO2. The electrochemical investigations of the new compositions have shown that the new phase Na4ZrSi3Om exhibits a remarkable ionic conductivity. As shown in fig. 3 the ionic conductivity at 300°C amounts to o- = 1.6 x 10 _3 ,(2-~ cm-L The activation energy for the ionic diffusion was calculated from the Arrhenius plots to 42 kJ/mol.
4. Na3aZrl.ssSi2.aP0.70., a representative of the new, high conducting solid solution system For the composition range 1.6 ~< x ~< 3 highly conducting compounds have been found in the new solid solution system. One of these compounds with an outstanding conductivity is Na3.~Zrl.55Si2.3P0.7Oll which corresponds nearly to a composition of x = 2.2 with a small silica deficit. This material could be reproducibly synthesized as a pure monophase with monoclinic crystal symmetry. High-density pellets 6o =
3.09 g/cm 3) have been prepared with a conventional sinter technique, the complex impedance diagrams have been measured at each temperature to obtain the conductivity data of these sinters, in addition the long-time chemical stability of this compound with molten sodium has been checked. Fig. 4 shows the Arrhenius representation of the ionic conductivity of Na3.1Zrl.55Siz.3P0.7Oll as function of the absolute temperature. The ionic conductivity at room temperature exceeds that of Nasicon by half a decade exhibiting a value between 2 and 3 × 10-3 J2 ~cm-~. These conductivity data are comparable to that of Na-flalumina. At 300°C a conductivity of 0.2 J2 ~cm-~ is obtained for Na3.1Zrl.55Si2.3P0.7Otl which is also comparable to that of/3-alumina. In table 1 conductivity data and activation energies f o r the ionic conduction of Nasicon, lithia- and magnesia-stabilized Na-/3-alumina and Na3.1Zrl.55Si2.3P0.7Ou a r e compared with values reported in the literature. It is obvious that this new compound exhibits
218
U. yon Alpen et al. / Electrochemical and structural parameters of Nasicon TEMPERATURE T (*C) 200 100
300 xx
i
i
50
25
i
,
Table 1 Conductivity and activation energy for Na-ion diffusion in Na31Zrl~sSi23P070~l, Nasicon and Li20- and MgO-stabilized Na-/3-alumina compared to values reported in the literature
(T~r(
0 ~ ~
T
OrR1 Ea ( ~ l cm i)(~(2 I cm i)(kJ/mol)
XXx x A Na3lZrl .. 55Si23P°7Oll Na3]Zr155Si2.3P,;Oll Nasicon [3] ref. [1] ref. [2] Na-13-alumina (Li20) [3] ref. [10] Na-/3-alumina (MgO) [3] ref.[ll]
1
[B-AI203 (Li20 doped)
t
u
N , ~
/
~-A1203 (MgO doped)
~-o
-1
1.5
2.0 2.5 3.0 INVERSE TEMPERATURE lIT ( 103/K )
Fig. 4. Arrhenius plot of the product of the ionic conductivity and the absolute temperature as function of the inverse absolute temperature for Na3 ~Zr] ~sSi23P. 7OH compared to the data of lithia- and magnesia-doped Na-/3-alumina.
extraordinary high conductivities especially at elevated temperatures, and therefore it may be well suited as a separator material in Na/S cells. The stability with molten sodium at a temperature of 300°C has been tested over a period of several weeks. No severe attack has been found in the material though a thin dark brown surface layer has been observed the origin of which is still unknown. It should be pointed out that the sintering temperature of this new compound is by far lower than that of/3-alumina, in addition the costs for the starting materials are low.
Acknowledgement The authors are gratefully indebted to H. Diem, Max-Planck-Institut, R. Brfiutigam and R. Titze, both at V A R T A , for their fruitful scientific and technical contributions to this
3.5
3 x l0 ~ 7.(1× 10 4 6.0× 10 a 6.8 x 10 a 2.7 x l(J ~ 2.2 x 10 ~
0.2 (L22 {).2
25.5 27 35 28.1) 34.7 3.49 x 1() -~ 16.6 0.2 15.3 17.6 6.2× I(1 ~ 21.t) 8.2× I(I ~ 23.4
program. We gratefully acknowledge financial support by the Federal German Government under contract number ET 5054 A.
References [1] H.Y.-P. Hong, Mat. Res. Bull. 11 (1976) 173; J.B. Goodenough, H.Y.-P. Hong and J.A. Kafalas, Mat. Res. Bull. 11 (1976) 203. [2] M.L. Bayard and G.G. Barna, J. Electroanal. Chem. 91 (1978) 2[)1. [3] U. von AIpen, M.F. Bell, R. Brfiutigam and H. LaigH6rstebrock, in: Fast ion transport in solids, eds. P. Vashishta, J.N. Mundy and G.K. Shenoy (North-Holland, 1979) p. 443. [4] J.A. Kafalas and R.J. Cava, in: Fast ion transport in solids, eds. P. Vashishta, J.N. Mundy and G.K. Shenoy (North-Holland, Amsterdam. 1979) p, 419. [5] H.Y.-P. Hong, J.A. Kafalas and M.L. Bayard, Mat. Res. Bull. 13 (1978) 757. [6] U. von Alpen, M.F. Bell and W. Wichelhaus, Mat. Res. Bull. 14 (1979) 1317. [7] J.P. Boilot, J.P. Salani& G. Desplanches and D. Le Potier, Mat. Res. Bull. 14 (1979) 1469. [8] J. D ' A n s and J. L6fl]er, Z. Anorg. Allg. Chem. 191 (19~1) 22. [9] D. Tranqui, J.J. Caponi, J.J. Joubert, R.D. "~,hannon and C.K. Jonson, in: Fast ion transport in solids, eds. P. Vashishta, J.N. Mundy and G.K. Shenoy (NorthHolland, Amsterdam, 1979) p. 439. [10] A.V. Virkar, G.R. Miller and R.S. Gordon, J. Am. Ceram. Soc. 61 (1978) 250. [11] G.T. May, J. Mat. Sci. 13 (1978) 261.
C O M P O S I T I O N A L D E P E N D E N C E OF T H E E L E C T R O C H E M I C A L AND S T R U C T U R A L P A R A M E T E R S IN T H E N A S I C O N S Y S T E M (Nal+xSi~Zr2P3-xOl2) U. V O N A L P E N V A R T A Batterie AG, D-6233 Kelkheim, FRG
and M.F. B E L L and H . H . H ( ~ F E R Max-Planck-Institut [iir Festk6rperforschung, D-7000 Stuttgart 80, F R G
Mixed crystals of the Nasicon composition Nai+xZr2SixP3-xOz2 0.4 ~
I. Introduction
It has recently been shown that materials with the composition Nal+xZr2SixP3-xO12 (0.4 ~< x <~ 2.8) belong to the best fast sodium ion conductors available today [1,2]. The ionic conductivity of the compositions exhibit a maximum for 1.8 ~< x ~< 2.2. Na3ZrzSi2POI: is the material with the highest conductivity, which is, at a t e m p e r a t u r e of 300°C, comparable to that of Na-/3 alumina [3,4]. This material is named Nasicon for sodium superionic conductor [5]. In fig. 1 the phase diagram of Nasicon is shown with the end compounds SiO2, NaeZrO3 and ZrP207. Nasicon solid solutions exist on the line which is defined by the end compounds N a 4 Z r 2 S i 3 O l 2 ( x = 3) and NaZr2P3Ol2 (x = 0). C o m p o u n d s in the composition range of 1.8 ~< x ~< 2 exhibit a monoclinic crystal symmetry while outside of these composition limits a r h o m b o h e d r a l symmetry is observed. From both conductivity m e a s u r e m e n t s and X-ray diffraction powder pattern analysis a second-order phase transition has been proposed for all the compounds with a monoclinic structure [6,7]. The change from the monoclinic into the rhombo-
hedral phase at the transition point between 150 and 200°C is accompanied by an anomaly in the specific heat [6] and the dilatomic curve [7]. This thermal anomaly may cause serious restrictions for the use of Nasicon tubes in Na/S cells because thermal shocks will cause a cracking of the separator tubes. The stability of Nasicon with molten sodium of 350°C is still the subject of intensive chemical and crystallographical investigations. Na2ZrO 3
No~,Zr2 Si3012
ZrP 2 07 Fig. 1. Phase diagram of Nasicon.
0167-2738/81/0000-0000/$02.50 O North-Holland Publishing C o m p a n y
216
U. yon Alpen et al. / Electrochemical and structural parameters of Nasicon
2. Preparation of Nasicon The preparation of Nasicon from the compounds Na2CO3, NI-I4H2PO4, ZrO2 and SiO2 as proposed by Hong [1] never leads to a pure monophase material. Nasicon prepared in this way contains considerable amounts of free ZrO2, the amount of which depends on the stoichiometry of the material and the temperature of the calcination process. It is well known that ZrO2 badly dissolves in silicate-containing compounds [8]. Another preparation method for Nasicon has been studied by starting with highly reactive gels from aqueous solutions consisting of elements of the final solution [7]. This method did not lead to ZrO2-free Nasicon, too. We have prepared Nasicon compounds including the end phases Na4ZrzSi3012 (x = 3) and NaZr2P3Ol2 (x = 0). Due to the low solubility of ZrO2 the end compounds x = 3 could be prepared in form of a pure monophase only using Zr(CsH702)4 instead of ZrO2 as proposed by Tranqui et al. [9]. Both end compounds did not reveal any considerable ionic conductivity, even at a temperature of 300°C. The preparation of mixed crystals of Nasicon in the composition range 1.8 ~< x ~< 2.4 mixing the end compounds in the stoichiometric composition did not result in ZrO2-free Nasicon, too. This implies that ZrO2 segregates even from pure mixtures. There may be three possible reasons to explain the fact that Nasicon can in no way be prepared as a pure phase: (i) Nasicon is characterized by an extremely small range of stoichiometry. (ii) Nasicon exists only as a high-temperature phase, then Nasicon is a metastable phase at ambient temperatures. (iii) Nasicon does not exist as a solid solution in the phase diagram on the line which is defined by the two end compounds Na4Zr2Si30~2 and NaZr2P3012. The validity of the third possible explanation has been proven by the discovery of the new solid solution of the composition Nal+xZr2 x/3SixP3 1012 21/3 for 0 ~< x ~< 3.
3. Structural and electrochemical parameters of the solid solution compounds of Nal+xZr2-x/~ixPs-xO12-z~/s including the end compounds NaZr~PsOI2 and NaIZrSisOI0 The new solid solution compounds have been investigated including the same end compound NaZrzP3012 for x = 0 as in the Nasicon system while on the sodium-rich side the corresponding end compound with less ZrO2, Na4ZrSi3Ol(~ has been chosen. It was found that in the composition range of 0 ~
ZrO 2
Na4Z~kSi3O12 Na4ZrSi3Ol°
NaZr2P3012
StO 2
Fig. 2. Location of Nasicon and the new mixed crystals in the quaternary system P2Os, SiO2, Na20 and ZrO2.
U. yon Alpen et al. / Electrochemical and structural parameters of Nasicon
217
1.111o II
'~
1. I l l e 11
:~.
! "-e.
i r
i. III~ I? b
= --
f -.
O
L' <
1. l l l e 13
I. l l l e - I 4
.
t
2
--
< £
EC£YL"
~
Fig. 3. A r r h e n i u s p l o t o f t h e p r o d u c t o f t h e i o n i c c o n d u c t i v i t y a n d t h e a b s o l u t e t e m p e r a t u r e as f u n c t i o n of t h e i n v e r s e a b s o l u t e t e m p e r a t u r e f o r Na4ZrSi3Om.
are shown in the phase diagram with the end compounds P205, SiO2, Na20 and ZrO2. The electrochemical investigations of the new compositions have shown that the new phase Na4ZrSi3Om exhibits a remarkable ionic conductivity. As shown in fig. 3 the ionic conductivity at 300°C amounts to o- = 1.6 x 10 _3 ,(2-~ cm-L The activation energy for the ionic diffusion was calculated from the Arrhenius plots to 42 kJ/mol.
4. Na3aZrl.ssSi2.aP0.70., a representative of the new, high conducting solid solution system For the composition range 1.6 ~< x ~< 3 highly conducting compounds have been found in the new solid solution system. One of these compounds with an outstanding conductivity is Na3.~Zrl.55Si2.3P0.7Oll which corresponds nearly to a composition of x = 2.2 with a small silica deficit. This material could be reproducibly synthesized as a pure monophase with monoclinic crystal symmetry. High-density pellets 6o =
3.09 g/cm 3) have been prepared with a conventional sinter technique, the complex impedance diagrams have been measured at each temperature to obtain the conductivity data of these sinters, in addition the long-time chemical stability of this compound with molten sodium has been checked. Fig. 4 shows the Arrhenius representation of the ionic conductivity of Na3.1Zrl.55Siz.3P0.7Oll as function of the absolute temperature. The ionic conductivity at room temperature exceeds that of Nasicon by half a decade exhibiting a value between 2 and 3 × 10-3 J2 ~cm-~. These conductivity data are comparable to that of Na-flalumina. At 300°C a conductivity of 0.2 J2 ~cm-~ is obtained for Na3.1Zrl.55Si2.3P0.7Otl which is also comparable to that of/3-alumina. In table 1 conductivity data and activation energies f o r the ionic conduction of Nasicon, lithia- and magnesia-stabilized Na-/3-alumina and Na3.1Zrl.55Si2.3P0.7Ou a r e compared with values reported in the literature. It is obvious that this new compound exhibits
218
U. yon Alpen et al. / Electrochemical and structural parameters of Nasicon TEMPERATURE T (*C) 200 100
300 xx
i
i
50
25
i
,
Table 1 Conductivity and activation energy for Na-ion diffusion in Na31Zrl~sSi23P070~l, Nasicon and Li20- and MgO-stabilized Na-/3-alumina compared to values reported in the literature
(T~r(
0 ~ ~
T
OrR1 Ea ( ~ l cm i)(~(2 I cm i)(kJ/mol)
XXx x A Na3lZrl .. 55Si23P°7Oll Na3]Zr155Si2.3P,;Oll Nasicon [3] ref. [1] ref. [2] Na-13-alumina (Li20) [3] ref. [10] Na-/3-alumina (MgO) [3] ref.[ll]
1
[B-AI203 (Li20 doped)
t
u
N , ~
/
~-A1203 (MgO doped)
~-o
-1
1.5
2.0 2.5 3.0 INVERSE TEMPERATURE lIT ( 103/K )
Fig. 4. Arrhenius plot of the product of the ionic conductivity and the absolute temperature as function of the inverse absolute temperature for Na3 ~Zr] ~sSi23P. 7OH compared to the data of lithia- and magnesia-doped Na-/3-alumina.
extraordinary high conductivities especially at elevated temperatures, and therefore it may be well suited as a separator material in Na/S cells. The stability with molten sodium at a temperature of 300°C has been tested over a period of several weeks. No severe attack has been found in the material though a thin dark brown surface layer has been observed the origin of which is still unknown. It should be pointed out that the sintering temperature of this new compound is by far lower than that of/3-alumina, in addition the costs for the starting materials are low.
Acknowledgement The authors are gratefully indebted to H. Diem, Max-Planck-Institut, R. Brfiutigam and R. Titze, both at V A R T A , for their fruitful scientific and technical contributions to this
3.5
3 x l0 ~ 7.(1× 10 4 6.0× 10 a 6.8 x 10 a 2.7 x l(J ~ 2.2 x 10 ~
0.2 (L22 {).2
25.5 27 35 28.1) 34.7 3.49 x 1() -~ 16.6 0.2 15.3 17.6 6.2× I(1 ~ 21.t) 8.2× I(I ~ 23.4
program. We gratefully acknowledge financial support by the Federal German Government under contract number ET 5054 A.
References [1] H.Y.-P. Hong, Mat. Res. Bull. 11 (1976) 173; J.B. Goodenough, H.Y.-P. Hong and J.A. Kafalas, Mat. Res. Bull. 11 (1976) 203. [2] M.L. Bayard and G.G. Barna, J. Electroanal. Chem. 91 (1978) 2[)1. [3] U. von AIpen, M.F. Bell, R. Brfiutigam and H. LaigH6rstebrock, in: Fast ion transport in solids, eds. P. Vashishta, J.N. Mundy and G.K. Shenoy (North-Holland, 1979) p. 443. [4] J.A. Kafalas and R.J. Cava, in: Fast ion transport in solids, eds. P. Vashishta, J.N. Mundy and G.K. Shenoy (North-Holland, Amsterdam. 1979) p, 419. [5] H.Y.-P. Hong, J.A. Kafalas and M.L. Bayard, Mat. Res. Bull. 13 (1978) 757. [6] U. von Alpen, M.F. Bell and W. Wichelhaus, Mat. Res. Bull. 14 (1979) 1317. [7] J.P. Boilot, J.P. Salani& G. Desplanches and D. Le Potier, Mat. Res. Bull. 14 (1979) 1469. [8] J. D ' A n s and J. L6fl]er, Z. Anorg. Allg. Chem. 191 (19~1) 22. [9] D. Tranqui, J.J. Caponi, J.J. Joubert, R.D. "~,hannon and C.K. Jonson, in: Fast ion transport in solids, eds. P. Vashishta, J.N. Mundy and G.K. Shenoy (NorthHolland, Amsterdam, 1979) p. 439. [10] A.V. Virkar, G.R. Miller and R.S. Gordon, J. Am. Ceram. Soc. 61 (1978) 250. [11] G.T. May, J. Mat. Sci. 13 (1978) 261.