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The study of structure and electrical properties of montmorillonite solid electrolyte
Solid State Ionies 36 (1989) 15-22 North-Holland, Amsterdam
THE STUDY OF STRUCTURE AND ELECIRICAL PROPERTIES OF MONTMORILLONITE SOLID ELECTROLYTE * Bin Z H U , Da-Zhi W A N G and Wen-hai Y U University of Science and Technologyof China, Hefei, AnhuL P.R. China Received 25 July 1988; accepted for publication 2 May 1989
MontmoriUonite, a natural aluminium-silicate, was investigated as a solid electrolyte. In the studied montmorillonite the charge density i.e cation density is low because of the Fe3+-A13+octahedral substitutions, the structure of iron is in octahedral coordination with two different symmetric environments, the information of the interlayer iron and other cations was also obtained. On the other hand, the open layered structure of montmorillonite leads up to its interlayer cations to be movable so that it is a good conductor for various cations such as H +, Mg2+, Zn2+, etc. The electrical property measurement of various cation-exchange montmorillonites shows a conductivity order H+, Zn2+ Mg2+ Fe3+ Cu2+ and the activation energyorder H + Zn2+ M~ + C'u2+Fe3+. Their room temperature conductivities and the activation energies are in the regions of 10-5-10 -4 (cm) -m and 0.2-0.5 eV respectively.
1. Introduction Montmorillonite, as a solid electrolyte, in the applications o f solid state battery, solid electro-chemical insertion and other fields exhibits a promising future [ 1 - 3 ] . In more recent years, the denatured montmoriUonite was found to be an excellent solid electrolyte for not only single-valence ions such as H +, Li + etc., but also the high-valence ions as Mg 2+, Zn 2+, etc. [ 2 ]. To investigate the conduction mechanism o f the cations the structure and electrical properties are studied further in this paper.
2. Experiments MontmoriUonite employed in this work was taken from Anhui, China. Various ion-exchange montmorillonite solid electrolyte samples were obtained from the raw material which was chemically denatured by ion-exchange in aqueous salt solutions. The acidity o f the exchange solution for various cations ( 1.0 N concentration) was p H value o f 1.0 by add* Project supported by the National Fund on Natural Sciences. t Author to whom all correspondence should be addressed.
ing into HC1 acid moderately. The ion exchange was carried out at 80°C, and each hour the ion-exchange solution was replaced by a fresh one; this treatment was repeated four times, followed by repeated washing and fdtrating in distilled water. The products, i.e. ion-exchange montmorillonite, or M-mont., were dried at 80°C and then ground and stored in air. Xray fluorescence ( X R F ) analyses o f the raw and various cation exchanged M-mont. samples were made by VF-320 X-ray fluorescence spectrometer. The structure analysis o f samples were performed using X-ray diffraction ( X R D , Geigerflex D / M A X - R A Xray diffractometer), differential thermal analysis (DTA, Shimadzu DT-30 thermal analyzer), infrared (IR, Nicolet 170 SX I R spectrophotometer), M6ssbauer (MB, MS-500 a conventional constant acceleration spectrometer, 57Co(Rh) source) and electron spin resonance (ESR, Bruker ER-20GODS R C - 1 0 / 1 2 spectrometer, X-band). The ac conductivities o f various M-mont. solid electrolyte samples were measured at 100 kHz with M / M - m o n t . / M cell, where M is the metal electrode being reversible to M-mont. solid electrolyte, by U X 16 A oscillate, U D 2 0 A detector amplifier and CD-6 admittance bridge and the signal voltage o f measurement was smaller than 100 mV. The dc conduc-
0 167-2738/89/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division )
16
B. Zhu et al. / Montmorillonite solid electrolyte
. /~d c i1-mont.
spring belectrode
Fig. 1. Sketch of sealed device of samples.
tivity measurement was made by YJ56 dc potentiostat, F L U K E numerical voltmeter and M3800 digital multimeter, the measurement signal voltage is 1.0 V. The samples were sealed in a special device (see fig. 1 ). The measurement of temperature dependence of ac and de conductivities were made in the range 283 K < T < 363 K (in dosed condition) and 283 K < T < 883 K (in air) respectively.
I
7
11
i
13
a
17
t
;
25
j
3
20( ° )
Fig. 2. XRD patterns of raw sample heated at various temperatures (see table 2 for description of curves).
pie and of it heated at various temperatures for one hour are shown in fig. 2, in which the peaks of the accompanying cristobalite are removed for clarity.
3. Results and discussions Table 2 001 reflection of raw mont. heated at various temperatures.
3.1. X R F and XRD The principal chemical contents of raw and various denatured M-mont. ( M = M g , Zn, Fe, Cu) sampies by X R F are listed in table 1, it is shown that in the denatured process M ~+ ( i = 2 , 3) ions were exchanged into the interlayer region of mont. so that the corresponding M contents were increased clearly, The water content is generally about 13-15% wt. to M-mont. samples. X-ray diffraction analyses indicate that the mont. structure ws retained for all Mmonts., only little differences in doo~ value being observed for them. The X R D patterns of the raw sam-
Order
doe~
Heated
no.
(nm)
temp. (oc)
a b m' c}m" d e f g
1.553 1.458 1.422 1.216 1.477 1.453
20 80
h
0.986 O.960
Note
original sample
120
double peaks
180 220 350 400
diffusion diffusion not observed
6O0
Table 1 Principal chemical content of various mont. samples. Content
SiO2
A1203
CaO
MgO
Fe304
ZnO
Others
77.0 79.5 77.0 75.6 77.1
12.1 8.92 12.0 11.9 12.0
1.10 0.06 0.10 0.02 0.10
0.67 0.22 1.34 0.24 0.20
2.14 2.25 2.54 4.46 2.30
0.06 5.36 0.06 0.06 0.06
5.93 3.69 6.63 7.07 8.24
(wt. %) raw mont. Zn-mont. Mg-mont. Fe-mont. Cu-mont.
B. Zhu et al. / Montmorillonite solid electrolyte
17
Mg-mont.
o
4()
~30
120
T(°C) t~
Fig. 3. DTA curves of various M-mont. samples.
The doo~ value and the change of 001 reflection corresponding to fig. 2 are listed in table 2, evidently the less the interlayer water content of mont. in the thermal dehydration process is, the smaller the doo~ value of it. doo~ changes from 15.5 to 9.6 A and shows a complicated dehydration phase transformation [ 4 ]. It is noticeable in fig. 2 that in the portion of 180350°C, the 001 reflection is changed from diffusion to non-observation and a disorder phase transformation of the direction of C axis occurred in the region.
1560
I 1 70
7~0
(cz~ -I wavenumber Fig. 4. ]R spectra for raw sample.
3.2. DTA
DTA curves of various M-monts. are shown in fig. 3. It is shown from fig. 3 that the temperatures of the endothermic valley are different to various M-monts. At the above temperature the dehydration of the ioncoordinated water occurs. Besides, two endothermic valleys of Cu-mont. are observed, where the first valley at 55°C and the second one at 135°C are concerned with the dehydration of the first and second ion-coordinated water sheet respectively. The very high endothermic temperature ( 135 °C) of the Cumont. sample shows the dehydration of Cu 2÷ ion-coordinated water to be the most difficult in all the Mmont. samples. 3.3. IR, M B and ESR
Fig. 4 shows the IR spectrum of the raw montmorillonite. The peaks at 1100 cm -~ and 790 cm -1 are due to the accompanying cristobalite. The ab-
sorption at 920 cm-~ is ascribed to the librational mode of the octahedral sheet structure hydroxyl groups A13+-OH-AIa+, which is weakend by the octahedral Fe 3+ for A13+ or Mg 2+ for A13+ substitutions and shifted to the lower frequencies, the absorptions of Fea+-OH-AI 3+ and Mg2+-OH-A1 a+ are in 870 and 840 cm-1 respectively [ 5 ]. In the spectrum of our studied mont. (see fig. 4), a weak peak of 840 c m - ' an a very weak shoulder of 870 c m vibration band are observed, it shows that the Fe a+ and Mg 2+ for A13+ substitutions are lower in our sample, which leads to a smaller interlayer cation concentration and charge density of the sheet framework because of the lower Mg 2+ for Al a+ isomorphous octahedral substitution. IR spectra of various M-mont. samples were studied. It was found that the IR spectra of all M-mont. samples are almost the same and the spectra are not sensitive to various ion exchanges. This result may arise from the low concentration of the interlayer
18
B. Zhu et al. / Montmorillonite solid electrolyte
cations so that the cation exchange does not influence the spectra. Fig. 5 shows MB spectra of the raw and Fe-mont. samples. AU data were computer fitted using the least squares assumed Lorentzian line-shapes. Decomposition of the spectra into three ferric subspectra, are shown in fig. 5. The fitted parameters are given in table 3, the isomer shifts which are quoted here relative to a-Fe show that three subspectra are due to the octahedral coordinated Fe 3+, where the doublets with the isomer shifts =0.263 m m / s and 0.233 mm/s, the quadrupole shifts =0.310 m m / s and 0.735 m m / s are correlated to Fe 3+ in the layer structure framework with two distinct distorted octahedral environments. The subspectrum identified by the largest quadrupole shift = 1.435 m m / s may result from the interlayer Fe 3+ as such a large quadrupole shift parameter suggests very strongly deformed octahedral environment; thus, it is very possible that Fe 3+ is in the interlayer region but in
the sheet structure. In addition, this subspectrum is clearly increased in Fe 3+ ion exchanged mont. sample (see fig. 5b), as a result of the interlayer Fe 3+ it is sensitive to Fe 3+ ion exchange. The ESR spectra of various cation-exchanged mont. samples are shown in fig. 6. The ESR spectrum of the original montmorillonite is characterized by features with g values approximately equal to 2.0, 3.7, 4.3, 9.6 and 15.8, it is similar to that of ref. [6]. The resonance of 3.7 is very weak in fig. 6, so it was not detected in most spectra. The signals of 15.8, 9.6 and 4.3 reflect the structural Fe 3+ localized in two distinct octahedral sites, especially, the signal of g ~ 4.3, which is a sharp peak, posseses the strong structural component and the character of much larger deformation octahedral environment. It is clearly seen from fig. 6 that the peak of g ~ 2.0, a broad signal, displays a sensitive change to various ion-exchanges. The intensity of g ~ 2.0 was enhanced by Fc 3+ and weakened by Li +, Cu 2+ etc., exchanged
rt
Fig. 5. MB spectra of samples: (a) raw sample, (b) Fe-mont.
Table 3 Parameters of M B and ESR spectra of samples.
Site and environment
In layered site 1
Structure site 2
Interlayer site 3
raw mont.
0.26 0.74 30 0.26 0.70 34 4.3
0.23 0.35 64 0.23 0.31 46 9.6, 15.8
0.26 1.44 6 0.26 1.43 20 2.0
Femont. g value
I.S. ( m m / s ) Q.S. ( m m / s ) I (%) I.S. ( m m / s ) Q.S. ( m m / s ) I (%)
Fig. 6. ESR spectra for various cation-mont, samples: (a) raw, (b) Li +-, (c) Fe 3+-, (d) Mn 2+-, (e) Cu 2+-, (f) Mg2+-, (g) Zn 2+(h) the raw heated at 350°C, (i) the raw heated at 600°C.
B. Zhu et al. / Montmorillonite solid electrolyte
into mont.. Furthermore, the characteristic ESR signals of Cu 2+, Mn 2+, and Cr 3+ appear in patterns (in these spectra the intensitives of them were decreased largely due to very low gains), where the four components of the hyperfine structure for Cu 2+ ( I = 3 / 2 ) and six lines hyperfine structure for Mn 2+ ( I = 5 / 2 ) are observed. Because the ion-exchange only occuffed in the interlayer of mont., the signal of g ~ 2.0 is ascribed to the contributions of the interlayer cations from the abovementioned experimental phenomena. It is in accordance with the theoretical result of ref. [7] due to a weak crystalline field environment in the interlayer region. The above ESR results are agreed with that gained in MB spectra on mont. and the corresponding parameters are listed in table 3. In addition, the ESR spectra of the heated various M-monts. were studied further, two changes were observed in the heated spectra, one is g ~ 4 . 3 signal was increased largely, others are the signals of 15.8, 9.6 and 2.0 were decreased and even not observed to all M-monts. From fig. 6i and j it is seen that for the mont. heated at 350°C the signal of g ~ 2 . 0 is weakened evidently but of g ~ 4 . 3 is enhanced, and the changes of that of g ~ 9.6 and 15.8 are small. These phenomena may be explained as follows: the potential field of the interlayer space was enhanced in dehydration process of heat treatment, it would convert the original signal o f F e 3+ in a weak field into that in a strong one. That is the reason why the resonance of g ~ 2.0 was decreased and that of 4.3 increased. Furthermore, in the heat-treated process the original Fe 3+ in the interlayer space would be heated and activated, then they would migrate into the layered structure of mont. where there is a proper structural environment, so that the structural signal of g ~ 4.2 would be increased and g ~ 2.0 decreased yet. When heated at 600°C, the signal of g ~ 4.3 was enhanced greatly and those of g ~ 2 . 0 , 9.6 and 15.8 were barely observed. These changes may be still interpreted as above. Moreover, in consideration with the structural O H groups the octahedral sheet of mont. would be removed at 600 ° C and led to a great distortion of the layered framework of mont., it would convert the signals of the structural Fe 3+ within the lower distorted sites such as g ~ 9.6 and 15.8 into ones in the greater distortion, i.e. g ~ 4 . 3 .
3. 4. Electrical properties
The parameters of the electrical properties to all M-mont. samples are listed in table 4. The dc conductivities of table 4 are gained from the plateaus of the curves in fig. 7. Fig. 7 shows the direct current polarization curves, namely the conductivity versus time. The data of the activation energy are derived from the Arrhenius curves in fig. 8 using the leastsquares fitting. Generally speaking, the ac conductivities are about one order of magnitude higher than dc ones, except for Zn-mont. (see table 4). Being with the reversible electrodes to M-monts. in dc measurements, the dc conductivities corresponding to the plateaus should be the contributions of M i÷ cations which are provided by the electrode of M metal for after a certain time interval, an ionic migration balance-concentration gradient (which is converse to the external field) is to be formed in mont. by the non-electrode ions such as H ÷ mainly. Only M i÷ ions can migrate under the external field driving at this time, so the conductivity, a stable motion of M i÷ along with the direction of the external field, is identified in the value of the plateau in the polarization curves (see fig. 7). On the other hand, the ac conductivity reflects the contributions of the conduction to all mobile ions in M-mont., where in our ion-exchange monts, they are H ÷ and M '÷ exchanged into M-mont. principally. Comparing ac and dc conduc,tivities, it shows clearly that the conductivity of ac, almost of H ÷, is higher about one order than that of M i÷. It is also reasonable that the ac activation energies are smaller than those of dc for a much stronger Coulomb interaction between the negative charges of the layer framework of mont. and M ~÷ ions. A very high potential barrier must be overcome in the migration o f M ~÷ ions. The ac and dc conductivities are changeable with the conditions of the ion-exchange such as the acidity, concentration of the exchange solution, the exchanged cations and the temperature, etc. The high dc conductivity of Zn-mont. can be considered under the exchanged conditions, mainly as the acidity. The sheet framework of mont. was destroyed and the isomorphous octahedral substitutions of Zn 2÷ for AP ÷ will occur so that the Zn content (see table 1 ) and dc conductivity of Zn-mont. are increased greatly due to the high Zn 2÷ ions concentration. Small differences in ac conductivity,
B. Zhu et al. / Montmorillonite solid electrolyte
20
Table 4 The electrical parameters for various M-mont. samples. Sample
Raw mont.
Zn-mont.
Mg-mont.
Fe-mont.
Cu-mont.
ac dc ac dc
10 -4 0.26 -
3.0X 10 -4 1.0× 10 -4 0.15, 0.38 0.26, 0.50
2.0×10 -4 3.0X 10 -5 0.25 0.29
8.0×10 -5 1.6× 10 -5 0.42 0.44
1.0Xl0 -4 3.2× 10 -6 0.20 0.37
( c m ) -~ (cm) - l (eV) (eV)
~tlom)-I _A I. Oxl O ~
- ~al-mont.
=
~
3.qx10-5
1.6 xl0-5
~_
~i,'e-mont.
3.2x10 -6
I
20
I
40
j Cu-mont.
I
60
~1~-mont.
I
8O
(rain)
Fig. 7. dc polarization curves of M-monts.
namely of H +, and large ones of dc conductivity, i.e. of M i+, to all M-mont. solid electrolytes can be generally predicted, table 4. The dc conductivity differences with various M-monts. can arise from many factors, at least from the following three: (1) Dissociation of water molecules in mont. whose degree is of the order of magnitude of 1% to total water [ 8 ]. (2) The migration ability of interlayer cations. (3) Hypothesis of hydrated interlayer cations act as Bronsted acids on hydrolysis: (M(H20)x)i+
+H20
= (M(H20)x_I)(,-l)+ +H30 + , the factors ( 1 ) and (2) are considered to be very important to dc conductivity, as it is not explained that the conductivity differs for general M-monts. according to the factor (3). In fact, there would be contradictory phenomenon, ref. [ 9 ] where the hydrolysis abilities of Cu 2+, Zn 2÷, etc., ions are clearly
higher than those of Li +, Na +, etc. alkali ions. It means that H + concentration, which is proportional to conductivity in Li, Na etc., clays, should be much smaller than that of Cu, Zn etc., clays if H ÷ is only from factor (3). Furthermore, the water content of Li, Na, etc., clays in ref. [ 9 ] is also much smaller than that of Cu, Zn, etc., clays, but the conductivities of the former ( ~ l 0 - 4 (~"~ cm) -1 ), are higher about one order of magnitude than those of the latter ( ~ 10- s ( ~ cm) - l ) inversely. In our dc measurements (since the factor ( 1 ) is the inherent feature of mont. which is dependent on the negative charge of the layer skeleton of mont.) on the interlayer cations and on temperature etc. the H + concentration is constant, the stable ionic migration in the external field can only come from the electrode cations i.e. M ~+ ions, even though there would be a few of Z n ( O H ) ÷ or M g ( O H ) ÷ ions migrating yet, so the differences of dc conductivities reflect those of the migration ability of interlayer cations. The dc measurements display a conductivity order
B. Zhu et al. / Montmorillonite solid electrolyte
(aom)-Ik A
x
Zn-mont. 14g-mont.
-I v
Fe-mont.
21
[ ~,
Zn-mol~t.
L
i~g-mont.
!
Pe-mont.
i
-2
-I
Cu-mont.
-2
,\
-3
-4
-5 I
2.8
I
3.O
I
3.2
I
3.4
m
2.8 (iO - I
-
!
I
3.o
3.2
I
3.4
I03"T
Fig. 8. The temperature dependence of conductivity for various M-mont. samples (in close): (a) ac measurement, ( b ) dc measurement.
H + > Zn 2+ > Mg 2+ > Fe 3+ > C u 2+ and an activation energy order H + < Z n 2+ < M g 2+ < C u 2+ < F e 3+. It is unexpected that an inverse order of the conductivity between Fe 3+ and Cu 2+, occurs as a result of a much stronger interaction between the ion-coordinated water and Cu 2+ ions (the dehydrated temperature of the close ion-coordinated water being as high as 135 °C (see table 2 and fig. 3)). The migration of Cu 2+ ions is very difficult, therefore a rapid decrease of dc conductivity in the initial interval and a very low conductivity plateau of Cu 2+ ion are observed in fig. 7d. One of the most interesting features of Znmont. is that a conversion point was detected at about 40°C in its Arrhenius curve (fig. 8). It shows that there are two distinct activation energies above and below 40 oC. This phenomenon can be explained with two kinds of hydration conditions and conduction ways of Zn 2+ in the interlayer space and certain temperature regions, their schemas are drawn in fig. 9. It is obvious that the much higher potential barrier must be overcome in the migration of Zn 2+ ion in fig. 9b than that in fig. 9a, certainly two different activation energies will be formed. In general, the above
Si-O ~heet
a
]
[
Si-O sheet
b
Fig. 9. Schematic of hydration condition and conduction way of interlayer cations.
case may appear to all M-mont. samples, but it will not be detected because of the following causes: ( 1 ) two kinds of hydrated conditions and conduction ways, arising from the distinct hydrated ions, accompany always in the measuring temperature ranges and (2) the temperature of the second hydration and conduction is higher than the dehydration temperature of the sample, so that the conductivity de-
22
B. Zhu et al. /Montmorillonite solid electrolyte
creases before the second case is to appear. The ac conductivity measurement was made further in the range of 283 K < T < 883 K (in air), the result of Mgmont. using graphite/Mg-mont./graphite is shown in fig. 10. In the air the conductivity of the sample decreases fast due to the dehydration of the sample when the temperature is higher than room temperature. Until the most water (which means the physical adsorption and non-coordinated or weak combined water of mont.) is dehydrated, the conductivity of the sample begins to increase at about 200 ° C. The activation energies are different obviously in the regions 200-400°C and above 400°C and the activation energy of the range 200-400°C is the lowest (see table 5 ). It is right in accordance with the range of the disorder-phase transformation of C-axis of mont. detected by X R D (see fig. 2), then one may deduce that there is a migration of the cations with the lowest potential barrier in the disorder phase of mont. 600
100
(C ° )
\
~e=)-lk -3
!
-4
A
/
-5 -6
./
-7
4. Summary (1) The interlayer cations concentration is very low because of few substitutions of Mg 2+ for AI3+ in the studied mont. (2) The structural Fe 3+ ions are localized in two distinct octahedral sites of the layer structure framework and the interlayer cations such as Fe 3+, Cu 2+, Mg 2+ etc. are in much stronger distortion, the weaker potential field and the more changeable environment, which is advantageous to the migration of the interlayer cations. (3) The conductivity of H + is higher about one order of magnitude than that of the interlayer M i+ cations for most mont. solid electrolytes. The conductivities of them are changeable with the ion-exchange conditions. (4) The conductivity order H + > Z n 2+ > M g 2+ > Fe 3+ > Cu 2+ and activation energy order H + < Zn 2+ < Mg 2+ < Cu 2+ < Fe 3+ were observed and the differences of dc conductivity reflect the migration ability of the interlayer cations. (5) Two hydration conditions and conduction ways of cations are proposed to account for the conversion in Arrhenius curves of mont. (6) There is a migration of cations with the lowest potential barrier in the C-axis in the direction of disorder phase of mont.
I
1
-8 /
References
-9 -10 -11 i
0.5
i
1.5 C~)~i5
i
3.~1o3/~
Fig. 10. Temperature dependence of ac conductivity for Mg-mont. (in air). Table 5 ac activation energies of Mg-mont. in various temperature ranges. Temp. range (K)
283< T<363
473< T<673
T>673
(in close)
(in air)
(in air)
a (eV)
0.26
0.23
0.69
[ 1 ] D.-Z Wang, W.-H. Yu, W.-Z. Yuan and B. Zhu, in: Materials for solid state batteries, eds. B.V.R. Chowdari and S. Radhakrishnan (World Scientific, Singapore, 1986) p. 461. [2] W.-N. Yu, D.-Z. Wang, B. Zhu, S.J. Wang and L-X. Xue, Solid State Commun. 61 (1987) 271. [ 3 ] B. Zhu, D.-Z. Wang and W.-H. Yu, Zn-mont. solid electrolyte and secondary batteries of Zn/V6Ol3, reported in: 6th Intern. Meeting on Solid State Ionics, Sept. 1987, GarmischPartenkirehen, FRG. [ 4 ] D.-Z. Wang, B. Zhu, W.-H. Yu and G.-E. Zhou, Acta Physica Sinica 36 (1987) 1004. [5] C. Cracium Spectrosc. Letters 17 (1984) 579. [6] B.A. Goodman, Clay Miner. 13 (1978) 351. [7] M.I. Seullane, J. Magn. Resort. 47 (1982) 383. [ 8 ] J.J. Fripiat, A. Jelli, G. Poncelet and J. Andre, J. Phys. Chem. 69 (1965) 2185. [9] R.C.T. Slade, J. Barker, P.P. Hirst, T.IC Halstead and P.L Reid, Solid State Ionics 24 (1987) 289.
THE STUDY OF STRUCTURE AND ELECIRICAL PROPERTIES OF MONTMORILLONITE SOLID ELECTROLYTE * Bin Z H U , Da-Zhi W A N G and Wen-hai Y U University of Science and Technologyof China, Hefei, AnhuL P.R. China Received 25 July 1988; accepted for publication 2 May 1989
MontmoriUonite, a natural aluminium-silicate, was investigated as a solid electrolyte. In the studied montmorillonite the charge density i.e cation density is low because of the Fe3+-A13+octahedral substitutions, the structure of iron is in octahedral coordination with two different symmetric environments, the information of the interlayer iron and other cations was also obtained. On the other hand, the open layered structure of montmorillonite leads up to its interlayer cations to be movable so that it is a good conductor for various cations such as H +, Mg2+, Zn2+, etc. The electrical property measurement of various cation-exchange montmorillonites shows a conductivity order H+, Zn2+ Mg2+ Fe3+ Cu2+ and the activation energyorder H + Zn2+ M~ + C'u2+Fe3+. Their room temperature conductivities and the activation energies are in the regions of 10-5-10 -4 (cm) -m and 0.2-0.5 eV respectively.
1. Introduction Montmorillonite, as a solid electrolyte, in the applications o f solid state battery, solid electro-chemical insertion and other fields exhibits a promising future [ 1 - 3 ] . In more recent years, the denatured montmoriUonite was found to be an excellent solid electrolyte for not only single-valence ions such as H +, Li + etc., but also the high-valence ions as Mg 2+, Zn 2+, etc. [ 2 ]. To investigate the conduction mechanism o f the cations the structure and electrical properties are studied further in this paper.
2. Experiments MontmoriUonite employed in this work was taken from Anhui, China. Various ion-exchange montmorillonite solid electrolyte samples were obtained from the raw material which was chemically denatured by ion-exchange in aqueous salt solutions. The acidity o f the exchange solution for various cations ( 1.0 N concentration) was p H value o f 1.0 by add* Project supported by the National Fund on Natural Sciences. t Author to whom all correspondence should be addressed.
ing into HC1 acid moderately. The ion exchange was carried out at 80°C, and each hour the ion-exchange solution was replaced by a fresh one; this treatment was repeated four times, followed by repeated washing and fdtrating in distilled water. The products, i.e. ion-exchange montmorillonite, or M-mont., were dried at 80°C and then ground and stored in air. Xray fluorescence ( X R F ) analyses o f the raw and various cation exchanged M-mont. samples were made by VF-320 X-ray fluorescence spectrometer. The structure analysis o f samples were performed using X-ray diffraction ( X R D , Geigerflex D / M A X - R A Xray diffractometer), differential thermal analysis (DTA, Shimadzu DT-30 thermal analyzer), infrared (IR, Nicolet 170 SX I R spectrophotometer), M6ssbauer (MB, MS-500 a conventional constant acceleration spectrometer, 57Co(Rh) source) and electron spin resonance (ESR, Bruker ER-20GODS R C - 1 0 / 1 2 spectrometer, X-band). The ac conductivities o f various M-mont. solid electrolyte samples were measured at 100 kHz with M / M - m o n t . / M cell, where M is the metal electrode being reversible to M-mont. solid electrolyte, by U X 16 A oscillate, U D 2 0 A detector amplifier and CD-6 admittance bridge and the signal voltage o f measurement was smaller than 100 mV. The dc conduc-
0 167-2738/89/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division )
16
B. Zhu et al. / Montmorillonite solid electrolyte
. /~d c i1-mont.
spring belectrode
Fig. 1. Sketch of sealed device of samples.
tivity measurement was made by YJ56 dc potentiostat, F L U K E numerical voltmeter and M3800 digital multimeter, the measurement signal voltage is 1.0 V. The samples were sealed in a special device (see fig. 1 ). The measurement of temperature dependence of ac and de conductivities were made in the range 283 K < T < 363 K (in dosed condition) and 283 K < T < 883 K (in air) respectively.
I
7
11
i
13
a
17
t
;
25
j
3
20( ° )
Fig. 2. XRD patterns of raw sample heated at various temperatures (see table 2 for description of curves).
pie and of it heated at various temperatures for one hour are shown in fig. 2, in which the peaks of the accompanying cristobalite are removed for clarity.
3. Results and discussions Table 2 001 reflection of raw mont. heated at various temperatures.
3.1. X R F and XRD The principal chemical contents of raw and various denatured M-mont. ( M = M g , Zn, Fe, Cu) sampies by X R F are listed in table 1, it is shown that in the denatured process M ~+ ( i = 2 , 3) ions were exchanged into the interlayer region of mont. so that the corresponding M contents were increased clearly, The water content is generally about 13-15% wt. to M-mont. samples. X-ray diffraction analyses indicate that the mont. structure ws retained for all Mmonts., only little differences in doo~ value being observed for them. The X R D patterns of the raw sam-
Order
doe~
Heated
no.
(nm)
temp. (oc)
a b m' c}m" d e f g
1.553 1.458 1.422 1.216 1.477 1.453
20 80
h
0.986 O.960
Note
original sample
120
double peaks
180 220 350 400
diffusion diffusion not observed
6O0
Table 1 Principal chemical content of various mont. samples. Content
SiO2
A1203
CaO
MgO
Fe304
ZnO
Others
77.0 79.5 77.0 75.6 77.1
12.1 8.92 12.0 11.9 12.0
1.10 0.06 0.10 0.02 0.10
0.67 0.22 1.34 0.24 0.20
2.14 2.25 2.54 4.46 2.30
0.06 5.36 0.06 0.06 0.06
5.93 3.69 6.63 7.07 8.24
(wt. %) raw mont. Zn-mont. Mg-mont. Fe-mont. Cu-mont.
B. Zhu et al. / Montmorillonite solid electrolyte
17
Mg-mont.
o
4()
~30
120
T(°C) t~
Fig. 3. DTA curves of various M-mont. samples.
The doo~ value and the change of 001 reflection corresponding to fig. 2 are listed in table 2, evidently the less the interlayer water content of mont. in the thermal dehydration process is, the smaller the doo~ value of it. doo~ changes from 15.5 to 9.6 A and shows a complicated dehydration phase transformation [ 4 ]. It is noticeable in fig. 2 that in the portion of 180350°C, the 001 reflection is changed from diffusion to non-observation and a disorder phase transformation of the direction of C axis occurred in the region.
1560
I 1 70
7~0
(cz~ -I wavenumber Fig. 4. ]R spectra for raw sample.
3.2. DTA
DTA curves of various M-monts. are shown in fig. 3. It is shown from fig. 3 that the temperatures of the endothermic valley are different to various M-monts. At the above temperature the dehydration of the ioncoordinated water occurs. Besides, two endothermic valleys of Cu-mont. are observed, where the first valley at 55°C and the second one at 135°C are concerned with the dehydration of the first and second ion-coordinated water sheet respectively. The very high endothermic temperature ( 135 °C) of the Cumont. sample shows the dehydration of Cu 2÷ ion-coordinated water to be the most difficult in all the Mmont. samples. 3.3. IR, M B and ESR
Fig. 4 shows the IR spectrum of the raw montmorillonite. The peaks at 1100 cm -~ and 790 cm -1 are due to the accompanying cristobalite. The ab-
sorption at 920 cm-~ is ascribed to the librational mode of the octahedral sheet structure hydroxyl groups A13+-OH-AIa+, which is weakend by the octahedral Fe 3+ for A13+ or Mg 2+ for A13+ substitutions and shifted to the lower frequencies, the absorptions of Fea+-OH-AI 3+ and Mg2+-OH-A1 a+ are in 870 and 840 cm-1 respectively [ 5 ]. In the spectrum of our studied mont. (see fig. 4), a weak peak of 840 c m - ' an a very weak shoulder of 870 c m vibration band are observed, it shows that the Fe a+ and Mg 2+ for A13+ substitutions are lower in our sample, which leads to a smaller interlayer cation concentration and charge density of the sheet framework because of the lower Mg 2+ for Al a+ isomorphous octahedral substitution. IR spectra of various M-mont. samples were studied. It was found that the IR spectra of all M-mont. samples are almost the same and the spectra are not sensitive to various ion exchanges. This result may arise from the low concentration of the interlayer
18
B. Zhu et al. / Montmorillonite solid electrolyte
cations so that the cation exchange does not influence the spectra. Fig. 5 shows MB spectra of the raw and Fe-mont. samples. AU data were computer fitted using the least squares assumed Lorentzian line-shapes. Decomposition of the spectra into three ferric subspectra, are shown in fig. 5. The fitted parameters are given in table 3, the isomer shifts which are quoted here relative to a-Fe show that three subspectra are due to the octahedral coordinated Fe 3+, where the doublets with the isomer shifts =0.263 m m / s and 0.233 mm/s, the quadrupole shifts =0.310 m m / s and 0.735 m m / s are correlated to Fe 3+ in the layer structure framework with two distinct distorted octahedral environments. The subspectrum identified by the largest quadrupole shift = 1.435 m m / s may result from the interlayer Fe 3+ as such a large quadrupole shift parameter suggests very strongly deformed octahedral environment; thus, it is very possible that Fe 3+ is in the interlayer region but in
the sheet structure. In addition, this subspectrum is clearly increased in Fe 3+ ion exchanged mont. sample (see fig. 5b), as a result of the interlayer Fe 3+ it is sensitive to Fe 3+ ion exchange. The ESR spectra of various cation-exchanged mont. samples are shown in fig. 6. The ESR spectrum of the original montmorillonite is characterized by features with g values approximately equal to 2.0, 3.7, 4.3, 9.6 and 15.8, it is similar to that of ref. [6]. The resonance of 3.7 is very weak in fig. 6, so it was not detected in most spectra. The signals of 15.8, 9.6 and 4.3 reflect the structural Fe 3+ localized in two distinct octahedral sites, especially, the signal of g ~ 4.3, which is a sharp peak, posseses the strong structural component and the character of much larger deformation octahedral environment. It is clearly seen from fig. 6 that the peak of g ~ 2.0, a broad signal, displays a sensitive change to various ion-exchanges. The intensity of g ~ 2.0 was enhanced by Fc 3+ and weakened by Li +, Cu 2+ etc., exchanged
rt
Fig. 5. MB spectra of samples: (a) raw sample, (b) Fe-mont.
Table 3 Parameters of M B and ESR spectra of samples.
Site and environment
In layered site 1
Structure site 2
Interlayer site 3
raw mont.
0.26 0.74 30 0.26 0.70 34 4.3
0.23 0.35 64 0.23 0.31 46 9.6, 15.8
0.26 1.44 6 0.26 1.43 20 2.0
Femont. g value
I.S. ( m m / s ) Q.S. ( m m / s ) I (%) I.S. ( m m / s ) Q.S. ( m m / s ) I (%)
Fig. 6. ESR spectra for various cation-mont, samples: (a) raw, (b) Li +-, (c) Fe 3+-, (d) Mn 2+-, (e) Cu 2+-, (f) Mg2+-, (g) Zn 2+(h) the raw heated at 350°C, (i) the raw heated at 600°C.
B. Zhu et al. / Montmorillonite solid electrolyte
into mont.. Furthermore, the characteristic ESR signals of Cu 2+, Mn 2+, and Cr 3+ appear in patterns (in these spectra the intensitives of them were decreased largely due to very low gains), where the four components of the hyperfine structure for Cu 2+ ( I = 3 / 2 ) and six lines hyperfine structure for Mn 2+ ( I = 5 / 2 ) are observed. Because the ion-exchange only occuffed in the interlayer of mont., the signal of g ~ 2.0 is ascribed to the contributions of the interlayer cations from the abovementioned experimental phenomena. It is in accordance with the theoretical result of ref. [7] due to a weak crystalline field environment in the interlayer region. The above ESR results are agreed with that gained in MB spectra on mont. and the corresponding parameters are listed in table 3. In addition, the ESR spectra of the heated various M-monts. were studied further, two changes were observed in the heated spectra, one is g ~ 4 . 3 signal was increased largely, others are the signals of 15.8, 9.6 and 2.0 were decreased and even not observed to all M-monts. From fig. 6i and j it is seen that for the mont. heated at 350°C the signal of g ~ 2 . 0 is weakened evidently but of g ~ 4 . 3 is enhanced, and the changes of that of g ~ 9.6 and 15.8 are small. These phenomena may be explained as follows: the potential field of the interlayer space was enhanced in dehydration process of heat treatment, it would convert the original signal o f F e 3+ in a weak field into that in a strong one. That is the reason why the resonance of g ~ 2.0 was decreased and that of 4.3 increased. Furthermore, in the heat-treated process the original Fe 3+ in the interlayer space would be heated and activated, then they would migrate into the layered structure of mont. where there is a proper structural environment, so that the structural signal of g ~ 4.2 would be increased and g ~ 2.0 decreased yet. When heated at 600°C, the signal of g ~ 4.3 was enhanced greatly and those of g ~ 2 . 0 , 9.6 and 15.8 were barely observed. These changes may be still interpreted as above. Moreover, in consideration with the structural O H groups the octahedral sheet of mont. would be removed at 600 ° C and led to a great distortion of the layered framework of mont., it would convert the signals of the structural Fe 3+ within the lower distorted sites such as g ~ 9.6 and 15.8 into ones in the greater distortion, i.e. g ~ 4 . 3 .
3. 4. Electrical properties
The parameters of the electrical properties to all M-mont. samples are listed in table 4. The dc conductivities of table 4 are gained from the plateaus of the curves in fig. 7. Fig. 7 shows the direct current polarization curves, namely the conductivity versus time. The data of the activation energy are derived from the Arrhenius curves in fig. 8 using the leastsquares fitting. Generally speaking, the ac conductivities are about one order of magnitude higher than dc ones, except for Zn-mont. (see table 4). Being with the reversible electrodes to M-monts. in dc measurements, the dc conductivities corresponding to the plateaus should be the contributions of M i÷ cations which are provided by the electrode of M metal for after a certain time interval, an ionic migration balance-concentration gradient (which is converse to the external field) is to be formed in mont. by the non-electrode ions such as H ÷ mainly. Only M i÷ ions can migrate under the external field driving at this time, so the conductivity, a stable motion of M i÷ along with the direction of the external field, is identified in the value of the plateau in the polarization curves (see fig. 7). On the other hand, the ac conductivity reflects the contributions of the conduction to all mobile ions in M-mont., where in our ion-exchange monts, they are H ÷ and M '÷ exchanged into M-mont. principally. Comparing ac and dc conduc,tivities, it shows clearly that the conductivity of ac, almost of H ÷, is higher about one order than that of M i÷. It is also reasonable that the ac activation energies are smaller than those of dc for a much stronger Coulomb interaction between the negative charges of the layer framework of mont. and M ~÷ ions. A very high potential barrier must be overcome in the migration o f M ~÷ ions. The ac and dc conductivities are changeable with the conditions of the ion-exchange such as the acidity, concentration of the exchange solution, the exchanged cations and the temperature, etc. The high dc conductivity of Zn-mont. can be considered under the exchanged conditions, mainly as the acidity. The sheet framework of mont. was destroyed and the isomorphous octahedral substitutions of Zn 2÷ for AP ÷ will occur so that the Zn content (see table 1 ) and dc conductivity of Zn-mont. are increased greatly due to the high Zn 2÷ ions concentration. Small differences in ac conductivity,
B. Zhu et al. / Montmorillonite solid electrolyte
20
Table 4 The electrical parameters for various M-mont. samples. Sample
Raw mont.
Zn-mont.
Mg-mont.
Fe-mont.
Cu-mont.
ac dc ac dc
10 -4 0.26 -
3.0X 10 -4 1.0× 10 -4 0.15, 0.38 0.26, 0.50
2.0×10 -4 3.0X 10 -5 0.25 0.29
8.0×10 -5 1.6× 10 -5 0.42 0.44
1.0Xl0 -4 3.2× 10 -6 0.20 0.37
( c m ) -~ (cm) - l (eV) (eV)
~tlom)-I _A I. Oxl O ~
- ~al-mont.
=
~
3.qx10-5
1.6 xl0-5
~_
~i,'e-mont.
3.2x10 -6
I
20
I
40
j Cu-mont.
I
60
~1~-mont.
I
8O
(rain)
Fig. 7. dc polarization curves of M-monts.
namely of H +, and large ones of dc conductivity, i.e. of M i+, to all M-mont. solid electrolytes can be generally predicted, table 4. The dc conductivity differences with various M-monts. can arise from many factors, at least from the following three: (1) Dissociation of water molecules in mont. whose degree is of the order of magnitude of 1% to total water [ 8 ]. (2) The migration ability of interlayer cations. (3) Hypothesis of hydrated interlayer cations act as Bronsted acids on hydrolysis: (M(H20)x)i+
+H20
= (M(H20)x_I)(,-l)+ +H30 + , the factors ( 1 ) and (2) are considered to be very important to dc conductivity, as it is not explained that the conductivity differs for general M-monts. according to the factor (3). In fact, there would be contradictory phenomenon, ref. [ 9 ] where the hydrolysis abilities of Cu 2+, Zn 2÷, etc., ions are clearly
higher than those of Li +, Na +, etc. alkali ions. It means that H + concentration, which is proportional to conductivity in Li, Na etc., clays, should be much smaller than that of Cu, Zn etc., clays if H ÷ is only from factor (3). Furthermore, the water content of Li, Na, etc., clays in ref. [ 9 ] is also much smaller than that of Cu, Zn, etc., clays, but the conductivities of the former ( ~ l 0 - 4 (~"~ cm) -1 ), are higher about one order of magnitude than those of the latter ( ~ 10- s ( ~ cm) - l ) inversely. In our dc measurements (since the factor ( 1 ) is the inherent feature of mont. which is dependent on the negative charge of the layer skeleton of mont.) on the interlayer cations and on temperature etc. the H + concentration is constant, the stable ionic migration in the external field can only come from the electrode cations i.e. M ~+ ions, even though there would be a few of Z n ( O H ) ÷ or M g ( O H ) ÷ ions migrating yet, so the differences of dc conductivities reflect those of the migration ability of interlayer cations. The dc measurements display a conductivity order
B. Zhu et al. / Montmorillonite solid electrolyte
(aom)-Ik A
x
Zn-mont. 14g-mont.
-I v
Fe-mont.
21
[ ~,
Zn-mol~t.
L
i~g-mont.
!
Pe-mont.
i
-2
-I
Cu-mont.
-2
,\
-3
-4
-5 I
2.8
I
3.O
I
3.2
I
3.4
m
2.8 (iO - I
-
!
I
3.o
3.2
I
3.4
I03"T
Fig. 8. The temperature dependence of conductivity for various M-mont. samples (in close): (a) ac measurement, ( b ) dc measurement.
H + > Zn 2+ > Mg 2+ > Fe 3+ > C u 2+ and an activation energy order H + < Z n 2+ < M g 2+ < C u 2+ < F e 3+. It is unexpected that an inverse order of the conductivity between Fe 3+ and Cu 2+, occurs as a result of a much stronger interaction between the ion-coordinated water and Cu 2+ ions (the dehydrated temperature of the close ion-coordinated water being as high as 135 °C (see table 2 and fig. 3)). The migration of Cu 2+ ions is very difficult, therefore a rapid decrease of dc conductivity in the initial interval and a very low conductivity plateau of Cu 2+ ion are observed in fig. 7d. One of the most interesting features of Znmont. is that a conversion point was detected at about 40°C in its Arrhenius curve (fig. 8). It shows that there are two distinct activation energies above and below 40 oC. This phenomenon can be explained with two kinds of hydration conditions and conduction ways of Zn 2+ in the interlayer space and certain temperature regions, their schemas are drawn in fig. 9. It is obvious that the much higher potential barrier must be overcome in the migration of Zn 2+ ion in fig. 9b than that in fig. 9a, certainly two different activation energies will be formed. In general, the above
Si-O ~heet
a
]
[
Si-O sheet
b
Fig. 9. Schematic of hydration condition and conduction way of interlayer cations.
case may appear to all M-mont. samples, but it will not be detected because of the following causes: ( 1 ) two kinds of hydrated conditions and conduction ways, arising from the distinct hydrated ions, accompany always in the measuring temperature ranges and (2) the temperature of the second hydration and conduction is higher than the dehydration temperature of the sample, so that the conductivity de-
22
B. Zhu et al. /Montmorillonite solid electrolyte
creases before the second case is to appear. The ac conductivity measurement was made further in the range of 283 K < T < 883 K (in air), the result of Mgmont. using graphite/Mg-mont./graphite is shown in fig. 10. In the air the conductivity of the sample decreases fast due to the dehydration of the sample when the temperature is higher than room temperature. Until the most water (which means the physical adsorption and non-coordinated or weak combined water of mont.) is dehydrated, the conductivity of the sample begins to increase at about 200 ° C. The activation energies are different obviously in the regions 200-400°C and above 400°C and the activation energy of the range 200-400°C is the lowest (see table 5 ). It is right in accordance with the range of the disorder-phase transformation of C-axis of mont. detected by X R D (see fig. 2), then one may deduce that there is a migration of the cations with the lowest potential barrier in the disorder phase of mont. 600
100
(C ° )
\
~e=)-lk -3
!
-4
A
/
-5 -6
./
-7
4. Summary (1) The interlayer cations concentration is very low because of few substitutions of Mg 2+ for AI3+ in the studied mont. (2) The structural Fe 3+ ions are localized in two distinct octahedral sites of the layer structure framework and the interlayer cations such as Fe 3+, Cu 2+, Mg 2+ etc. are in much stronger distortion, the weaker potential field and the more changeable environment, which is advantageous to the migration of the interlayer cations. (3) The conductivity of H + is higher about one order of magnitude than that of the interlayer M i+ cations for most mont. solid electrolytes. The conductivities of them are changeable with the ion-exchange conditions. (4) The conductivity order H + > Z n 2+ > M g 2+ > Fe 3+ > Cu 2+ and activation energy order H + < Zn 2+ < Mg 2+ < Cu 2+ < Fe 3+ were observed and the differences of dc conductivity reflect the migration ability of the interlayer cations. (5) Two hydration conditions and conduction ways of cations are proposed to account for the conversion in Arrhenius curves of mont. (6) There is a migration of cations with the lowest potential barrier in the C-axis in the direction of disorder phase of mont.
I
1
-8 /
References
-9 -10 -11 i
0.5
i
1.5 C~)~i5
i
3.~1o3/~
Fig. 10. Temperature dependence of ac conductivity for Mg-mont. (in air). Table 5 ac activation energies of Mg-mont. in various temperature ranges. Temp. range (K)
283< T<363
473< T<673
T>673
(in close)
(in air)
(in air)
a (eV)
0.26
0.23
0.69
[ 1 ] D.-Z Wang, W.-H. Yu, W.-Z. Yuan and B. Zhu, in: Materials for solid state batteries, eds. B.V.R. Chowdari and S. Radhakrishnan (World Scientific, Singapore, 1986) p. 461. [2] W.-N. Yu, D.-Z. Wang, B. Zhu, S.J. Wang and L-X. Xue, Solid State Commun. 61 (1987) 271. [ 3 ] B. Zhu, D.-Z. Wang and W.-H. Yu, Zn-mont. solid electrolyte and secondary batteries of Zn/V6Ol3, reported in: 6th Intern. Meeting on Solid State Ionics, Sept. 1987, GarmischPartenkirehen, FRG. [ 4 ] D.-Z. Wang, B. Zhu, W.-H. Yu and G.-E. Zhou, Acta Physica Sinica 36 (1987) 1004. [5] C. Cracium Spectrosc. Letters 17 (1984) 579. [6] B.A. Goodman, Clay Miner. 13 (1978) 351. [7] M.I. Seullane, J. Magn. Resort. 47 (1982) 383. [ 8 ] J.J. Fripiat, A. Jelli, G. Poncelet and J. Andre, J. Phys. Chem. 69 (1965) 2185. [9] R.C.T. Slade, J. Barker, P.P. Hirst, T.IC Halstead and P.L Reid, Solid State Ionics 24 (1987) 289.