High ionic conductivity of hydrated Li0.5FeOCl

Solid State Ionics 177 (2006) 1099 – 1104 www.elsevier.com/locate/ssi

High ionic conductivity of hydrated Li0.5FeOCl Aurora Sagua a,d , Alberto Rivera b,c , Carlos León b , Jacobo Santamaría b , Jesús Sanz c , Emilio Morán a,⁎ a

Departamento de Química Inorgánica, Facultad de Ciencias Químicas, Universidad Complutense, 28040 Madrid, Spain (EU) b Departamento de Física Aplicada, Facultad de Ciencias Físicas, Universidad Complutense, 28040 Madrid, Spain (EU) c Instituto de Ciencia de los Materiales de Madrid, C.S.I.C. 28049 Cantoblanco, Madrid, Spain (EU) d Departamento de Química e Ingeniería Química, Universidad Nacional del Sur, 8000 Bahía Blanca, Argentina Received 28 July 2005; received in revised form 13 February 2006; accepted 9 March 2006

Abstract Iron oxychloride has been lithiated by the reaction with n-butyllithium and thereafter exposed to air. Lithium intercalation increases several orders of magnitude of the electrical conductivity of the pristine material although the intercalate remains a semiconductor. This phase, after being exposed to atmospheric humidity becomes an ionic conductor, with a conductivity comparable to that of some molten salts, and does not show electronic conduction in the whole range of temperatures of measurement (150–300 K), a strong non-Arrhenius behaviour being observed. Impedance spectroscopy and NMR techniques, among others, have been used to follow this behaviour. © 2006 Elsevier B.V. All rights reserved. Keywords: Solid electrolytes; Iron oxychloride; Water intercalation; Ionic conductivity; Lithium intercalation; Layered compounds; Nuclear magnetic resonance

1. Introduction Among layered compounds containing transition metals and, therefore suitable for intercalation chemistry, iron oxychloride has been widely used as a host and can be considered as a classic material for that purpose. Actually, a great variety of species, redox active or not, inorganic or organic, simple ions or quite complex molecules have been intercalated in the interlayer, van der Waals space, of this solid matrix: from the very simple alkaline ions to quite complex organosulfur compounds such as TTF [1–7]. The redox intercalation chemistry of FeOCl is well documented and quite similar to that of the chalcogenides; however some aspects are different and some still remains unclear. The intercalation of lithium in this material was first reported by Palvadeau et al. [2], who performed it by chemical and electrochemical means but there is some controversy in the literature about the total amount of lithium which can be introduced: 0.5 according to Palvadeau et al. [2] and up to 2 after

⁎ Corresponding author. E-mail addresses: [email protected] (A. Sagua), [email protected] (E. Morán). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.03.031

Kanamura et al. [8]. In a previous paper we have reexamined the reaction of iron oxychloride with n-butyllithium, important changes being found both in the structure and in the transport properties of lithiated compounds as compared to the pristine material. From the structural and microstructural points of view lithiation is not at all a simple process: complex changes, including inter alia staging, superstructures, etc., seem to take place [9]. On the other hand it is worth to point that the lithiated materials are not very crystalline and easily degrade, something which obviously makes their study difficult. In spite of these materials being so sensitive towards hydrolysis, some hydrated alkali intercalation compounds have been reported [10]; they can be formulated as M0.14(H2O)yFeOCl, with M(Li,Na,K,Cs) and they show large interlayer spacings which are dependent on the conditions of preparation; this behaviour is in marked contrast with that shown by chalcogenides. Thus, it seems that, under particular conditions of synthesis, water can be incorporated in this structure without destroying it, as it happens for instance in clays such as montmorillonites where phenomena implying cationic mobility such as ionic exchange are wellknown [11]. Finally it has to be recalled that, recently, superconductivity has been found at 5 K upon sodium extraction and water

A. Sagua et al. / Solid State Ionics 177 (2006) 1099–1104

intercalation in a layered sodium cobalt oxide, originally semiconducting [12], something which illustrates how dramatic changes in transport properties can be induced by “soft chemistry” routes in layered materials. Thus, the aim of this work has been to study, by combining different techniques, namely NMR and impedance spectroscopy, the lithium ionic mobility in lithiated, hydrated or not, iron oxychloride materials. In this paper we report the important changes in the conductivity that take place in FeOCl upon lithium intercalation and, especially, after the ulterior water adsorption. Worth to be noted, the changes seem to be, up to a certain degree reversible. 1.1. Experimental section 1.1.1. Synthesis FeOCl was prepared by heating Fe2O3 with FeCl3 in a sealed Pyrex ampoule following the procedure given in Ref. [2]. Lithium intercalation has been performed by chemical reaction with 0.4 M n-butyllithium under a dry N2 atmosphere, at room temperature, stirring for 4 days. The resulting black microcrystalline solid was washed with n-hexane and dried at room temperature for three hours under vacuum, the sample being stored in a glove box under argon atmosphere. Hydration took place after the sample was exposed to room conditions (30% relative humidity in air) for 12 h. 1.1.2. Characterization Chemical analysis was performed by means of Inductive coupled plasma (ICP) in a Jovin Yvon instrument (JY-70 plus) to determine the Fe and Li composition. Light elements analysis (C, H) was performed in order to look for possible remaining organic matter and energy dispersive spectroscopy (EDS), within the electron microscope, was also done to check the overall atomic composition. The stability of the samples between room temperature and 600 °C was followed by means of TGA and DTA experiments performed in a Stanton instrument working at 10 °C/min under flowing nitrogen. X-ray powder diffraction (XRD) was performed in a Siemens D500 apparatus with a Ni filter and Cu Kα radiation. The lithiated sample required a long time of X-ray exposition and had to be covered with a Berylium foil, to ensure its protection against oxidation and hydrolysis. The XRD pattern of the hydrated product was performed in conventional room conditions. Impedance spectroscopy measurements were conducted using an automatically controlled HP4284A and HP4285 precision LCR meter in the frequency range from 20 Hz to 30 MHz and temperatures ranging from 150 to 400 K. Samples were cold pressed into cylindrical pellets of 8 mm diameter and 0.8 mm thickness, the electrodes being defined by using silver paint. Pellets were mounted in a sealed stainless steel holder through which high purity dry nitrogen was circulated to ensure an inert atmosphere and to avoid water condensation. Infrared spectra, aiming to know the nature of groups present in the hydrated oxychloride, were obtained in a Prospect-IR

(Midac Corp.) apparatus, the sample being dispersed in a dry KBr pellet. Nuclear magnetic resonance (NMR) was used to go deeper into the study of the Li mobility observed by impedance spectroscopy. 1H and 7Li MAS-NMR spectra of lithiated FeOCl samples, with variable degree of hydration, were recorded at room temperature in a MSL-400 Bruker spectrometer working at 400.13 and 155.45 MHz. Spectra were taken after irradiation of the sample with a π/2 pulse (6 and 3 μs respectively). The spinning rates used in MAS experiments were 4 or 10 kHz and the frequency filter window used 500 kHz. The number of scans was 400 and the time between accumulations was in 3s. The analysis of the spectra was carried out with the Winfit (Bruker) program, which allows the multiplication of the FID signal (free induction decay) with a trapezoidal function (noise filtering), the Fourier transformation of this signal and the subsequent absorption profile determination. 2. Results and discussion Fig. 1 shows the powder XRD patterns of the original oxychloride, that of the lithium intercalate (of approximate composition Li0.5FeOCl as determined by ICP analysis), another corresponding to the same material after being exposed to air humidity and finally, after being dried under vacuum at a moderate temperature. The measured cell parameters of the original material are a = 3.759 (1) Å; b = 7.918(1) Å and c = 3.296(1) Å, (Space Group Pmnm) in fairly good agreement with those reported in the literature (JCPDS cards n° 39–0612; 72–0619 and 73–2229). However, there are some discrepancies in intensities of (0 k 0) reflections, which show a marked preferential orientation effect along the b axis due to the layered nature of this material. It can be observed that upon lithiation the XRD patterns present some features similar to the original one, in particular the (010) reflection remains in the same position, meaning that the cell is not expanded along the b axis, as it would be expected in layered materials upon intercalation. Nevertheless some other peaks do change and, in general the

(010)

Intensity (a.u.)

1100

FeOCl

(020)(100) (040)

Li0.5FeOCl

Be

(004)

(311)

Be

Li0.5FeOCl (hydrated)

Be Li0.5FeOCl (redryed)

5

10

15

20

25 2θ

30

35

40

45

Fig. 1. Powder X-ray diffraction patterns (Cu, Kα radiation) corresponding to the starting iron oxychloride, its lithiated form, upon water intercalation and dried again.

A. Sagua et al. / Solid State Ionics 177 (2006) 1099–1104

1101

100

6 4

95

2 0

90

-2 85

DTA (μV)

TG (%)

Li0.5FeOCl

-4 100

200

300

400

500

T (°C) 2

90

0

Li0.5FeOCl•yH2O

80

-2

70

-4

60

-6 100

200

300

400

DTA (μV)

TG (%)

100

500

T (°C)

Fig. 2. Thermal analysis plots (TGA and DTA) corresponding to the hydrated-lithiated iron oxychloride.

characteristic vibrations of OH− groups around 3400 and 1600 cm− 1, but, after water adsorption, these adsorptions peaks do appear. In both cases, in the low frequency region, the vibrations associated to metal-oxygen or metal-chlorine bonds are evident (a narrow band at 475 cm− 1 in the original oxychloride broadens and gives a wide one centered at 516 cm− 1). As regarding the transport properties, in Fig. 3 the conductivities of different samples (original, lithiated, lithiated after water intercalation and fully dried) are plotted against inverse temperature. In this connection, it is known that FeOCl is a semiconductor showing a thermally activated electronic conductivity; Arrhenius type, for which an activation energy of 0.29 eV is obtained. There is no sign of blocking effects at the grain boundaries, pointing to an electrical conductivity due to electronic charge carriers. Dry lithiated samples show a thermally activated temperature dependence, with a lower 0 -1

c

-2 -3 log (σdc / (Scm-1))

crystalinity is worsened. In a previous paper [3] other authors have reported a possible b’ ∼ 3b superstructure for a lithiated material of formula Li0.18FeOCl but the figures of merit obtained with such unit cell were not satisfactory in our case, the better fittings being obtained with an 3a × 4b × 3c orthorhombic cell, 11.289(5) Å × 31.44(1)Å × 10.070(8)Å together with a change in the Space Group to Fmmm, probably due to a change in the stacking of the FeOCl layers. We already reported these results in a previous paper [9] and supported by electron microscopy observations. In particular electron diffraction patterns show 3-fold superstructures along the a⁎ or c⁎ axis in the lithiated materials and some streaking hints of a 4-fold superstructure along the b⁎ axis of the pristine material. Moreover the basal-plane layers of lithiated materials do not appear planar but corrugated, meaning that lithium induces quite important structural changes worth of a further and deeper study. In particular in situ diffraction measurements with synchrotron radiation while performing electrochemical intercalation of lithium would shed light on the structural rearrangements produced. Once water adsorption takes place, the material becomes almost amorphous although some weak peaks such as that one appearing at 2θ ∼ 10° are still noticeable, probably due to some remnant, not reacted material. Finally, the material dried for 3 h under vacuum shows some degree of recrystallization as evidenced by its XRD pattern with features similar to that of the Li0.5FeOCl. However, after several hydration/drying cycles the crystalinity of the samples decreases considerably impeding a detailed analysis of these materials. Fig. 2 shows the thermal analysis data (TG and DTA) of the lithium intercalated sample after 12 h of exposition to air. Two endothermic peaks appear near 100 °C corresponding to the loss of water upon heating, which can be as high as 35% for the material heated up to 400 °C. This weight loss corresponds to 3.3 moles of water per formula, which approximately fits to 6 water molecules per lithium ion, pointing to an octahedral coordination of the intercalated cations. To corroborate the hydration process, conventional IR experiments were carried out: the spectrum of the lithium intercalate does not show the

-4 d -5 b -6 -7 -8

a

-9 2

3

4

5

6

7

8

1000 / T (K-1)

Fig. 3. Conductivity plots as a function of the inverse temperature for a) the starting iron oxychloride; b) the lithiated form; c) the hydrated and d) the dehydrated samples.

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activation energy of 0.17 eV, indicating that lithiation of FeOCl improves by several orders of magnitude of the electronic conductivity. Contribution of lithium ions to dc conductivity must be negligible since no blocking effects are detected. The increase in electronic conductivity has to be assigned to the Fe3+ to Fe2+ reduction process, pointing to an electron hopping mechanism. The reduction of iron upon lithium intercalation has been corroborated by Mössbauer spectroscopy by Slade et al. [13]. Spectral parameters reported in the literature [3], deduced from Mössbauer spectra, are intermediate between those of Fe2+ and Fe3+ pointing to a fast electron hoping mechanism. After hydration of Li0.5FeOCl, a totally different scenario is observed: the transport dynamics changes, there is a total inhibition of electronic conductivity and only ionic conductivity is observed. The ionic conductivity is evidenced by the blocking of the real part of the conductivity and by a strongly nonArrhenius temperature dependence, similar to that observed in other fast ionic conductors, in the low frequency region of the high temperatures region. It is remarkable for the high dc conductivity value measured at room temperature, about 10− 2 S/cm. As temperature is decreased, dc conductivity decreases rapidly, going well below the values obtained for the dry sample at the lowest temperatures, indicating that there is no trace of a superimposed electronic conduction. It is found that electrical conductivity of hydrated Li0.5FeOCl samples show liquid-like features. The strong non-Arrhenius behaviour, as well as the high dc conductivity value at room temperature, are both characteristics of highly ionic conducting melts. In fact the observed temperature dependence can be fitted, as it is usually claimed for melts, with a Vogel-Fulcher–Tamman law ⁎ exp(− A / (T − T0)), with T0 = 135 K and a low activation σ0 = σ∞ energy for the high temperature limit, A = 667 K. Local fit of the activation energy at room temperature gives a value of 0.2 eV,

similar to the values reported for the fast ionic conductors. Finally, we have measured the conductivity of dried (at 400 K) Li0.5FeOCl samples. Dc conductivity values of these samples are very close to those of the lithium intercalated oxychloride before water adsorption, pointing to some degree of reversibility in the transport properties of these samples after some hydration/dehydration cycles. Nuclear magnetic resonance was used to follow the lithium mobility described by impedance spectroscopy. 7Li (I = 3 / 2) MAS-NMR spectra of the lithiated sample, outgassed at 10− 3 Torr or dried at 100 °C, are formed by a central line (1 / 2,− 1 / 2 transition) shifted at 60 ppm from the resonance frequency and a set of spinning side bands (3 / 2, 1 / 2 and − 3 / 2, − 1 / 2 transitions) equally separated by 10 kHz (spinning rate of the sample), covering a 256 kHz region (Fig. 4). Line width of components, 5 kHz, are important indicators that residual magnetic interactions are still relevant in this sample. Hydrated samples display important modifications in NMR spectra: in the less hydrated sample (5 min exposed to atmospheric humidity), the 7Li MAS-NMR spectrum is formed by a set of narrow components covering a 93 kHz experimental region. This signal is centered at the resonance frequency indicating that paramagnetic interactions have decreased considerably in hydrated samples as a consequence of the hydration process. In this sample, Li ions, octahedrally coordinated to water molecules, move to the center of the interlayer space. When the hydration of sample progresses, the envelope of spinning side bands narrows (20 kHz) to finally give a lithium spectrum formed by a single narrow component, at the resonance frequence, characteristic of a liquid sample. 1 H MAS-NMR spectra, corresponding to the FeOCl sample submitted to the same treatments, are given in Fig. 5. In dried samples 1H NMR spectra have very low intensities, indicating that the amount of OH− / H2O is very low in the starting sample. 360 min.

7Li-MAS-NMR

30 min.

5 min.

Dried 300 K

600

400

200

0

-200

-400

-600

(ppm)

Fig. 4. 7Li-MAS NMR spectra of the lithiated sample submitted to increasing hydration times.

A. Sagua et al. / Solid State Ionics 177 (2006) 1099–1104

1103

1H-MAS-NMR

360 min

30 min

5 min

Dried, 300 K

250

200

150

100

50

0 (ppm)

-50

-100

-150

-200

-250

Fig. 5. 1H-MAS NMR spectra of the lithiated sample submitted to increasing hydration times. The two detected signals have been ascribed to water adsorbed inside (the interlamellar space) and outside (surface) of particles.

In the partially hydrated sample, the spectrum is formed by a signal at 30 ppm that spreads over an important experimental region (200 kHz); however spinning side bands of this signal are very narrow. Both facts suggest that water molecules adsorbed in the interlamellar space of these materials, are submitted to important paramagnetic interactions and display an appreciable mobility. On the other hand, the formation of some amount of OH− groups during the hydrolysis of the sample, cannot be disregarded. In hydrated samples, 1H NMR spectra are formed by two components at 30 and 0 ppm, with narrow spinning side bands patterns covering an experimental region of 50 kHz. Relative intensity of the 0 ppm component increases with the water content of the sample, decreasing the spectral region covered by two components with the degree of hydration. In all cases, the region covered by the 1H NMR is considerably broader than the one covered by the 7Li NMR signal. Taking into account the above observations, some conclusions can be drawn. First of all, the 7Li-NMR spectrum of the dried lithium FeOCl sample is formed by a very broad envelope that cannot be explained by quadrupolar interactions of Li with electric field gradients at occupied sites, or by dipolar interactions between Li ions. The observed broadening has been associated with paramagnetic interactions (FeOCl is an antiferromagnet with TN = 77 K, see Ref. [3] that produces important fields at sites occupied by lithium. In this sample, the lithium mobility must be low as suggested by the line broadening observed in spinning side bands. When water is adsorbed in the interlamellar space of the lithiated FeOCl sample, the experimental envelope and line width of spinning side bands are considerably reduced. Both facts indicate an important increase on Li mobility induced by interlamellar water. However, the water mobility displays a lower mobility

than lithium as deduced from broadening effects detected in both NMR signals. In hydrated samples, two signals are detected at 30 and 0 ppm in the 1H NMR spectra, their relative intensities changing slowly with the water content. The signal centered at 30 ppm could correspond to water molecules adsorbed in the interlamellar space and the signal centered at 0 ppm could be tentatively assigned to water molecules adsorbed at the external surface of particles. In these samples, the exchange rate between both types of water is low, suggesting that interlamellar water is strongly retained inside the particles. From facts considered herein, it seems probable that interactions between water molecules and interlamellar oxygens of FeOCl are more important than those between water molecules and Li ions. This could explain important paramagnetic interactions detected in the proton signal and the outstanding mobility detected for Li ions. Based on these observations, we have concluded that ion conductivity is mainly due to hydrated Li+ ions in Li0.5FeOCl. From above results, we can state that lithium insertion in iron oxychloride changes its transport properties; more strikingly, samples exposed to atmospheric moisture become ionic conductors with a remarkably high lithium conductivity, similar to that of molten salts. Finally, changes induced, first by lithium and thereafter by water intercalation, seem to be reversible, which could be important in technological applications. Acknowledgements One of the authors (Prof. Aurora Sagua) would like to thank both EEC (ALFA program) and Universidad Nacional del Sur (UNS), Argentina for the grant received. The authors would to like thank the Spanish Ministry for Science and Education for providing financial support (MAT2001-3713-C0-04 and

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MAT2004-03070-C05-05). Professor Miguel Angel AlarioFranco (Universidad Complutense de Madrid, UCM) is gratefully acknowledged for the helpful discussions and for carefully reading this manuscript. References [1] A.J. Jacobson, in: A.K. Cheetham, P. Day (Eds.), Chapter 6 in “Solid State Chemistry Compounds”, 3d. edition, Oxford University Press, 1996. [2] P. Palvadeau, L. Coic, J. Rouxel, Mater. Res. Bull. 13 (1978) 221. [3] J. Rouxel, P. Palvadeau, Rev. Chim. Minér. 19 (1982) 317. [4] A. Weiss, E. Sick, Naturforsch 33 (1978) m 1087. [5] S.M. Kauzlarich, B.K. Teo, B.A. Averill, Inorg. Chem. 25 (1986) 1209. [6] S.M. Kauzlarich, J.L. Stanton, J. Farber Jr., B.A. Averill, J. Am. Chem. Soc. 108 (1986) 7946.

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