Pseudo-equilibrium phase diagrams for PEO–Zn salts-based electrolytes

Solid State Ionics 116 (1999) 293–300

Pseudo-equilibrium phase diagrams for PEO–Zn salts-based electrolytes M.J.C. Plancha*, C.M. Rangel, C.A.C. Sequeira Instituto Nacional de Engenharia e Tecnologia Industrial, IMP/DM-Electrochemistry of Materials, Azinhaga dos Lameiros a` Estrada do Pac¸o do Lumiar, 22, 1699 Lisboa Codex, Portugal Received 4 August 1998; received in revised form 7 September 1998; accepted 14 September 1998

Abstract Poly(ethylene oxide) (PEO)-based electrolytes with Zn(II) salts systems have been re-examined and discrepancies found in the literature are discussed in terms of our own experimental results. Our electrolytes were prepared at room temperature, using acetonitrile as solvent for film-casting and the data were obtained with films without any pre-heating. The electrolytes were characterized by X-ray diffraction, TGA / DTA, SEM and EIS, in the range of concentrations n 5 4–16. An intermediate compound with an n 5 4 composition appears to exist in the PEO n ZnCl 2 system and a pseudo-equilibrium phase diagram has been drawn. Ongoing work for the PEO n ZnI 2 system, has provided evidence for the existence of an intermediate compound with the composition in the interval n 5 4–8. A preliminary phase diagram is discussed.  1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Poly(ethylene oxide)-based polymer electrolytes; Conductivity; Pseudo-equilibrium phase diagrams

1. Introduction The interest in polymer-ion salt systems is continuously growing due to the potential applicability of these materials as solid electrolytes in a variety of electrochemical devices [1–5]. Equilibrium phase diagrams for polymer electrolytes are thought to be a convenient way of condensing relevant information, even though due to their dependence on the nature of the casting solvent and preparation methods, and on the thermal history of the polymer, they cannot be regarded as true equilibrium phase diagrams [6]. In the literature, phase diagrams have been drawn *Corresponding author.

for PEO-based polymers with several monovalent cation salts like Li 1 , Na 1 , K 1 , NH 41 [7–10]. All studied systems revealed the presence of one or several intermediate compounds and the existence of eutectic reactions. It is known that ionic conduction takes place preferentially in the amorphous phase, as demonstrated some time ago by Gorecki and coworkers [11]. Variations observed in the electrolyte conductivity as a function of temperature have been analysed in terms of the nature, composition, and proportion of the different phases present in the electrolyte. Several studies have been made with divalent cation complexes [12–21]. However, work on phase diagrams is somewhat scarce. The PEO–ZnCl 2 system was studied before by several authors, [6,22–

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24], but only Bermudez et al. [22] have constructed a phase diagram. However, results obtained by Glasse et al. [6], Yang and Farrington [23] and Wendsjo¨ and Yang [24] are not in agreement with this phase diagram. The pre-heating treatment performed on the samples, before the thermal analysis, is perhaps one of the possible causes for the discrepancies observed. The two previous different studies [6,23] showed only slight differences in the melting temperature of the crystalline compounds. For the PEO–ZnI 2 system there are also important discrepancies between the results obtained by different authors [6,23,25], thought also to be related to the electrolyte preparation method. Evidence for the existence of an intermediate compound and the value of its melting temperature is not always clear. Cole et al. [25] found a crystalline phase containing only PEO with the salt totally confined to the amorphous phase, while Glasse et al. and Yang and Farrington [6,23] show data for a crystalline complex, even though the melting temperatures reported differ. The effects of the water content and of the drying temperatures on casting of the polymer electrolytes form the basis of some of the discrepancies mentioned above. Glasse et al. [6] found that the melting temperature of the crystalline material is depressed by the presence of the water in undried samples, giving a lower melting temperature. It is reported, that the amount of the different crystalline compounds present in the samples increases with the drying temperature of the casting, affecting the electrolyte conductivity values. In the present work, PEO–ZnCl 2 and PEO–ZnI 2 systems are re-examined and inconsistencies found in the literature are discussed in terms of our own experimental results, obtained, using for the casting of the films acetonitrile as solvent, at room temperature and in the absence of any pre-heating treatment.

2. Experimental The PEO n ZnCl 2 and PEO n ZnI 2 electrolytes, with n 5 4, 8, 12 and 16, were prepared by the wellknown casting procedure, commonly used for the preparation of PEO-based complexes. Appropriate amounts of powdered salts and PEO were dissolved in acetonitrile and allowed to stir until complete

dissolution. Methanol / acetonitrile mixed solvent was used for the n 5 4 compositions. Solid films were then isolated from the viscous solutions by pouring them into clean glass Petri dishes, placed in a desiccator containing molecular sieves, at room temperature, and evaporating off the solvent. X-ray diffraction measurements were performed using a Rigaku model D/ Max III C automated diffractometer with graphite monochromated Co radiation. The films were mounted on aluminium plates with a window for the exposure of X-rays. All measurements were made in air and at room temperature, and the scan speed and range were 28 min 21 and 5–508, respectively. TGA / DTA curves were obtained using a Stanton Redcroft 706 Temperature Programmer and Data Acquisition System calibrated with Zn standard. The films were cut into pieces (typically 10 mg) and loaded to aluminium pans. An atmosphere of nitrogen was used and the heating rate was 108C min 21 . In some cases, thermal cycles (cooling after first heating and second heating) were performed. Cooling naturally took about 30 min. The surface morphology of the polymer electrolytes was examined with a JFM 35 CF JEOL Scanning Electron Microscope. Prior to observation the samples were gold-coated. Electrochemical measurements were performed with EIS using a two-electrode sandwich-type cell with non-blocking Zn electrodes. The impedance measurements were carried out with a Solartron 1250 frequency response analyser and a 1286 electrochemical interface, both coupled to an HP 9000 microcomputer.

3. Results and discussion A pseudo-equilibrium phase diagram for the PEO– ZnCl 2 system, based on X-ray diffraction and differential thermal analysis data, is shown in Fig. 1. Superimposed, in the same figure, are values obtained by another author [6], from differential scanning calorimetry (DSC) and variable temperature polarized microscopy (VTPM) experiments. This phase diagram, with compositions expressed as the mass fraction of zinc chloride, has two major characteristics: the presence of a single salt-rich

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Fig. 1. Pseudo-equilibrium phase diagram drawn for PEO–ZnCl 2 system. L stands for the liquid-like amorphous phase and P(EO 4 ZnCl 2 ) is the intermediate compound suggested by the results. The transition temperatures were observed by DTA (our results) – (G); VTPM [6] – (x); DSC [6] – (h).

intermediate compound P(EO 4 ZnCl 2 ), which melts at about 1738C, and the presence of an eutectic between PEO and P(EO 4 ZnCl 2 ), with a melting temperature of about 508C for a composition near n 5 16. Bermudez et al. [22] have worked on the same system and have proposed a different phase diagram; they have found an eutectic mixture of two intermediate compounds for electrolytes with compositions between n 5 4–6 and n 5 22. For the PEO 4 ZnCl 2 electrolyte they have found free crystalline zinc chloride, unlike us and others [23,24]. Moreover, the conductivity behaviour of their samples is not in agreement, at least for some compositions, with the phase diagram proposed by themselves. A possible cause is the pre-heating that they did before the DSC analysis that affect the crystallinity of the electrolytes. By contrast, some other authors [6,23,24] have results that confirm the phase diagram herein drawn.

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All the X-ray diffraction patterns were recorded at room temperature. For all compositions, a pattern associated with a crystalline polymer salt complex was found. Upon going to more concentrated electrolytes, the relative intensity of this pattern gradually increased up to the n 5 4 composition, whilst the characteristic peaks of uncomplexed PEO, decreased in intensity up to the n 5 8 composition. The diffraction pattern for the n 5 4 composition electrolyte is identified as corresponding totally to the intermediate compound. Since no other peaks appeared, n 5 4 is taken as the stoichiometry of the crystalline polymer salt complex formed between PEO and ZnCl 2 . Differential thermal analysis (DTA) coupled with thermogravimetric analysis (TGA) confirmed this conclusion, showing a single sharp endotherm with an onset at about 1738C, for the PEO 4 ZnCl 2 electrolyte. The TGA / DTA results and the X-ray diffraction pattern for this electrolyte, are presented in Fig. 2. The first heating cycle in the thermogram, also shows a broad peak appearing at lower temperatures. This is due to loss of water and solvent as there is a decrease in weight of the sample in the same temperature region. In a second heating cycle, after cooling of the sample, the peak and the weight loss are no longer apparent. A peak due to recrystallization of the intermediate compound is clearly seen at about 648C. For the PEO 8 ZnCl 2 electrolyte, the thermograms show two melting peaks: one at low temperature (37.68C), assigned to the melting of the eutectic formed between the pure PEO and the intermediate compound, P(EO 4 ZnCl 2 ); at high temperature (1438C), assigned to the melting of P(EO 4 ZnCl 2 ). For the more diluted electrolytes, only one melting peak was visible at a temperature of around 508C, probably because the amount of the intermediate compound is not sufficiently high to be detectable by DTA. The PEO–ZnCl 2 system was characterized by electrochemical impedance spectroscopy (EIS) as well. Fig. 3 shows the a.c. conductivity results, collected using Zn electrodes, between room temperature and 1508C. The conductivity values, s, were calculated using Eq. (1), where l and A are the thickness and the area of the polymer electrolyte, respectively, R b is the polymer electrolyte resistance

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Fig. 2. Thermal and X-ray analysis for PEO 4 ZnCl 2 electrolyte: (a) TGA (dashed line) / DTA (continuous line) scans, with heating rate of 108C / min (A, first heating; B, second heating); (b) X-ray diffraction pattern with scan speed of 28 / min.

estimated from the a.c. impedance data after fitting to experimental curves done using a simulation computer program based on the equivalent circuit model. Adjustment of a.c. impedance-related parameters uses the non-linear least-squares fitting technique [17]. l (cm) s 5 ]]]]]] R b (V ) 3 A (cm 2 )

Fig. 3. Variation in the electrolyte conductivity plotted versus the inverse of the temperature for the PEO–ZnCl 2 system: n54 (3); n58 (h); n512 (쏻); n516 (*).

(1)

For the PEO 8 ZnCl 2 and PEO 12 ZnCl 2 electrolytes, the Arrhenius law was obeyed for two different temperature regions, separated at the transition temperature. This corresponds to the melting temperature of the eutectic, predicted by the phase diagram discussed above. The activation energy values found in this work, for the higher temperature regions, range from 4.1 to 38.5 kJ mol 21 . At low temperatures, higher values, between 61.9 and 68.1 kJ mol 21 were determined. This is justified because at lower temperatures, the conductivity is restricted to the low percentage of amorphous phase present together with the crystalline mixture of uncomplexed PEO and the intermediate compound. After the melting event, the salt concentration of the amorphous conducting phase

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decreases and there is an increase in the local mobility of the polymer chains, resulting in lower activation energies for the ionic conduction. The VTF conductivity behaviour exhibited by the n54 composition electrolyte, through the whole temperature range studied, may reveal a relatively high degree of amorphousness. Traces of humidity left in the material may be responsible for this, as water destroys the crystallinity and this may not have been sufficiently high to lead to an Arrhenius behaviour. It is to be noted that, in the low temperature region, the higher conductivity values (Fig. 3) correspond to the more concentrated electrolytes. This feature can be correlated, in part, with the surface roughness of the electrolytes, as observed by scanning electron microscopy. The SEM micrographs are

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presented for all compositions in Fig. 4. The more concentrated electrolytes exhibit a flat surface morphology (Fig. 4a,b), while for higher concentrations of PEO, a noticeable increase in surface roughness is observed (Fig. 4c,d). So, the contact area used to calculate the conductivity (s ) in the PEO 12 ZnCl 2 and PEO 16 ZnCl 2 electrolytes, was higher than the real area, giving lower s values. This difference in the contact area decreases as the temperature is raised, because the films were heated in contact with the measuring electrodes, modifying the spherulitic structure of the electrolytes and giving as a result, a flatter surface. The same conductivity / morphology dependence was obtained at low temperatures for the PEO n ZnI 2 system, where we can observe, for example, the flat surface of the PEO 4 ZnI 2 electrolyte, which presents

Fig. 4. SEM micrographs of: (a) PEO 4 ZnCl 2 ; (b) PEO 8 ZnCl 2 ; (c) PEO 12 ZnCl 2 ; (d) PEO 16 ZnCl 2 .

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the maximum conductivity values. In Fig. 5, the SEM micrographs for this system are presented and the conductivity temperature dependence can be seen in Fig. 6. The conductivity of the n54 composition electrolyte follows the Arrhenius law at two different temperature ranges separated by a knee at a specific temperature. For the other compositions of electrolytes, at low temperatures, the same conductivity behaviour is followed, but after a region where the conductivity increases steeply, due to the formation of a liquid phase when the pure PEO and / or the intermediate compound melts, the VTF law is now obeyed. The completely liquid state in this temperature region for these composition electrolytes, is the reason for the lower activation energies found (in the range 3.6–4.6 kJ mol 21 ), compared with the PEO–

ZnCl 2 system, where there is still crystalline material present, except for the PEO 16 ZnCl 2 electrolyte above the melting temperature of the eutectic. The conductivity results for both polymer–Zn salt systems can be compared with those obtained by others [6,22,23,26,27] taking into account the different sample preparation methods and experimental conditions, namely drying and pre-heating treatments which are known to affect the crystallinity, and as a consequence, the conductivity. The impedance results are in accordance with MacDonald’s model, [28,29] with the presence of a high-frequency semicircle in the complex a.c. plots, at low temperatures correspondent to the electrolyte resistance, which disappeared as the temperature was increased. A second semicircle appearing at medium frequencies and assigned to the interfacial impe-

Fig. 5. SEM micrographs of: (a) PEO 4 ZnI 2 ; (b) PEO 8 ZnI 2 ; (c) PEO 12 ZnI 2 ; (d) PEO 16 ZnI 2 .

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Fig. 6. Variation in the electrolyte conductivity plotted versus the inverse of the temperature for the PEO–ZnI 2 system: n54 (3); n58 (h); n512 (쏻); n516 (*).

dance, was followed at low frequencies by an arc with a 458 branch curving towards the real axis and corresponding to the diffusion impedance. Comparing the conductivity results obtained in this work for the PEO–ZnCl 2 system with those obtained by Yang and Farrington [23], we observed that our values are about 10–100 times higher, the PEO 4 ZnCl 2 electrolyte showing the major difference. They obtained the n524 composition electrolyte as the most conductive electrolyte. A possible explanation is the different water content in the samples and the more pronounced effect of the moisture present in the more concentrated electrolytes, as was observed for PEO–Pb, Zn(CF 3 SO 3 ) 2 systems [21]. While our samples were not totally free from water and solvent, about 8% in average (as analysed by TGA), those prepared by the authors mentioned were heated to 1108C for 24 h, attaining clean anhydrous conditions. In our electrolytes, not only the crystallinity is reduced, the mobility of the polymer chains is also increased by the plasticization

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effect of the volatile materials present, giving higher s values. For the same reasons, the PEO 8 ZnI 2 electrolyte prepared by Glasse et al. [6] gives at 408C a s value of about 5310 29 S cm 21 , which is 703 lower than the obtained in this work, at the same temperature. However, at 1408C, the same value of conductivity is found, as the samples are now both amorphous and without water and solvent. The crystallinity and thermal transitions for the PEO–ZnI 2 system were also analysed. X-ray diffraction analysis showed the existence of an intermediate compound with concentration in the range n54–8, designated as C1 and present in all electrolytes studied. The PEO 4 ZnI 2 electrolyte seems to have, apart from the already mentioned intermediate compound, another, with a composition n, 4 (C2). For the other electrolytes, uncomplexed PEO was found together with the C1 complex. Characteristic peaks from free salt did not appear for any electrolyte in the range of compositions studied. From TGA / DTA experiments, it was not possible to deduce all melting temperatures, because there was a superimposition of endothermic peaks, due to the evaporation of solvent and water and to the melting of the different crystalline phases. With the data available, a preliminary pseudoequilibrium phase diagram for the PEO-ZnI 2 system was constructed shown in Fig. 7. It should be noticed that, the melting temperatures obtained by Glasse et al. [6] from DSC experiments with PEO n ZnI 2 films, cast from water and undried (i.e. in equilibrium with atmospheric moisture), are in accordance with this phase diagram, suggesting that the crystalline content can be considered similar in both electrolyte systems. In order to complete the preliminary diagram drawn here, more work is needed, in particular cycle thermal analysis to obtain the undefined temperature phase transitions. An extension of the study to different compositions is also necessary to obtain the exact stoichiometries of the C1 and C2 intermediate compounds. As it is known, water can play a major role, not only on the structure (crystallinity), but also on the conductivity and charge carrier mobility. Polymer electrolytes, with different water contents and dried, i.e. after pumping away the water, are being tested,

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Fig. 7. Preliminary phase diagram of the PEO–ZnI 2 system. C1 and C2 are the intermediate compounds with compositions in the range n54–8 and n,4, respectively. The symbol ↓ means that the transition temperature is below the represented point.

to get a better idea of the effect of humidity on the conductivity behaviour of our PEO–Zn electrolytes. It is expected that some of the inconsistencies observed in prior studies, can in this way be adequately explained.

Acknowledgements JNICT is acknowledged for providing of a grant to M.J. Plancha.

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