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Enhancement of ionic conductivity by dispersed oxide inclusions: Influence of oxide isoelectric point and cation size
Solid State lonics 26 (1988) 5-10 North-Holland, Amsterdam
E N H A N C E M E N T O F IONIC C O N D U C T I V I T Y BY D I S P E R S E D OXIDE INCLUSIONS: I N F L U E N C E OF O X I D E ISOELECTRIC POINT A N D CATION SIZE A.K. SHUKLA *, R. M A N O H A R A N and J.B. G O O D E N O U G H Centerfor Materials Science and Engineering, ETC 5.160, University of Texas at Austin, Austin, TX 78712, USA
Received 9 November 1987; accepted for publication 27 November 1987
Doping of a halide salt by dispersion of oxide particles rather than substitutional impurities is a prtwen method of enhancing the extrD.sic ionic conductivity of the host. The conductivity mechanism in any space-charge layer at tl-e oxide/host interface is determined by the chemical reactions at the interface. These interactions are discassed by analogywith the particle hydrates. The enhancement of the F--ion conductivity in PbF2is predicted to be via F--ion vacancies in the space-chargeregion and to increase with decreasing oxide isoelectric point and increasing normal anion coordination of the oxide cation. This prediction accounts satisfac:orily for the relative enhancement factors measured for PbF2 containing dispersed CeO2, SiO2, ZrO2 and AI203.
1. Introduction
Since the observation o f Liang [ 1 ] that the dispersion of small A1203 parlicles into otherwise pure Lil enhances the Li+-ion conductivity of the host, a number of experimental [2-11 ] and theoretical [ 12-14 ] studies have beea devoted to the phenomenon. The following obervations and ideas appear le be the most significant: (1) The effect is greater the smaller the dispersoid particle size. (2) In the case of A1203 particles in AgI, dispersc,id pazticlcs containing b o u n d surface water were found to be more effective than predried particles. (3) The enhancement exhibits a maximum between 10 and 40 mole percent dispersoid, the maximum value and composition depending on other factors. (4) The enhancement with respect to the pure phase is greatest at lower temperatures where a significant decrease in the activation energy for fl~e conductivity is obscured. ( 5 ) An enhanceme at is observed for anionic ( F - ion) conductors as well as cationic conductors of
quite different mobile-ion electronegativity, e.g., Li + versus Ag ÷ ions. (6) The phenomenology o f the enhancement mechanism can be modelled - with an adjustable parameter - in terms of an increase in the defect concentration responsible for extrinsic conduction in a narrow space-charge layer in the host at the particlehost interface. (7) A maximum in the enhancement as a function of sintering temperature appears to reflect changes in the particle distribution within the host. To date, iittle, if any, consideration has been given to the influence of the chemical characteristics of the dispersoids that maximize the width of the spacecharge region in the host and thus optimize the conduetivity enhancement. In this paper, we emphasize the analogy between the ionic-conductivity enhancement under consideration and that of proton-conductivity enhancement in the particle hydrates [ 15 t. In the next section we use this analog3, to discuss some candidate chcmicai processes that can give rise to a space-charge layer. We then present some preliminary data that show a dependence of the phenomenon on the isoelectdc point, i.e., point of zero zeta potential (pzzp), and the ionic radius of the oxide cation.
* On leave from Solid State and Structural Chemistry Unit, Indian Intitute of Science, Bangalore, India. 0 167-2738/88/$ 03.50 O Elsevier Science Publishers B.V. (North-HoUand Physics Publishing Division)
6
A.K. Shukla et al./Enhancement of ionic conductivity
2. Chemical plocesses and interfaces
H20
Hi
/HH
2.1. Particlehydrates
H I
A particle hydrate is a composite consisting of colloidal-sized particles dispersed in an immobilized aqueous solution. The solution is immobilized by the formation of hydrogen-bond networks that bridge from particle to particle. Proton conduction occur~ in the immobilized aqueous matrix; it may also occur in the particles if they happen to be protonic conductors [ 15 ]. Whether the proton conduction in the aqueous matrix occurs via an HsO+-concentration or an OH--concentration mechanism depends upon the pH of the solution relative to the pzzp, i.e., the pH at which the particles carry zero charge [ 15 ]. As indicated schematically in fig. 1, if the pH of the solution is less than the pzzp of the particles, then the particlesurface accept proto~ from the ~queous matrix;the particle,.,thus carry a net positivecharge and the aqueous matrix a net negat.lvc charge. The matrix then exchanges anions more readily than cations. Conversely, ifthe .oH of the solution is greater tha~. the pzzp of the particles, then the panicle surfa~.,:~,iona~e protons (from the boand surface water) to the aqueous matrix. In this case the particles carry a net negative charge and the aqueous matrix a net positive charge, so the matrix exchanges cations move readily than anions. If the aqueous matrix is initially pure water ( p / t = 7), then the analogy with a colloidal-particle dispersoid in a stoichiometric salt is nearly exact except for the crystalline character o f the salt. If the oxide present as dispersed colloidal particles has a pzzp < 7, the aqueous matrix is made acidic and the oxide is said to be acidic; a pzzp > 7 makes the matrix alkaline, and the oxide is said to be basic. An acidic matrix is analogous to an initially stoichiometric salt having a space-charge layer made positive by the presence of interstitial, mobile protons (Frenkel defects); an alkaline matrix to a salt having a space-charge layer made negative by the introduction of mobile proton vacancies (Schottky defects). The maximum protonic conductivity in the matrix is associated with a maximum d,:fect concentration and hence with a pzzp for the oxide t.hat is most removed from pH= 7. Moreover, the smaller the oxide
I~1 HaO+
H
H20
Ha0
H'(~/~-I'1
H20 H30+ H I
H20
H~H
HaO H~, sH ---.I~
H,~ i~~H
_H
(a)
H ~.H -
H
o
H¢ -'~)'~-."H
H20 H-
o.
.
-H
H
-H H
H-
~' H20 H I~I H"O-H H "~/ HzO H-_~H OH" H . H20
OH" .H
H
H(b)
-H I
H
Fig. 1.Schematicrepresentationof the structureof: (a) an acidic parUc~e(AP) and (b) a basic-particle(BP) hydrate. particles, the larger is their surface-to-volume ratio and hence the larger the proton transfer from or to the particle surfaces per unit mass o f dispersoid.
2.2. Salts containing dispersed oxides The relevant reactions occurring at the interface of an oxide panicle dispersed in a halide are compared in table 1 with the corresponding surface reactions in the panicle hydrates. We consider two types of halides, the M+-ion conductors illustrated by MX (M = Li or Ag and X = Br or I) and the F--ion conductors represented by PbF2. Wet panicles are those that have been exposed to
A.K. Shukla et aL/Enhancement ~,: wnic conductivity
7
Table 1 Surface reactions Oxide particle" )
Ho~t (x--Brorl)
Surface reaction
Net charge
ion
particle Particle hydrates acidic
basic
(H20),+H20~(OH-),+H~O + ( O H - ) , + H,O.-. (O 2- ),+ H;O+
H+
H,O
(OH-)
+HaO~ (H~O),+OH(O 2- ),+H20~ (OH-),+':-~H-
v~
MX
( H20 ), + 2MX--) (OM2) ,+ 2HXt (HzO),+MX-* (X-),+H:Ot+M 3 ( O H - ) , + M X - ) ( 0 2 . ),+~-{X~ +M~"
0
( 0 2- )~+MX~- (OM-),~,- , (;~ (Vo),+MX--* ( X - ) , + M i '
+
W/acidic
PbF2
( H 2 0 ) ~+ ½PbF,--) ( O H - )~+ HFt + V~ ( O H - ) ~ + ½PbF2-) (O 2- ) ~+ HF~' +V~-"
W/basic D
PbF2 PbF2
( H 2 0 ) t + ½PbF2--) ( H 2 0 } ? + ( F - ) s + V ~ (Vo)~+ ½ P b F ~ ( F - ) ~ + V ;
~) W=wet, D=dry.
sc b)
H20
Halides ~ ith dispersed oxides W MX
D
Mobile
-
0
+ +
M3 M3
-
v~,
+
M~+
+
v~-
+ +
v¢v~
b~ sc=chargelayer.
the air at room temperature before dispersion in the salt; dr), particles have been predried before dispersion. In a wet particle, the surface cations having an incomplete anion coordination bind the oxygen o f a water molecule so as to complete their coordination; the proton of the water may distribute themselves over the surface to present a surface consisting primarily o f O 2- and O H - anions. A dry particle is one where the bound surface water has been driven off; in this case, a surface cation with d~.ficient anion coordination may induce a reversible restructuring o f the surface that, on contact with a salt, returns to normal with the capture o f an anion from the sa!t. The reversible restructuring o f the surface is not indicatea in table 1; the surface-oxygen deficiency for full oxygen coordination at a surface cation is simply
~.::- ::]cs attract Li 4 ions from the salt via the re~ c t i o n s
( ".: - ), + LiX ~- ( OLi- ), + Vci,
(2)
u>.~ii the electrochemical potential for Li + in the oxide oecomes equal to that in the salt via the spacecha~e-layer capacitance created by the Li+-ion transfer. Therefore the enhanced extrinsic Li+-ion cenducfivity should be similar for wet and dry oxide l;arficles dispersed in a LiX halide, and the Li+-ion conduction in the space-charge region should be via Li+-ion vacancies V~-i. The Ag + ion, on the other hand, does not displace a surface proton; in this case, reaction (1) is replaced by
~UULr.;U a a ul~; IL~;UU~I SpeCieS [ V0 / s .
The lithium salts LiI and LiBr must be distinguished from the silver salts AN and AgBr. The Li + ions cat, displace surface protons from a wet particle to give ~:he reaction ( H20)~ + 2LiX--, (OLi2) ~+ 2HX {,
( 1)
which does not introduce a space-charge layer in the host. Howev~.." the oxide ions on the wet and dry
Ti~c competitive reaction (O 2 - ) ~ + A g X a ( O A g - ) ~ + V g ,
(4)
which would be tbllowed by Ag~+ + V ~ - A g ,
(5)
is shifted strongly to the left, so the enhancement due
8
A.K. Shukla et al./Enhancement of ionic conductivity
to reaction (3) is not suppressed by reaction (5). For a dry particle, the reaction (Vo) ~+ A g X ~ ( X - )s +Ag + ,
(6)
must alo be shifted strongly to the left for A1203 particles in AgBr since little enhancement is oberved in this case. We therefore would predict that the enhancement of Ag+-ion conductivity in AgBr for only wet A1203 particles results from the dominance of reaction (3) and hence that the enhanced extrinsic conductivity is associated with interstitial Ag + ions, Ag+ , in the space-charge layer. In the case of PbF2 containing a dispersed oxide, the reaction
(Vo)s + ½PbF2~- (F-)s + V f ,
(7)
would be shifted more strongly to the right. Therefore a similar enhancement can be expected for wet and dry particles since the reaction (OH-)., + ~PbF2--, (O2-).~ + HFI' + V ~ ,
(8)
would give a similar concentration of fluoride-ion vacancies in the space-charge region. Moreover, as with the silver salts, the reaction (O2-) s + PbF2 ~- (OPb)s + 2 F , ,
(9)
would be shifted strongly to the left, so the reaction Vf- + F 7 ~ F
(10)
is not available to annihilate the V~ population. Two factors are operative to shift reactions (7) and/or (8) to the right: (i) the more acidic the oxide, the stronger an F - ion is bound to the particle surface as ( F - L and the more strongly a proton is attracted from the surface to form HF; (ii) the larger the surface cation of the oxide, the g:eatec its normal anion coordination and hence the concentration o/" anion-vacancy sites (Vo)~ and/or bound water molecules (H20)~ at the Surfaces. In order to test this prediction, several oxides ~CeO,, ZrO_,, SiO_,, and Al20~), all of about t3.2 um average particle size, were dispersed in PbF2. The respective isoelectric points for these oxides are 7, 4, 2, and 9 [ 16 ]; their cationic radii are 6.87, 0.39, 0.26, and 0,39/k [ 17]. According to the above argument, the maximum enhancement should occur for the maximum generation of V~- species at the interface surfaces and hence for the oxide with the greatest cat-
ionic radius and smallest isoelectric point. A measured F--ion conductivity enhancement that varies as ZrO2 > SiO2 >CeO., > A1203, see fig. 2, indicates that, in addition to the acidic character of the oxide, the ionic radius makes a noticeable contribution.
3. Experimental 3. I. Specimen preparation
H i g h - p u n y (Puratronic grade), anhydrous samples of PbF2, COO2, ZrO2, SiOz, and Al:O3 from Johnson-Mathey Chemicals were used. Therrnogravimetnc analyses (TGA) in the temperature range 25-600°C were made with a Perkin Elmer 7-series Thermal Analysis System; the only evidence of lattice or adsorbed water found after exposure to air at room temperature was 0. I and 0.2 wt% in AI203 and SiO2. The average particle size ofaU the powders was 0.2 gm. Each oxide was dispersed in separate samples of PbF. by mechanically mixing weighed oxide/PbF2 ratios of the powders in an agate mortar. No new phase was detected by X-ray powder diffraction in any mixture. Disc-shaped, dense pellets of 8 mm in diameter were made from the intimate mixtures by double-end compression in a steel die; they were cold-pressed at 1200 MPa. Gold electrodes about 250 A thick were sputtered onto the two disc faces with a Pelco Sputter-Coater Model-3 machine. 3.2. Measurement
Two-probe ac-impedance measurements were made in the frequency range 10-~-105 Hz by placing the specimen between silver electrodes of a measuring cell that made contact with the two gold-coated surfaces. The temperature of the cell was controlled to within + 1 °C; the ac impedance was measured with the setup shown in fig. 3. The variation of the conductivity ~ as a fnnction of frequency is sbow.q in fig. 4 for PbF2 and PbF2 containing various mole% dispersed ca. 0.2 ~tm CeO2 palaicles. The maximum conductivity occurs near l 0 mole% CeO> At 105 Hz, the highest frequency used, any polarization resistance makes a negligible contribution. At 298 K and 105 Hz, the conductivity of
A.K. Shukla et aL/Enhancement of ionic conductivity
9
X PbF 2 + AI203 2.0
0 p13F2 +CeO 2 [] PbF 2 + SiQ2 Z~ PbF 2 + Zr02
,Sr-. o..
1.5 x
1.0 I
I
I
5
10
15
Moie % of the Dispersoid Fig, 2. Normalized conductivity enhancement of r b F : versL~.~dispersoid concentration for various dispersed oxides of mean particle diameter 0.2 ~tm.
Potentiostat/Galv~ I
E
Measurement Cell
Interface
Key Board
Li" i
I'%Y! i C.~ alqneE
Prea_mplifieri Lock - In Amplifier
Printer
Fig. 3. Schematic for ac-impedance measurement apparatus.
A.K. Shukla et aL/Enhancement of ionic conductivity
10
larger ionic radius of ZP + is compatible with eightfold anion coordination in ZrOz; the Si4+ ion preferentially occupies tetrahedral sites. Thus the reversal in enhaacement factors based on isoelectric point for ZrO2 and SiO2 can be reconciled by invoking a contribution from the difference in cationic size.
-8
--
--
~
w
u
References O
°
PbF2+5 tool % CoO2 o PbF2 +7.5 too! % CaO2
•* PbF2 +10 mol % CaO2 e PbF2+15 mol % CeO2 =5
-1
,
|
0
1
-..|-
2
|
i
3
4
5
Log Frequency(Hz) .-~ Fig. 4. Frequency dependence o f the conductivity o f PbF2 and
PbFz dispersed with various concentration of CeOz at 298 K.
PbF2 dispersed with 7.5 and 10 mole% CeO2 iS about
1.4 times larger than ~.hat of PbF2. A comparison of these results with those obtained for Zr02, Si02, a~ldA1203aprti.~les dispersed in PbF_~, fig. 2, shows that the maxim~:n enhancement occurs ibr ZiG: ~a PbF~. Although SiO2 has a somewhat smaller isoelectfic point (2 versus 4 for ZrO2), the
[ 1] C.C. Liang, 3. Eleetrochem. See. 120 (1973) 1289. [2] S. Pack, B.B. Owens and 3.B. Wagner, J. Electroehem. SOe. 127 (1950) 2177. [ 31 P. Hartwig and W. Weppner, Solid State lonics 3/4 (1981 ) 249. [41 A. Hooper, J. Power Sources 9 (1983) 161. [5] O. Nakamura and J.B. Goodenough, Solid State lonics 7 (1982) 119. [6] T. Jow and 3.B. Wagner, J. Electrochem. SOc. 126 (1979) 1963. [7] J.B. Wagner, Meter. Res. Bull. 15 (1980) 1691. [8] K, Shahi and J.B. Wagner, J. Solid State Chem. 42 (1982) 107. [9] K, Shahi and J.B. Wagner, J. Phys, Chem. Solids 43 (1982) 713. [ 10] N, Vaidehi, R. Akila, A.K. Shukla and ICT. Jacob, Mater. Res. Bull. 21 (1986) 909. [ 11 ] M.R.W. Chang, Masters Thesis (Arizona State University, 1987). [ 12] A. Stoneham, E. Wade and J.A. Kilmer, Mater. Res. Bull 21 (1979) 661. [ 13] L Maler, J. Phys. Chem. Solids 46 (1985) 309. [ 14] J. Maier, J. Electrochem. Soc. 134 (1987) 1524. [ 15] W.A. England, M.G. Cross, A. Hamnett, P.3. Wiseman and J.B. Goodenough, Solid State tonics I (1980) 231. [ 16] G.A. Parks, Chem. Rex,. 65 (1965) 177. [ 17] R.D. Shannon, Acta Crys. A32 (! 976) 75 i.
E N H A N C E M E N T O F IONIC C O N D U C T I V I T Y BY D I S P E R S E D OXIDE INCLUSIONS: I N F L U E N C E OF O X I D E ISOELECTRIC POINT A N D CATION SIZE A.K. SHUKLA *, R. M A N O H A R A N and J.B. G O O D E N O U G H Centerfor Materials Science and Engineering, ETC 5.160, University of Texas at Austin, Austin, TX 78712, USA
Received 9 November 1987; accepted for publication 27 November 1987
Doping of a halide salt by dispersion of oxide particles rather than substitutional impurities is a prtwen method of enhancing the extrD.sic ionic conductivity of the host. The conductivity mechanism in any space-charge layer at tl-e oxide/host interface is determined by the chemical reactions at the interface. These interactions are discassed by analogywith the particle hydrates. The enhancement of the F--ion conductivity in PbF2is predicted to be via F--ion vacancies in the space-chargeregion and to increase with decreasing oxide isoelectric point and increasing normal anion coordination of the oxide cation. This prediction accounts satisfac:orily for the relative enhancement factors measured for PbF2 containing dispersed CeO2, SiO2, ZrO2 and AI203.
1. Introduction
Since the observation o f Liang [ 1 ] that the dispersion of small A1203 parlicles into otherwise pure Lil enhances the Li+-ion conductivity of the host, a number of experimental [2-11 ] and theoretical [ 12-14 ] studies have beea devoted to the phenomenon. The following obervations and ideas appear le be the most significant: (1) The effect is greater the smaller the dispersoid particle size. (2) In the case of A1203 particles in AgI, dispersc,id pazticlcs containing b o u n d surface water were found to be more effective than predried particles. (3) The enhancement exhibits a maximum between 10 and 40 mole percent dispersoid, the maximum value and composition depending on other factors. (4) The enhancement with respect to the pure phase is greatest at lower temperatures where a significant decrease in the activation energy for fl~e conductivity is obscured. ( 5 ) An enhanceme at is observed for anionic ( F - ion) conductors as well as cationic conductors of
quite different mobile-ion electronegativity, e.g., Li + versus Ag ÷ ions. (6) The phenomenology o f the enhancement mechanism can be modelled - with an adjustable parameter - in terms of an increase in the defect concentration responsible for extrinsic conduction in a narrow space-charge layer in the host at the particlehost interface. (7) A maximum in the enhancement as a function of sintering temperature appears to reflect changes in the particle distribution within the host. To date, iittle, if any, consideration has been given to the influence of the chemical characteristics of the dispersoids that maximize the width of the spacecharge region in the host and thus optimize the conduetivity enhancement. In this paper, we emphasize the analogy between the ionic-conductivity enhancement under consideration and that of proton-conductivity enhancement in the particle hydrates [ 15 t. In the next section we use this analog3, to discuss some candidate chcmicai processes that can give rise to a space-charge layer. We then present some preliminary data that show a dependence of the phenomenon on the isoelectdc point, i.e., point of zero zeta potential (pzzp), and the ionic radius of the oxide cation.
* On leave from Solid State and Structural Chemistry Unit, Indian Intitute of Science, Bangalore, India. 0 167-2738/88/$ 03.50 O Elsevier Science Publishers B.V. (North-HoUand Physics Publishing Division)
6
A.K. Shukla et al./Enhancement of ionic conductivity
2. Chemical plocesses and interfaces
H20
Hi
/HH
2.1. Particlehydrates
H I
A particle hydrate is a composite consisting of colloidal-sized particles dispersed in an immobilized aqueous solution. The solution is immobilized by the formation of hydrogen-bond networks that bridge from particle to particle. Proton conduction occur~ in the immobilized aqueous matrix; it may also occur in the particles if they happen to be protonic conductors [ 15 ]. Whether the proton conduction in the aqueous matrix occurs via an HsO+-concentration or an OH--concentration mechanism depends upon the pH of the solution relative to the pzzp, i.e., the pH at which the particles carry zero charge [ 15 ]. As indicated schematically in fig. 1, if the pH of the solution is less than the pzzp of the particles, then the particlesurface accept proto~ from the ~queous matrix;the particle,.,thus carry a net positivecharge and the aqueous matrix a net negat.lvc charge. The matrix then exchanges anions more readily than cations. Conversely, ifthe .oH of the solution is greater tha~. the pzzp of the particles, then the panicle surfa~.,:~,iona~e protons (from the boand surface water) to the aqueous matrix. In this case the particles carry a net negative charge and the aqueous matrix a net positive charge, so the matrix exchanges cations move readily than anions. If the aqueous matrix is initially pure water ( p / t = 7), then the analogy with a colloidal-particle dispersoid in a stoichiometric salt is nearly exact except for the crystalline character o f the salt. If the oxide present as dispersed colloidal particles has a pzzp < 7, the aqueous matrix is made acidic and the oxide is said to be acidic; a pzzp > 7 makes the matrix alkaline, and the oxide is said to be basic. An acidic matrix is analogous to an initially stoichiometric salt having a space-charge layer made positive by the presence of interstitial, mobile protons (Frenkel defects); an alkaline matrix to a salt having a space-charge layer made negative by the introduction of mobile proton vacancies (Schottky defects). The maximum protonic conductivity in the matrix is associated with a maximum d,:fect concentration and hence with a pzzp for the oxide t.hat is most removed from pH= 7. Moreover, the smaller the oxide
I~1 HaO+
H
H20
Ha0
H'(~/~-I'1
H20 H30+ H I
H20
H~H
HaO H~, sH ---.I~
H,~ i~~H
_H
(a)
H ~.H -
H
o
H¢ -'~)'~-."H
H20 H-
o.
.
-H
H
-H H
H-
~' H20 H I~I H"O-H H "~/ HzO H-_~H OH" H . H20
OH" .H
H
H(b)
-H I
H
Fig. 1.Schematicrepresentationof the structureof: (a) an acidic parUc~e(AP) and (b) a basic-particle(BP) hydrate. particles, the larger is their surface-to-volume ratio and hence the larger the proton transfer from or to the particle surfaces per unit mass o f dispersoid.
2.2. Salts containing dispersed oxides The relevant reactions occurring at the interface of an oxide panicle dispersed in a halide are compared in table 1 with the corresponding surface reactions in the panicle hydrates. We consider two types of halides, the M+-ion conductors illustrated by MX (M = Li or Ag and X = Br or I) and the F--ion conductors represented by PbF2. Wet panicles are those that have been exposed to
A.K. Shukla et aL/Enhancement ~,: wnic conductivity
7
Table 1 Surface reactions Oxide particle" )
Ho~t (x--Brorl)
Surface reaction
Net charge
ion
particle Particle hydrates acidic
basic
(H20),+H20~(OH-),+H~O + ( O H - ) , + H,O.-. (O 2- ),+ H;O+
H+
H,O
(OH-)
+HaO~ (H~O),+OH(O 2- ),+H20~ (OH-),+':-~H-
v~
MX
( H20 ), + 2MX--) (OM2) ,+ 2HXt (HzO),+MX-* (X-),+H:Ot+M 3 ( O H - ) , + M X - ) ( 0 2 . ),+~-{X~ +M~"
0
( 0 2- )~+MX~- (OM-),~,- , (;~ (Vo),+MX--* ( X - ) , + M i '
+
W/acidic
PbF2
( H 2 0 ) ~+ ½PbF,--) ( O H - )~+ HFt + V~ ( O H - ) ~ + ½PbF2-) (O 2- ) ~+ HF~' +V~-"
W/basic D
PbF2 PbF2
( H 2 0 ) t + ½PbF2--) ( H 2 0 } ? + ( F - ) s + V ~ (Vo)~+ ½ P b F ~ ( F - ) ~ + V ;
~) W=wet, D=dry.
sc b)
H20
Halides ~ ith dispersed oxides W MX
D
Mobile
-
0
+ +
M3 M3
-
v~,
+
M~+
+
v~-
+ +
v¢v~
b~ sc=chargelayer.
the air at room temperature before dispersion in the salt; dr), particles have been predried before dispersion. In a wet particle, the surface cations having an incomplete anion coordination bind the oxygen o f a water molecule so as to complete their coordination; the proton of the water may distribute themselves over the surface to present a surface consisting primarily o f O 2- and O H - anions. A dry particle is one where the bound surface water has been driven off; in this case, a surface cation with d~.ficient anion coordination may induce a reversible restructuring o f the surface that, on contact with a salt, returns to normal with the capture o f an anion from the sa!t. The reversible restructuring o f the surface is not indicatea in table 1; the surface-oxygen deficiency for full oxygen coordination at a surface cation is simply
~.::- ::]cs attract Li 4 ions from the salt via the re~ c t i o n s
( ".: - ), + LiX ~- ( OLi- ), + Vci,
(2)
u>.~ii the electrochemical potential for Li + in the oxide oecomes equal to that in the salt via the spacecha~e-layer capacitance created by the Li+-ion transfer. Therefore the enhanced extrinsic Li+-ion cenducfivity should be similar for wet and dry oxide l;arficles dispersed in a LiX halide, and the Li+-ion conduction in the space-charge region should be via Li+-ion vacancies V~-i. The Ag + ion, on the other hand, does not displace a surface proton; in this case, reaction (1) is replaced by
~UULr.;U a a ul~; IL~;UU~I SpeCieS [ V0 / s .
The lithium salts LiI and LiBr must be distinguished from the silver salts AN and AgBr. The Li + ions cat, displace surface protons from a wet particle to give ~:he reaction ( H20)~ + 2LiX--, (OLi2) ~+ 2HX {,
( 1)
which does not introduce a space-charge layer in the host. Howev~.." the oxide ions on the wet and dry
Ti~c competitive reaction (O 2 - ) ~ + A g X a ( O A g - ) ~ + V g ,
(4)
which would be tbllowed by Ag~+ + V ~ - A g ,
(5)
is shifted strongly to the left, so the enhancement due
8
A.K. Shukla et al./Enhancement of ionic conductivity
to reaction (3) is not suppressed by reaction (5). For a dry particle, the reaction (Vo) ~+ A g X ~ ( X - )s +Ag + ,
(6)
must alo be shifted strongly to the left for A1203 particles in AgBr since little enhancement is oberved in this case. We therefore would predict that the enhancement of Ag+-ion conductivity in AgBr for only wet A1203 particles results from the dominance of reaction (3) and hence that the enhanced extrinsic conductivity is associated with interstitial Ag + ions, Ag+ , in the space-charge layer. In the case of PbF2 containing a dispersed oxide, the reaction
(Vo)s + ½PbF2~- (F-)s + V f ,
(7)
would be shifted more strongly to the right. Therefore a similar enhancement can be expected for wet and dry particles since the reaction (OH-)., + ~PbF2--, (O2-).~ + HFI' + V ~ ,
(8)
would give a similar concentration of fluoride-ion vacancies in the space-charge region. Moreover, as with the silver salts, the reaction (O2-) s + PbF2 ~- (OPb)s + 2 F , ,
(9)
would be shifted strongly to the left, so the reaction Vf- + F 7 ~ F
(10)
is not available to annihilate the V~ population. Two factors are operative to shift reactions (7) and/or (8) to the right: (i) the more acidic the oxide, the stronger an F - ion is bound to the particle surface as ( F - L and the more strongly a proton is attracted from the surface to form HF; (ii) the larger the surface cation of the oxide, the g:eatec its normal anion coordination and hence the concentration o/" anion-vacancy sites (Vo)~ and/or bound water molecules (H20)~ at the Surfaces. In order to test this prediction, several oxides ~CeO,, ZrO_,, SiO_,, and Al20~), all of about t3.2 um average particle size, were dispersed in PbF2. The respective isoelectric points for these oxides are 7, 4, 2, and 9 [ 16 ]; their cationic radii are 6.87, 0.39, 0.26, and 0,39/k [ 17]. According to the above argument, the maximum enhancement should occur for the maximum generation of V~- species at the interface surfaces and hence for the oxide with the greatest cat-
ionic radius and smallest isoelectric point. A measured F--ion conductivity enhancement that varies as ZrO2 > SiO2 >CeO., > A1203, see fig. 2, indicates that, in addition to the acidic character of the oxide, the ionic radius makes a noticeable contribution.
3. Experimental 3. I. Specimen preparation
H i g h - p u n y (Puratronic grade), anhydrous samples of PbF2, COO2, ZrO2, SiOz, and Al:O3 from Johnson-Mathey Chemicals were used. Therrnogravimetnc analyses (TGA) in the temperature range 25-600°C were made with a Perkin Elmer 7-series Thermal Analysis System; the only evidence of lattice or adsorbed water found after exposure to air at room temperature was 0. I and 0.2 wt% in AI203 and SiO2. The average particle size ofaU the powders was 0.2 gm. Each oxide was dispersed in separate samples of PbF. by mechanically mixing weighed oxide/PbF2 ratios of the powders in an agate mortar. No new phase was detected by X-ray powder diffraction in any mixture. Disc-shaped, dense pellets of 8 mm in diameter were made from the intimate mixtures by double-end compression in a steel die; they were cold-pressed at 1200 MPa. Gold electrodes about 250 A thick were sputtered onto the two disc faces with a Pelco Sputter-Coater Model-3 machine. 3.2. Measurement
Two-probe ac-impedance measurements were made in the frequency range 10-~-105 Hz by placing the specimen between silver electrodes of a measuring cell that made contact with the two gold-coated surfaces. The temperature of the cell was controlled to within + 1 °C; the ac impedance was measured with the setup shown in fig. 3. The variation of the conductivity ~ as a fnnction of frequency is sbow.q in fig. 4 for PbF2 and PbF2 containing various mole% dispersed ca. 0.2 ~tm CeO2 palaicles. The maximum conductivity occurs near l 0 mole% CeO> At 105 Hz, the highest frequency used, any polarization resistance makes a negligible contribution. At 298 K and 105 Hz, the conductivity of
A.K. Shukla et aL/Enhancement of ionic conductivity
9
X PbF 2 + AI203 2.0
0 p13F2 +CeO 2 [] PbF 2 + SiQ2 Z~ PbF 2 + Zr02
,Sr-. o..
1.5 x
1.0 I
I
I
5
10
15
Moie % of the Dispersoid Fig, 2. Normalized conductivity enhancement of r b F : versL~.~dispersoid concentration for various dispersed oxides of mean particle diameter 0.2 ~tm.
Potentiostat/Galv~ I
E
Measurement Cell
Interface
Key Board
Li" i
I'%Y! i C.~ alqneE
Prea_mplifieri Lock - In Amplifier
Printer
Fig. 3. Schematic for ac-impedance measurement apparatus.
A.K. Shukla et aL/Enhancement of ionic conductivity
10
larger ionic radius of ZP + is compatible with eightfold anion coordination in ZrOz; the Si4+ ion preferentially occupies tetrahedral sites. Thus the reversal in enhaacement factors based on isoelectric point for ZrO2 and SiO2 can be reconciled by invoking a contribution from the difference in cationic size.
-8
--
--
~
w
u
References O
°
PbF2+5 tool % CoO2 o PbF2 +7.5 too! % CaO2
•* PbF2 +10 mol % CaO2 e PbF2+15 mol % CeO2 =5
-1
,
|
0
1
-..|-
2
|
i
3
4
5
Log Frequency(Hz) .-~ Fig. 4. Frequency dependence o f the conductivity o f PbF2 and
PbFz dispersed with various concentration of CeOz at 298 K.
PbF2 dispersed with 7.5 and 10 mole% CeO2 iS about
1.4 times larger than ~.hat of PbF2. A comparison of these results with those obtained for Zr02, Si02, a~ldA1203aprti.~les dispersed in PbF_~, fig. 2, shows that the maxim~:n enhancement occurs ibr ZiG: ~a PbF~. Although SiO2 has a somewhat smaller isoelectfic point (2 versus 4 for ZrO2), the
[ 1] C.C. Liang, 3. Eleetrochem. See. 120 (1973) 1289. [2] S. Pack, B.B. Owens and 3.B. Wagner, J. Electroehem. SOe. 127 (1950) 2177. [ 31 P. Hartwig and W. Weppner, Solid State lonics 3/4 (1981 ) 249. [41 A. Hooper, J. Power Sources 9 (1983) 161. [5] O. Nakamura and J.B. Goodenough, Solid State lonics 7 (1982) 119. [6] T. Jow and 3.B. Wagner, J. Electrochem. SOc. 126 (1979) 1963. [7] J.B. Wagner, Meter. Res. Bull. 15 (1980) 1691. [8] K, Shahi and J.B. Wagner, J. Solid State Chem. 42 (1982) 107. [9] K, Shahi and J.B. Wagner, J. Phys, Chem. Solids 43 (1982) 713. [ 10] N, Vaidehi, R. Akila, A.K. Shukla and ICT. Jacob, Mater. Res. Bull. 21 (1986) 909. [ 11 ] M.R.W. Chang, Masters Thesis (Arizona State University, 1987). [ 12] A. Stoneham, E. Wade and J.A. Kilmer, Mater. Res. Bull 21 (1979) 661. [ 13] L Maler, J. Phys. Chem. Solids 46 (1985) 309. [ 14] J. Maier, J. Electrochem. Soc. 134 (1987) 1524. [ 15] W.A. England, M.G. Cross, A. Hamnett, P.3. Wiseman and J.B. Goodenough, Solid State tonics I (1980) 231. [ 16] G.A. Parks, Chem. Rex,. 65 (1965) 177. [ 17] R.D. Shannon, Acta Crys. A32 (! 976) 75 i.