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Intercalation of lithium in a protonic solid electrolyte
Materials'Scienceand Engineering, BI8 (1993) 72-77
72
Intercalation of lithium in a protonic solid electrolyte G. Herren PR1NSO CON1CET (Fellowship of the Bunge & Born Foundation), Zufriategui 4380, (1603) Villa Martelli, Buenos Aires (Argentina)
N. E. Wals6e de Reca PRINSO (Program of Research in Solid State Physics) C1TEFA-CONICET, Zufriategui 4380, (1603) Villa Martelli, Buenos Aires (Argentina) (Received January 9, 1992; in revised form September 1.1992)
Abstract HUO2PO4"4H20 (HUP) could be intercalated with lithium by direct reaction in n-butyl-lithium in hexane solution. yielding a hydrated substituted lamellar solid (HUP)~ which exhibits lower conductivity than the pure (HUP)p. Both materials were carefully characterized by X-ray diffractometry, differential scanning calorimetry, IR spectroscopy and electron scanning microscopy. Lithium concentrations in (HUP)~ were determined by atomic absorption spectroscopy. Characterization experiments revealed the swelling of (HUP), and the change in volume of the unit cell was evaluated by determining the refined parameters of (HUP)p and (HUP)~. In addition, partial dehydration and a change in colour of the original crystals were observed. Reversibility of the lithiation was proved. A possible mechanism is considered and Li-H 20 solvation is suggested.
1. Introduction The solid electrolyte HUO2PO4"4H20 (HUP) is one of the best known proton solid conductors (oi = (3-6) x 10- 3 S cm- 1) at room temperature and is employed in micro-ionic devices such as sensors, supercapacitors, electrochromic displays, batteries, etc.
[1-3]. The H U P structure contains sheets of (UO2PO4)',', separated by a two-level layer of water molecules (Fig. 1). The oxygen atoms of water in this layer are arranged in squares and they are hydrogen bonded to uranyl slabs. The high proton conductivity is attributed to a high H + concentration, existing as H3 O+ in the interlamellar network. A Grotthus-type mechanism of conduction was proposed by Howe and Shilton [4] involving two steps: a proton jump followed by a reorientation. In the first step (intermolecular mechanism), the proton is transported by hopping to the hydrogen bond. In the second step, the subsequent reorientation of neutral molecules (H20) thus formed occupies the vacant proton positions. H3 O+ can be partially or totally substituted with a cation M + (M + -=K +, Na ÷, Li +, Ag +, NH4 + ) and the resulting compounds (MUP) also exhibit ionic conductivity [5]. Vibrational studies enabled Phan-Thi and coworkers [6-10] to conclude that HUP exhibits a 0921-5107/93/$6.00
bidimensional quasi-liquid state of protonated species in a superprotonic phase at room temperature. They also discussed the phase transitions and conductivity mechanisms of H U P and M U P through detailed vibrational studies. The aim of this work was to study lithium intercalation in H U P by direct reaction with n-butyl-lithium in inert atmosphere at room temperature. The pure material (HUP)p and the intercalated material (HUP)~ were characterized by X-ray diffractometry (XRD), IR spectroscopy, scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). Conductivity data obtained by complex impedance spectroscopy (CIS) during previous work at our laboratory are discussed. The chemical analysis of lithium in (HUP)i was performed using atomic absorption spectroscopy (AAS).
2. Experimental procedure
2.1. HUP synthesis H U P crystals exhibit a (4/mmm) tetragonal structure and belong to the spatial group P4/ncc (DS4h) with Z = 2, a = 6.995 A and c = 17.491 A [11]. HUP was obtained as lemon-yellow microcrystals by precipitation at room temperature from equimolar © 1993 - Elsevier Sequoia.All rights reserved
G. Herren, N. E. Walsdede Reca /
'°°2 P°,"/
I
(uo2.Poo-
I . - ' I
Fig. 1. Model of HUP structure. The white sheets correspond to (UO2PO4)~- slabs which are separated by water molecules arranged around a square. H,O moleculesare hydrogen bonded to the uranyl slabs. (0.5-2 M) solutions of uranyl-nitrate and phosphoric acid [11-13]. After 24 h, layered crystals with areas ranging from approximately 5 ,um2 to approximately 100 /~m2 and thickness between 0.5 and 5 /~m were obtained. Crystals were separated from the mother solution, washed several times with H3PO 4 solution, maintaining pH 2.5, and kept in a glass cell with controlled humidity [13]. Cylindrical HUP specimens 1.1 cm in diameter and 0.1 cm thick were obtained by pressing the crystals at approximately 1000 kg cm -2. Preferable orientation of crystals was observed by SEM with their c axis parallel to the pressing direction of the pellets [ 13-14]. 2.2. HUP intercalation HUP specimens were immersed in butyl-lithium solutions in hexane (0.26 M) for different times at room temperature in a dry-box under purified N 2 flow. A possible reaction of the organometallic compound with HUP could be
2C4H9Li + HUO2PO4.4H20 LiUO2PO 4"4H20 + CsHls
( 1)
Specimens were washed with hexane. The resulting lithium concentration in (HUP)i was analysed using AAS. 2.3. Materials characterization X-ray diffractometry was carried out with a Philips Electronics Inc. diffractometer, Model PW 1050/25, on (HUP)p and (HUP)i specimens employing Ka Cu radiation and a nickel filter. (HUP)i specimens were protected with a thin mylar sheet. A1203 was employed as reference material. Conductivity values a of (HUP)p and (HUP)i which are reported in this work are taken from experiments described elsewhere [13]. Conductivity measurements in both solid electrolytes employing several electrodes (including platinum sheets) were performed using com-
Intercalation of lithium
73
plex impedance spectroscopy between 265 and 330 K. A 4800 Hewlett Packard impedancemeter on line with an Apple II Plus minicomputer through an analogue to digital conversor takes the impedance Z and phase angle ¢ values for each frequency 09 transforming them to real Re (Z) and imaginary Im (Z) parts (Nyquist plot). The frequency range is 10 Hz~<~0~<200 kHz. Conditions of experiments enable the bulk nature of conduction to be established and to determine that values of a and the activation energy for conduction E are not influenced by electrode or interface effects. The effects of voids and grain boundaries must be taken into account in pressed specimens. The first contribution can be considered negligible because of the densities of (HUP)p and (HUP)i: 95% and 94% respectively. IR spectra of (HUP)p and (HUP)i specimens (crystal suspensions in Nujol and Fluorolube) were obtained with a Perkin-Elmer spectrometer, Model 399 B, in a wavenumber K range 200 cm- 1 < K < 4000 cm- 1. S E M images were obtained in the secondary electron mode with a Philips 505 microscope, employing an accelerating voltage of 20 kV. DSC experiments were performed with 910 DuPont equipment employing 0.5 g of specimen in each run and varying temperatures from 268 to 315 K [ 13].
3. Results and discussion
Properties of the (HUP)p pellets changed after intercalation. (HUP)p turned from a bright lemon yellow colour to pale yellow. (HUP)i pellets showed considerable swelling parallel to the cylinder axis. As mentioned above, pellets exhibited considerable stacking order due to pressing. Swelling may be assigned to the separation of layers parallel to the basal planes of crystals. Figures 2(a) and 2(b) are SEM images of (HUP)p and (HUP)i while Figs. 3(a) and 3(b) are SEM images of successive stages of the cleavage of a single crystal parallel to its basal plane. In previous work performed at our laboratory [13, 14], cleavage of single crystals of HUP (and of the isomorphous HUAs) was observed parallel to the basal plane (001) because of partial water loss. No broken crystals or exfoliation were observed in this case. X-ray diffractograms of (HUP)p and (HUP)i are shown in Figs. 4(a) and 4(b). In Table 1, the lattice parameter data d(A) of HUP [11] are given together with the experimental d I (A) and d 2 (A) data for (HUP)p and (HUP)i, showing the shift of peaks due to intercalation. Crystallinity of specimens was partially preserved after intercalation, as can be proved by comparison of Figs. 4(a) and 4(b); both patterns could be indexed with tetragonal symmetry. Refined parameters of the unit
74
G. Herren, N. E. Wals6e de Reca
Fig. 2. (a) SEM image of (HUP)p, the bar represents 10/~m; (b) SEM image of (HUP)~,the bar represents 10/~m.
cells of (HUP)p and (HUP)i were obtained by least square analysis [15, 16]. Lattice parameters for (HUP)p are a I = (6.96 _+0.06) A and c 1= (8.68 + 0.08) A, while those of (HUP)i are a 2 = ( 6 . 9 0 + 0 . 0 6 ) A and c, = (9.00 _+0.08) ,/k. The calculated volumes of elemental cells of (HUP)p and (HUP)i are VI = 420.8 A3 and V2 = 428.4 A 3, showing an expansion of the (HUP)~ cell of about 1.77%. This variation in volume, causing swelling of the pellets could be assigned fundamentally to a 3.55% expansion of the lattice parallel to the c axis. This was revealed by the doublet (002) peak which can be observed in the (HUP)~ diffractogram. However, the refined a 2 parameter revealed that it was only slightly affected by the intercalation process because the decrease in its value (0.92%) was within the error of parameter measurement. The marked intensity of peaks in the (HUP)p diffractogram could be due to the considerable perfection of the crystal lattice and to stacking order due to pressing. The rather sharp peaks and broadened diffraction lines at high values
/
Intercalation o f lithium
Fig. 3. (a) SEM image of the (110) plane of an HUP single crystal at the beginning of cleavage parallel to the basal plane, the bar represents 10/~m; (b) SEM image of the same specimen with a more pronounced cleavage by dehydratation, the bar represents 10 #m.
and the presence of a spectral doublet can be assigned to less perfect crystals and to partial dehydratation of (HUP)p due to its reaction with lithium. The lithium undergoes an intercalative ion-exchange reaction with H 3 0 +; AAS showed a lithium concentration of 20-24/~g ml-~ in (HUP)i even after eliminating a surface layer to ensure the presence of lithium in the bulk. This shows an Li + substitution from 1.30% to 1.56%. Nevertheless, the intercalation of the small Li + ion with an ionic radius of 0.60 A [17] cannot by itself be responsible for layer separation swelling of pellets. Conductivity data o and the activation energy for conduction E of (HUP)p [4, 5, 12, 13], of LiUP (LiUO: PO4"4H20) [5] and of (HUP)i [13] are shown in Table 2. The lower E value corresponds to phase I, above T t = 274.5 K (T t is the temperature of the displacive transformation) and the higher E data correspond to phase II below T t [13].
G. Herren, N. E. Wals6e de Reca
13.70~ /
110/,I
~.~
111211004)
~2~
I 1/ 7;22 ni 132°°
J{002)
0
[
.A ''°''
//
k, 1(11o111
/
~'k
834~ J 10021
0..
o
o (1121 3.0OA
2.9311 :
(104) o "I~9A
(200i o
30
4.15A (1121(00/,,I it o /'~I,I L,.55A / I~ i
~, ./
3i?A
25
# /1/
'\/i ,(1o) 5.<.,.,~`1°2~
15 D.
20
\
10 28
Fig. 4. X-ray diffractograms of (a) (HUP)p and (b) (HUP)i.
/
75
Intercalation of fithium
Non-Arrhenius behaviour is observed around the transition. T h e highly conducting low activation energy phase is observed for the highly disordered phase, in which the water molecules and the M ÷ cation are in quasi-liquid state, as revealed by spectroscopy, X-rays [18-20] and N M R [21]. Protons are the mobile species and they j u m p associated with H 2 0 molecules, while H 2 0 and H 3 0 + m o v e simultaneously [22]. T h e degree of o r d e r in the lattice influences the value of o as well as the characteristics of the displacive transformation. At lower temperatures (phase II) the order increases and an M+-cation j u m p associated with an H 2 0 j u m p is the m o r e p r o b a b l e mechanism [5]. In the following we shall only refer to the conduction mechanism around r o o m t e m p e r a t u r e ( 2 8 0 - 3 1 0 K). T h e ordering energy of the lattice depends on conservation of the square symmetry built up by the water molecules arranged in the H U P crystal (Fig. 1). O n c e an H 2 0 molecule is substituted either by a cation smaller than H 2 0 (Na ÷, Ag +) or by a cation larger than H 2 0 (NH4 ÷ ), the symmetry is reduced and lattice disorder
TABLE 1. Lattice parameters of pure and intercalated HUP JCPDS 29-670
(HUP)p (exp)
hkl
d (]k )
I/I(,
d,
002
8.75
100
102
5.43
110
4.91
70 60
112 004 104 200 202 122 212
4.32
70
3.68 3.50 3.25
100 80 90
2.93
60
(,/it)
(HUP)i (exp) Ill o
d 2 (]k )
1/11,
8.76
S
9.01 8.34
S S
5.44 4.92 4.59 4.35 4.28 3.70 3.50 3.24
M
5.44
W
M M S S S M M
4.92 4.55 4.15 4.13 3.79 3.47 3.22
VW M M M M M W
3.08 2.94
W W
3.08 2.93
W W
S strong, M medium, W weak, VW very weak.
TABLE 2. Conductivity o data, activation energies of displacive transformation II ~ I and transformation temperature of pure and intercalated HUP, results from other research (with references) and from this work are given Material (pellet)
o (S cm- 1)
E (eV)
Temperature (K)
(HUP)p
4 x 10 -3 (1-5) × 10 -3"
0.32 0.35 _+0.03 0.65 _+0.05 0.30_+0.03 0.68 _+0.06 0.30 _+0.02 0.30 0.60 + 0.05
> 275 < 270 > 275 < 268 > 312 310 < 305
(1.4-5) x 10 -3" (HUP)i
(2.4-3.0) x 10 -6 4 x 10 -6 --
aDepending on sample orientation.
Reference 4-12 5 13 5 5 13
76
G. Herren, N. E. WalsOe de Reca
appears. In the case of substitution with a cation of similar size (K + ), the order is preserved. Li +, because of its very small ionic size, may be located at the centre of the square of water distribution at different z values [22], also preserving the square symmetry. The Li + concentration measured by AAS (1.30%-1.56% of Li + substitution) seems to corroborate this assumption. Comparing o and E values for (HUP)p and (HUP)i in Table 2, it is seen that similar E values correspond to conductivities differing by three orders of magnitude. These differences may be assigned to the pre-exponential factor in the Arrhenius expression for variation of conductivity with absolute temperature T: o = o 0 exp( - E / k T ) where k is the Boltzmann constant. Several parameters affecting the pre-exponential factor change if a proton of (HUP)p exchanges with Li+: the jump distance, the correlation factor and the configurational entropy. Notwithstanding, the nearly equal values of E above T~ for (HUP)p and (HUP)i, shows that the same conduction mechanism operates in both materials. Differences in conductivity data can show that only a fraction of the total number of protons that is presumably available effectively participates in the conduction process, this number being quite different in (HUP)p and (HUP)i. Similar mechanisms of conduction and cation exchange were observed in fl-aluminas and glasses [23]. In all cases, the magnitude of E is consistent with a Grotthus conduction mechanism [24] in which the proton moves in a two-step process: (1) translation from an oxonium ion to a water molecule by hopping, and (2) subsequent reorientation (rotation) of H , O molecules thus formed, in order to take the next proton. Since step (2) is the slower step of the mechanism, it should be rate determining, and E corresponds to the energy necessary for hydrogen-bond breaking involved in this step. Upon intercalation of 1.30%-1.56% of Li +, nearly all the centres of the square configuration of H20 molecules would be occupied by the cation, thus making the rotation of H~O molecules more difficult.
/
Intercalation of fithium
However, the activation energy remains low, indicating that the same process is still sufficient to describe the conduction mechanism. Assuming that the mobile species in (HUP)i is Li(H20) +, comparison of its ionic radius with that of H 3 0 + shows that the former could favour migration. With regard to the proportion of ions available for conduction, this is determined by the stoichiometry and will be mainly dependent (as in fl/fl"aluminas) on vacant interstitial sites. When they are occupied with Li + (in a significant proportion), the lattice symmetry is modified (even though the Li + ion does not strengthen the lattice because of its small ionic radius), thus making cation mobility more difficult. The interaction between occupied sites (due primarily to the Coulomb potential) may also result in some degree of ordering among the sites, causing an increase in ordering and poorer conductivity. In the case of LiUP [5-25] it has been proved that the degree of order decreases because of Li + -H20 solvation. The IR spectra (Fig. 5) of both (HUP)p (solid line) and (HUP)i (dotted line) at room temperature showed no significant differences between the initial specimen and the intercalated specimen. Retention of the lamellar structure was confirmed by the sharp phosphate stretching band at 1000 c m - ' [26]. Weak dehydration was found by DSC experiments when comparing the results of both specimens. Although there was evidence of hydration of (HUP)p and (HUP)i from the band at 3250 cm ' when the (HUP)i specimens were exposed to a humid atmosphere for a few minutes, DSC and IR experiments proved that they were rehydrated. No additional bands characteristic of carbon-carbon bonds due to n-butyllithium or to CsH,8 (according to reaction (1)) could be detected, probably because of low concentrations. As has already been discussed, the degree of order thus depends not only on the size of the exchange cation but also on its polarization, on the crystalline fields due to the (UO2PO4)',',- slabs, on the hydrogen bond strength and on the probability of undergoing an Li+-H2 O solvation process [25]. IR peaks correspondr
,'f'",
,."" "",
80
60
40
20
4O0O
30CO
Fig. 5. IR spectra of (HUP)e (
20OO
) and (HUP)i (---).
1600
1200
800
K (c rrr' I
400
G. Herren, N. E. Walsde de Reca
ing to L i O H and L i ( O H ) H 2 0 were found at 3678 and 3574 cm -1 respectively [27]. Deformation of H U P lattice by intercalation does not seem to be related to lithium exchange with protons. It can be better assigned to a lithium solvation process.
/
Intercalation of lithium
77
C I T E F A for use of the IR equipment, and to Ldo. J. Casanova (CITEFA) for his discussion of refining of parameters. We thank C O N I C E T for the Grant Ner. 3-913 5/86 given to one of us (N.E.W.R.).
References 4. Conclusions Pure (HUP)p was synthesized and carefully characterized before intercalation by direct reaction with nbutyl-lithium in inert atmosphere and at room temperature. Both (HUP)p and intercalated material (HUP)i were characterized by SEM, X-ray diffraction, IR spectroscopy, and DSC. T h e lithium concentration was determined by A A S ( 2 0 - 2 4 big m l - 1). After intercalation, the (HUP)~ pellets showed a change in colour, swelling and cleavage parallel to the basal planes of crystals, as revealed by SEM. X-ray diffractometry revealed that crystallinity was partially preserved after lithiation. Refined parameters of (HUP)p and (HUP)i elemental tetragonal unit cells showed a volume expansion of the (HUP)~ cell of 1.77% which can be assigned to an expansion of the lattice parallel to the c axis after intercalation. T h e decrease in a2 of the (HUP)~ cell was within the error of determination. Wide peaks for (HUP)~ indicated partial water loss, as was also proved by DSC. Colour change and cleavage of crystals can also be related to dehydration. IR spectra of (HUP)p and (HUP)i showed that the layered structure was preserved and that the dehydration was only partial and reversible. This was also proved using DSC after exposing (HUP)i to a humid atmosphere. No evidence of CsHls intercalation was observed. Lithium, because of its charge and its small ionic radius, can easily intercalate, causing minimal distortion of the (HUP)i lattice and preserving the lattice order. However, the subsequent L i - H 2 0 solvation process increases disorder. Differences in data for (HUP)p and (HUP)i are discussed according to parameters affecting the conduction mechanism when (HUP)p is intercalated with Li +, in comparison with a similar mechanism for f l / f l " - a l u m i n a s and glasses.
Acknowledgments We are deeply indebted to Dr. R. Brec of the Laboratoire de Chimie des Solides, IPCM, Universit6 de Nantes, France, for his valuable discussion of the manuscript, to Dra. P. Perazzo and to Dr. D. Batistoni ( C N E A ) for their assistance in X-ray diffractometry and in AAS, to the Analytical Chemistry Lab. of
1 P. Childs, A. Howe and M. Shilton, J. Power Sources, 3 (1978)105. 2 A. T. Howe, S. H. Sheffield, P. E. Childs and M. G. Shilton, Thin Solid Films, 6-7(1980) 365. 3 Ph. Colomban and G. Velasco, Recherche, 148(1983) 1292. 4 A.T. Howe and M. Shilton, J. Solid State Chem., 28 (1979) 345. 5 M. Phan-Thi and Ph. Colomban, Solid State lonics, 15 (1985) 113. 6 M. Phan-Thi, G. Velasco, Ph. Colomban and A. Novak, Solid State lonics, 9-10 ( 1985) 1055. 7 R. Mercier, M. Phan-Thi and Ph. Colomban, Solid State lonics, 15(1985) 113. 8 M. Phan-Thi, A. Novak and Ph. Colomban, in J. B. Goodenough, J. Jensen and A. Potier (eds.), Solid State Protonic Conductors, Vol. Ili, Odensee University Press, Odensee, 1984, p. 143. 9 M. Phan-Thi and Ph. Colomban, J. Less-Common Met., 108 (1985) 189. 10 M. Phan-Thi, Ph. Colomban and A. Novak, J. Phys. Chem. Solids, 46(5)(1985)565. 11 F. Weigel and G. Hoffmann, J. Less-Common Met., 44 (1976) 99. JCPDS, Crystallography Table 29-670, JCPDS, Swarthmore, PA, 1979. 12 M. G. Shilton and A. T. Howe, Mater. Res. Bull., 12 (1977) 701. 13 M. A. Pokropek, Study of HUO2"POz'4HzO (HUP) properties and its application in a hydrogen sensor, Physics Licenciature Thesis, Faculty of Exact and Natural Sciences, University of Buenos Aires, Buenos Aires, 1988. 14 N. E. Wals6e de Reca and J. I. Franco, Crystallogr. Rev., 2 (5) (1992) 239-328. 15 Ch. Barret, Structure of Metals, McGraw Hill, New York, 1952, 147. 16 W. Parrish and A. J. C. Wilson, Precision measurement of lattice parameters of polycrystalline specimens. In International Tables for X-ray Crystallography, Kynoch Press, Birmingham, 1967, p. 287. 17 A. West, Solid State Chemistry and its Applications, Wiley, New York, 1984, p. 270. 18 M. Ross and H. T. Evans, Am. MineraL, 49(1964) 1578. 19 J. Beintema, Rec. Tray. Chim. Pays Bas., 57(1983) 155. 20 B. Morosin, Acta Crystallogr. B, 34(1978) 32-37. 21 P. E. Childs, T. K. Halstead, A. T. Howe and M. G. Shilton, Mater. Res. BulL, 13 (1978) 609. 22 K. Metcalfe and T. K. Halstead, SolidState lonics, 26 (1988) 209. 23 A.S. Nowick and W. K. Lee, in A. L. Laskar and S. Chandra (eds.), Superionic Solids and Solid Electrolytes. Academic Press, New York, 1989, p. 381. 24 Y. Tsai, S. Smoot, D. H. Whitmore, J. Tarczon and W. Halperin, Solid State Ionic's, 9-10 (1983) 1033. 25 W.L. Hase and Da Fei Feing, J. Chem. Phys., 75 ( 1981 ) 738. 26 M.M. Alken, R. N. Biagioni and A. B. Ellis, lnorg. Chem., 22 (1983)4128. 27 R.M. Hexter, J. Chem. Phys., 34 ( 1961 ) 941.
72
Intercalation of lithium in a protonic solid electrolyte G. Herren PR1NSO CON1CET (Fellowship of the Bunge & Born Foundation), Zufriategui 4380, (1603) Villa Martelli, Buenos Aires (Argentina)
N. E. Wals6e de Reca PRINSO (Program of Research in Solid State Physics) C1TEFA-CONICET, Zufriategui 4380, (1603) Villa Martelli, Buenos Aires (Argentina) (Received January 9, 1992; in revised form September 1.1992)
Abstract HUO2PO4"4H20 (HUP) could be intercalated with lithium by direct reaction in n-butyl-lithium in hexane solution. yielding a hydrated substituted lamellar solid (HUP)~ which exhibits lower conductivity than the pure (HUP)p. Both materials were carefully characterized by X-ray diffractometry, differential scanning calorimetry, IR spectroscopy and electron scanning microscopy. Lithium concentrations in (HUP)~ were determined by atomic absorption spectroscopy. Characterization experiments revealed the swelling of (HUP), and the change in volume of the unit cell was evaluated by determining the refined parameters of (HUP)p and (HUP)~. In addition, partial dehydration and a change in colour of the original crystals were observed. Reversibility of the lithiation was proved. A possible mechanism is considered and Li-H 20 solvation is suggested.
1. Introduction The solid electrolyte HUO2PO4"4H20 (HUP) is one of the best known proton solid conductors (oi = (3-6) x 10- 3 S cm- 1) at room temperature and is employed in micro-ionic devices such as sensors, supercapacitors, electrochromic displays, batteries, etc.
[1-3]. The H U P structure contains sheets of (UO2PO4)',', separated by a two-level layer of water molecules (Fig. 1). The oxygen atoms of water in this layer are arranged in squares and they are hydrogen bonded to uranyl slabs. The high proton conductivity is attributed to a high H + concentration, existing as H3 O+ in the interlamellar network. A Grotthus-type mechanism of conduction was proposed by Howe and Shilton [4] involving two steps: a proton jump followed by a reorientation. In the first step (intermolecular mechanism), the proton is transported by hopping to the hydrogen bond. In the second step, the subsequent reorientation of neutral molecules (H20) thus formed occupies the vacant proton positions. H3 O+ can be partially or totally substituted with a cation M + (M + -=K +, Na ÷, Li +, Ag +, NH4 + ) and the resulting compounds (MUP) also exhibit ionic conductivity [5]. Vibrational studies enabled Phan-Thi and coworkers [6-10] to conclude that HUP exhibits a 0921-5107/93/$6.00
bidimensional quasi-liquid state of protonated species in a superprotonic phase at room temperature. They also discussed the phase transitions and conductivity mechanisms of H U P and M U P through detailed vibrational studies. The aim of this work was to study lithium intercalation in H U P by direct reaction with n-butyl-lithium in inert atmosphere at room temperature. The pure material (HUP)p and the intercalated material (HUP)~ were characterized by X-ray diffractometry (XRD), IR spectroscopy, scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). Conductivity data obtained by complex impedance spectroscopy (CIS) during previous work at our laboratory are discussed. The chemical analysis of lithium in (HUP)i was performed using atomic absorption spectroscopy (AAS).
2. Experimental procedure
2.1. HUP synthesis H U P crystals exhibit a (4/mmm) tetragonal structure and belong to the spatial group P4/ncc (DS4h) with Z = 2, a = 6.995 A and c = 17.491 A [11]. HUP was obtained as lemon-yellow microcrystals by precipitation at room temperature from equimolar © 1993 - Elsevier Sequoia.All rights reserved
G. Herren, N. E. Walsdede Reca /
'°°2 P°,"/
I
(uo2.Poo-
I . - ' I
Fig. 1. Model of HUP structure. The white sheets correspond to (UO2PO4)~- slabs which are separated by water molecules arranged around a square. H,O moleculesare hydrogen bonded to the uranyl slabs. (0.5-2 M) solutions of uranyl-nitrate and phosphoric acid [11-13]. After 24 h, layered crystals with areas ranging from approximately 5 ,um2 to approximately 100 /~m2 and thickness between 0.5 and 5 /~m were obtained. Crystals were separated from the mother solution, washed several times with H3PO 4 solution, maintaining pH 2.5, and kept in a glass cell with controlled humidity [13]. Cylindrical HUP specimens 1.1 cm in diameter and 0.1 cm thick were obtained by pressing the crystals at approximately 1000 kg cm -2. Preferable orientation of crystals was observed by SEM with their c axis parallel to the pressing direction of the pellets [ 13-14]. 2.2. HUP intercalation HUP specimens were immersed in butyl-lithium solutions in hexane (0.26 M) for different times at room temperature in a dry-box under purified N 2 flow. A possible reaction of the organometallic compound with HUP could be
2C4H9Li + HUO2PO4.4H20 LiUO2PO 4"4H20 + CsHls
( 1)
Specimens were washed with hexane. The resulting lithium concentration in (HUP)i was analysed using AAS. 2.3. Materials characterization X-ray diffractometry was carried out with a Philips Electronics Inc. diffractometer, Model PW 1050/25, on (HUP)p and (HUP)i specimens employing Ka Cu radiation and a nickel filter. (HUP)i specimens were protected with a thin mylar sheet. A1203 was employed as reference material. Conductivity values a of (HUP)p and (HUP)i which are reported in this work are taken from experiments described elsewhere [13]. Conductivity measurements in both solid electrolytes employing several electrodes (including platinum sheets) were performed using com-
Intercalation of lithium
73
plex impedance spectroscopy between 265 and 330 K. A 4800 Hewlett Packard impedancemeter on line with an Apple II Plus minicomputer through an analogue to digital conversor takes the impedance Z and phase angle ¢ values for each frequency 09 transforming them to real Re (Z) and imaginary Im (Z) parts (Nyquist plot). The frequency range is 10 Hz~<~0~<200 kHz. Conditions of experiments enable the bulk nature of conduction to be established and to determine that values of a and the activation energy for conduction E are not influenced by electrode or interface effects. The effects of voids and grain boundaries must be taken into account in pressed specimens. The first contribution can be considered negligible because of the densities of (HUP)p and (HUP)i: 95% and 94% respectively. IR spectra of (HUP)p and (HUP)i specimens (crystal suspensions in Nujol and Fluorolube) were obtained with a Perkin-Elmer spectrometer, Model 399 B, in a wavenumber K range 200 cm- 1 < K < 4000 cm- 1. S E M images were obtained in the secondary electron mode with a Philips 505 microscope, employing an accelerating voltage of 20 kV. DSC experiments were performed with 910 DuPont equipment employing 0.5 g of specimen in each run and varying temperatures from 268 to 315 K [ 13].
3. Results and discussion
Properties of the (HUP)p pellets changed after intercalation. (HUP)p turned from a bright lemon yellow colour to pale yellow. (HUP)i pellets showed considerable swelling parallel to the cylinder axis. As mentioned above, pellets exhibited considerable stacking order due to pressing. Swelling may be assigned to the separation of layers parallel to the basal planes of crystals. Figures 2(a) and 2(b) are SEM images of (HUP)p and (HUP)i while Figs. 3(a) and 3(b) are SEM images of successive stages of the cleavage of a single crystal parallel to its basal plane. In previous work performed at our laboratory [13, 14], cleavage of single crystals of HUP (and of the isomorphous HUAs) was observed parallel to the basal plane (001) because of partial water loss. No broken crystals or exfoliation were observed in this case. X-ray diffractograms of (HUP)p and (HUP)i are shown in Figs. 4(a) and 4(b). In Table 1, the lattice parameter data d(A) of HUP [11] are given together with the experimental d I (A) and d 2 (A) data for (HUP)p and (HUP)i, showing the shift of peaks due to intercalation. Crystallinity of specimens was partially preserved after intercalation, as can be proved by comparison of Figs. 4(a) and 4(b); both patterns could be indexed with tetragonal symmetry. Refined parameters of the unit
74
G. Herren, N. E. Wals6e de Reca
Fig. 2. (a) SEM image of (HUP)p, the bar represents 10/~m; (b) SEM image of (HUP)~,the bar represents 10/~m.
cells of (HUP)p and (HUP)i were obtained by least square analysis [15, 16]. Lattice parameters for (HUP)p are a I = (6.96 _+0.06) A and c 1= (8.68 + 0.08) A, while those of (HUP)i are a 2 = ( 6 . 9 0 + 0 . 0 6 ) A and c, = (9.00 _+0.08) ,/k. The calculated volumes of elemental cells of (HUP)p and (HUP)i are VI = 420.8 A3 and V2 = 428.4 A 3, showing an expansion of the (HUP)~ cell of about 1.77%. This variation in volume, causing swelling of the pellets could be assigned fundamentally to a 3.55% expansion of the lattice parallel to the c axis. This was revealed by the doublet (002) peak which can be observed in the (HUP)~ diffractogram. However, the refined a 2 parameter revealed that it was only slightly affected by the intercalation process because the decrease in its value (0.92%) was within the error of parameter measurement. The marked intensity of peaks in the (HUP)p diffractogram could be due to the considerable perfection of the crystal lattice and to stacking order due to pressing. The rather sharp peaks and broadened diffraction lines at high values
/
Intercalation o f lithium
Fig. 3. (a) SEM image of the (110) plane of an HUP single crystal at the beginning of cleavage parallel to the basal plane, the bar represents 10/~m; (b) SEM image of the same specimen with a more pronounced cleavage by dehydratation, the bar represents 10 #m.
and the presence of a spectral doublet can be assigned to less perfect crystals and to partial dehydratation of (HUP)p due to its reaction with lithium. The lithium undergoes an intercalative ion-exchange reaction with H 3 0 +; AAS showed a lithium concentration of 20-24/~g ml-~ in (HUP)i even after eliminating a surface layer to ensure the presence of lithium in the bulk. This shows an Li + substitution from 1.30% to 1.56%. Nevertheless, the intercalation of the small Li + ion with an ionic radius of 0.60 A [17] cannot by itself be responsible for layer separation swelling of pellets. Conductivity data o and the activation energy for conduction E of (HUP)p [4, 5, 12, 13], of LiUP (LiUO: PO4"4H20) [5] and of (HUP)i [13] are shown in Table 2. The lower E value corresponds to phase I, above T t = 274.5 K (T t is the temperature of the displacive transformation) and the higher E data correspond to phase II below T t [13].
G. Herren, N. E. Wals6e de Reca
13.70~ /
110/,I
~.~
111211004)
~2~
I 1/ 7;22 ni 132°°
J{002)
0
[
.A ''°''
//
k, 1(11o111
/
~'k
834~ J 10021
0..
o
o (1121 3.0OA
2.9311 :
(104) o "I~9A
(200i o
30
4.15A (1121(00/,,I it o /'~I,I L,.55A / I~ i
~, ./
3i?A
25
# /1/
'\/i ,(1o) 5.<.,.,~`1°2~
15 D.
20
\
10 28
Fig. 4. X-ray diffractograms of (a) (HUP)p and (b) (HUP)i.
/
75
Intercalation of fithium
Non-Arrhenius behaviour is observed around the transition. T h e highly conducting low activation energy phase is observed for the highly disordered phase, in which the water molecules and the M ÷ cation are in quasi-liquid state, as revealed by spectroscopy, X-rays [18-20] and N M R [21]. Protons are the mobile species and they j u m p associated with H 2 0 molecules, while H 2 0 and H 3 0 + m o v e simultaneously [22]. T h e degree of o r d e r in the lattice influences the value of o as well as the characteristics of the displacive transformation. At lower temperatures (phase II) the order increases and an M+-cation j u m p associated with an H 2 0 j u m p is the m o r e p r o b a b l e mechanism [5]. In the following we shall only refer to the conduction mechanism around r o o m t e m p e r a t u r e ( 2 8 0 - 3 1 0 K). T h e ordering energy of the lattice depends on conservation of the square symmetry built up by the water molecules arranged in the H U P crystal (Fig. 1). O n c e an H 2 0 molecule is substituted either by a cation smaller than H 2 0 (Na ÷, Ag +) or by a cation larger than H 2 0 (NH4 ÷ ), the symmetry is reduced and lattice disorder
TABLE 1. Lattice parameters of pure and intercalated HUP JCPDS 29-670
(HUP)p (exp)
hkl
d (]k )
I/I(,
d,
002
8.75
100
102
5.43
110
4.91
70 60
112 004 104 200 202 122 212
4.32
70
3.68 3.50 3.25
100 80 90
2.93
60
(,/it)
(HUP)i (exp) Ill o
d 2 (]k )
1/11,
8.76
S
9.01 8.34
S S
5.44 4.92 4.59 4.35 4.28 3.70 3.50 3.24
M
5.44
W
M M S S S M M
4.92 4.55 4.15 4.13 3.79 3.47 3.22
VW M M M M M W
3.08 2.94
W W
3.08 2.93
W W
S strong, M medium, W weak, VW very weak.
TABLE 2. Conductivity o data, activation energies of displacive transformation II ~ I and transformation temperature of pure and intercalated HUP, results from other research (with references) and from this work are given Material (pellet)
o (S cm- 1)
E (eV)
Temperature (K)
(HUP)p
4 x 10 -3 (1-5) × 10 -3"
0.32 0.35 _+0.03 0.65 _+0.05 0.30_+0.03 0.68 _+0.06 0.30 _+0.02 0.30 0.60 + 0.05
> 275 < 270 > 275 < 268 > 312 310 < 305
(1.4-5) x 10 -3" (HUP)i
(2.4-3.0) x 10 -6 4 x 10 -6 --
aDepending on sample orientation.
Reference 4-12 5 13 5 5 13
76
G. Herren, N. E. WalsOe de Reca
appears. In the case of substitution with a cation of similar size (K + ), the order is preserved. Li +, because of its very small ionic size, may be located at the centre of the square of water distribution at different z values [22], also preserving the square symmetry. The Li + concentration measured by AAS (1.30%-1.56% of Li + substitution) seems to corroborate this assumption. Comparing o and E values for (HUP)p and (HUP)i in Table 2, it is seen that similar E values correspond to conductivities differing by three orders of magnitude. These differences may be assigned to the pre-exponential factor in the Arrhenius expression for variation of conductivity with absolute temperature T: o = o 0 exp( - E / k T ) where k is the Boltzmann constant. Several parameters affecting the pre-exponential factor change if a proton of (HUP)p exchanges with Li+: the jump distance, the correlation factor and the configurational entropy. Notwithstanding, the nearly equal values of E above T~ for (HUP)p and (HUP)i, shows that the same conduction mechanism operates in both materials. Differences in conductivity data can show that only a fraction of the total number of protons that is presumably available effectively participates in the conduction process, this number being quite different in (HUP)p and (HUP)i. Similar mechanisms of conduction and cation exchange were observed in fl-aluminas and glasses [23]. In all cases, the magnitude of E is consistent with a Grotthus conduction mechanism [24] in which the proton moves in a two-step process: (1) translation from an oxonium ion to a water molecule by hopping, and (2) subsequent reorientation (rotation) of H , O molecules thus formed, in order to take the next proton. Since step (2) is the slower step of the mechanism, it should be rate determining, and E corresponds to the energy necessary for hydrogen-bond breaking involved in this step. Upon intercalation of 1.30%-1.56% of Li +, nearly all the centres of the square configuration of H20 molecules would be occupied by the cation, thus making the rotation of H~O molecules more difficult.
/
Intercalation of fithium
However, the activation energy remains low, indicating that the same process is still sufficient to describe the conduction mechanism. Assuming that the mobile species in (HUP)i is Li(H20) +, comparison of its ionic radius with that of H 3 0 + shows that the former could favour migration. With regard to the proportion of ions available for conduction, this is determined by the stoichiometry and will be mainly dependent (as in fl/fl"aluminas) on vacant interstitial sites. When they are occupied with Li + (in a significant proportion), the lattice symmetry is modified (even though the Li + ion does not strengthen the lattice because of its small ionic radius), thus making cation mobility more difficult. The interaction between occupied sites (due primarily to the Coulomb potential) may also result in some degree of ordering among the sites, causing an increase in ordering and poorer conductivity. In the case of LiUP [5-25] it has been proved that the degree of order decreases because of Li + -H20 solvation. The IR spectra (Fig. 5) of both (HUP)p (solid line) and (HUP)i (dotted line) at room temperature showed no significant differences between the initial specimen and the intercalated specimen. Retention of the lamellar structure was confirmed by the sharp phosphate stretching band at 1000 c m - ' [26]. Weak dehydration was found by DSC experiments when comparing the results of both specimens. Although there was evidence of hydration of (HUP)p and (HUP)i from the band at 3250 cm ' when the (HUP)i specimens were exposed to a humid atmosphere for a few minutes, DSC and IR experiments proved that they were rehydrated. No additional bands characteristic of carbon-carbon bonds due to n-butyllithium or to CsH,8 (according to reaction (1)) could be detected, probably because of low concentrations. As has already been discussed, the degree of order thus depends not only on the size of the exchange cation but also on its polarization, on the crystalline fields due to the (UO2PO4)',',- slabs, on the hydrogen bond strength and on the probability of undergoing an Li+-H2 O solvation process [25]. IR peaks correspondr
,'f'",
,."" "",
80
60
40
20
4O0O
30CO
Fig. 5. IR spectra of (HUP)e (
20OO
) and (HUP)i (---).
1600
1200
800
K (c rrr' I
400
G. Herren, N. E. Walsde de Reca
ing to L i O H and L i ( O H ) H 2 0 were found at 3678 and 3574 cm -1 respectively [27]. Deformation of H U P lattice by intercalation does not seem to be related to lithium exchange with protons. It can be better assigned to a lithium solvation process.
/
Intercalation of lithium
77
C I T E F A for use of the IR equipment, and to Ldo. J. Casanova (CITEFA) for his discussion of refining of parameters. We thank C O N I C E T for the Grant Ner. 3-913 5/86 given to one of us (N.E.W.R.).
References 4. Conclusions Pure (HUP)p was synthesized and carefully characterized before intercalation by direct reaction with nbutyl-lithium in inert atmosphere and at room temperature. Both (HUP)p and intercalated material (HUP)i were characterized by SEM, X-ray diffraction, IR spectroscopy, and DSC. T h e lithium concentration was determined by A A S ( 2 0 - 2 4 big m l - 1). After intercalation, the (HUP)~ pellets showed a change in colour, swelling and cleavage parallel to the basal planes of crystals, as revealed by SEM. X-ray diffractometry revealed that crystallinity was partially preserved after lithiation. Refined parameters of (HUP)p and (HUP)i elemental tetragonal unit cells showed a volume expansion of the (HUP)~ cell of 1.77% which can be assigned to an expansion of the lattice parallel to the c axis after intercalation. T h e decrease in a2 of the (HUP)~ cell was within the error of determination. Wide peaks for (HUP)~ indicated partial water loss, as was also proved by DSC. Colour change and cleavage of crystals can also be related to dehydration. IR spectra of (HUP)p and (HUP)i showed that the layered structure was preserved and that the dehydration was only partial and reversible. This was also proved using DSC after exposing (HUP)i to a humid atmosphere. No evidence of CsHls intercalation was observed. Lithium, because of its charge and its small ionic radius, can easily intercalate, causing minimal distortion of the (HUP)i lattice and preserving the lattice order. However, the subsequent L i - H 2 0 solvation process increases disorder. Differences in data for (HUP)p and (HUP)i are discussed according to parameters affecting the conduction mechanism when (HUP)p is intercalated with Li +, in comparison with a similar mechanism for f l / f l " - a l u m i n a s and glasses.
Acknowledgments We are deeply indebted to Dr. R. Brec of the Laboratoire de Chimie des Solides, IPCM, Universit6 de Nantes, France, for his valuable discussion of the manuscript, to Dra. P. Perazzo and to Dr. D. Batistoni ( C N E A ) for their assistance in X-ray diffractometry and in AAS, to the Analytical Chemistry Lab. of
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