Model composite polymer electrolytes containing triphenylborane

Electrochimica Acta 50 (2005) 3934–3941

Model composite polymer electrolytes containing triphenylborane ˙ M. Marcinek ∗ , G.Z. Zukowska, W. Wieczorek Warsaw University of Technology, ul. Noakowskiego 3, 00-664 Warsaw, Poland Received 9 October 2004; received in revised form 4 January 2005; accepted 10 February 2005

Abstract Anion trapping materials are presently of major interest as additives to polymer electrolytes. The paper describes the influence of triphenylborane on the physicochemical properties (conductivity, viscosity) of the PEODME-LiX electrolytes. The effect of such type of additive was discussed for various triphenylborane/LiX (X = I− , ClO4 − , BF4 − , CF3 SO3 − , (SO2 CF3 )2 N− ) molar ratios. A possible mechanism for anion–triphenylborane interactions is postulated on the basis of conductivity and FT-IR spectroscopy results. The present work is a beginning of the systematic studies on the effect of the Lewis acidity of an additive on the properties of composite polymeric electrolytes. © 2005 Elsevier Ltd. All rights reserved. Keywords: Composite polymer electrlytes; Triphenylborane; Lewis acid

1. Introduction The next important step in the development of novel polymer electrolytes will be to obtain a highly single cation conductive systems. This concept is exemplified with the use of an anion trapping compound as additive. In the current literature studies on the use of cation receptors such as crown ethers, cryptands and calixarenes in low molecular weight non-aqueous solutions [1–4] as well as polymer electrolytes are widely described. Studies on the use of anion receptors in polymer electrolytes are very limited. Papers dealing with this subject are based on either theoretical predictions [5] or studies on addition of boron family compounds, as well as cyclic or linear aza-ether structures with electronwithdrawing groups to oligoethers. The new types of ionic conducting polymers with grafted anion receptors based on aza-ether structures have been also described [6–9]. Anion receptors based on boron compounds were applied to the solutions of lithium salts in aprotic (inert) electrolyte based on low molecular weight solvents [10,11] as well as in gel polyelectrolytes [12]. Boron based aza-ether compounds (borane, borate complexes) have been studied by McBreen ∗

Corresponding author. Tel.: +48 22 6605739. E-mail address: [email protected] (M. Marcinek).

0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.02.045

and co-workers [6–9,11,12] using mainly near edge X-ray absorption fine structure spectroscopy (NEXAFS) Zhou and MacFarlane used 7 Li and 11 B NMR by [13]. These studies showed that the degree of complexation of Cl− or I− anions strongly depends on the structure of the boron compounds. Also the dramatic enhancement in ionic conductivity upon the addition of boron compounds has been noticed in these electrolytes [6–9,11,12]. The present paper describes the physicochemical properties of composite electrolytes based on PEODME Mw = 500 (n ≈ 11–12) complexes with different salts LiI, LiCF3 SO3 , LiN(SO2 CF3 )2 (LiTFSI), LiBF4 and LiClO4 used as ionic dopants. We have selected triphenylborane as a complexing agent in polymer electrolyte for several reasons. As an additive the triphenylborane (of the structure shown in Fig. 1) is a weak Lewis acid relatively stable when kept under argon. It is expected, due to its bulkiness, to act as an anion trap and therefore it should promote the fraction of the conductivity devoted to lithium cations. Furthermore it is a weak acid and there is no risk of destroying of the polymer matrix, which might have an effect on the conductivity of the electrolyte studied as well as on the detailed analysis of the conduction mechanism in these composite electrolytes.

M. Marcinek et al. / Electrochimica Acta 50 (2005) 3934–3941

3935

2.4. Impedance spectroscopy between blocking electrodes

Fig. 1. Scheme of triphenylborane.

Ionic conductivity was determined using the impedance spectroscopy method in the temperature range from −10 to 70 ◦ C. The samples were sandwiched between stainless steel blocking electrodes and placed in a temperature-controlled oven. The experiments were performed in a constant volume cylindrical cell with an electrode diameter equal to 7.8 mm and electrodes separation of 1.6 mm. The impedance measurements were carried out on a computer-interfaced Solartron-Schlumberger 1255 impedance analyzer over the frequency range 1 Hz to 1 MHz. 2.5. FT-IR spectroscopy

BPh3 has a relatively simple structure, which enables analysis of direct interactions between an additive, polymer matrix and salt in order to establish the mechanism of the anion complexation in polymer electrolytes. It is also commercially available and chemically safe. Therefore it is a good candidate for systematic studies on the effect of the Lewis acidity of an additive on conductivity and microstructure of polymer electrolytes.

2. Experimental 2.1. Polymer matrices Poly(ethylene oxide) dimethyl ether (PEODME, Mw = 500, Aldrich, liquid at room temperature) was filtered, than repeatedly freeze-dried using several freeze–pump–thaw cycles on a vacuum line. Afterward it was dried under high vacuum (10−5 Torr) at ∼60 ◦ C for 72 h. 2.2. Salts Different salts: LiClO4 , Li(CF3 SO2 )2 N (LiTFSI), LiCF3 SO3 , LiI, LiBF4 were dried under vacuum at 80 ◦ C for 48 h prior to their dissolution in the polyether. 2.3. Samples preparation procedure After drying procedure, still under vacuum, the polymer was transferred to an argon filled dry-box (moisture content lower than 2 ppm) where the salts were dissolved into the polymer using a magnetic stirrer. The salt concentrations were equal to 0.2, 1 and 3 mol/kg of polymer, respectively. Samples were prepared by direct dissolution of the salt in the polyether. Samples with LiClO4 and LiCF3 SO3 were heated at 50 ◦ C to facilitate the dissolution process. Triphenylborane was used as received from Aldrich in argon filled dry box atmosphere to the samples in 1:1 (BPh3 :salt) proportion.

Infrared absorption spectra were recorded on a computerinterfaced Perkin-Elmer 2000 FT-IR system with a wavenumber resolution of 2 cm−1 . FT-IR studies were performed in the −20 to 70 ◦ C temperature range. Spectra were performed for sample in a form of a thin film sandwiched between two NaCl plates (high salt concentration) or placed in a cuvette with a 0.1 mm spacer. Cells filled with electrolytes were placed in the FT-IR temperature-controlled unit; the accuracy of the temperature was estimated to be ±1 ◦ C. Samples were placed in a vacuum isolated holder, cooled with liquid nitrogen and then slowly heated and stabilized for 5 min at each particular temperature. 2.6. DSC DSC data were obtained in the −150 to 150 ◦ C temperature range using UNIPAN 610 scanning calorimeters with low-temperature measuring heads and liquid nitrogen cooling elements. Samples were loaded into titanium pans and stabilised by cooling from room temperature down to −150 ◦ C. They were then heated at 20 ◦ C/min to 150 ◦ C. Melting temperature calibration was performed using metallic indium. An empty pan was used as a reference. 2.7. Rheological experiments Rheological experiments were conducted at 25 ◦ C a Bohlin Visco 88BV viscometer in two coaxial cylinders geometry was used and the measurements were performed within a shear rate range of 24–1200 cm−1 .

3. Results and discussion 3.1. Conductivity Fig. 2a–e presents the temperature dependence of the conductivity for the samples containing PEODME doped with 0.2 mol/kg of the different lithium salts (LiI, LiBF4 , LiClO4 , LiTFSI, LiCF3 SO3 ) and for the analogue composites with the

3936

M. Marcinek et al. / Electrochimica Acta 50 (2005) 3934–3941

Fig. 2. Conductivity vs. temperature for PEODME-LiX-BPh3 (molar ratio 1:1). Salt content equal to 0.2 mol/kg. LiX = LiI (a), LiBF4 (b), LiClO4 (c), LiTFSI (d), LiCF3 SO3 (e).

M. Marcinek et al. / Electrochimica Acta 50 (2005) 3934–3941

3937

Fig. 3. Conductivity as a function of inverse temperature for PEODMELiCF3 SO3 and PEODME-LiCF3 SO3 -BPh3 (salt content equal to 3 mol/kg).

Fig. 4. Influence of the different amount of the triphenylborane on the PEODME-LiCF3 SO3 (1 mol/kg of polymer) electrolyte.

equimolar (with respect to the salt) addition of the triphenylborane compound. All conductivity plots are characterized by a monotonic decrease in the conductivity with decrease in temperature. In the range of temperature (∼0–5 ◦ C depending on the salt concentration) where the crystallization of the electrolyte might occur a rapid (∼3 orders of magnitude) drop in the conductivity is observed. The critical phase point is shifted toward lower temperatures with an increase in the salt concentration (see Fig. 3). As can be easily seen the influence of the triphenylborane on the conductivity strongly depends on the structure and type of the anion used. For the composite systems with LiI, LiBF4 and LiClO4 the differences are almost invisible or very slight at the temperatures above room temperature range. For the samples containing LiCF3 SO3 and LiTFSI in the 10–70 ◦ C temperature range the addition of the BPh3 compound results in the most visible (in comparison with other salts) decrease of the conductivity. The decrease of the conductivity value is more visible for the composite electrolytes with the salt to boron additive molar ration = 1:2 (see Fig. 2c and d). At temperatures lower than 10 ◦ C conductivities of the BPh3 containing composites independently of the salt used are higher than for the pure systems. It might be connected with the lower crystallinity of the samples with the boron compound as shown clear on the basis of DSC studies. The conductivity measurements have been repeated after one month in order to verify the reproducibility of the results and no significant differences in comparison with previous measurements have been noticed. The temperature dependence of the conductivity for the sample with 3 mol kg−1 of lithium triflate is presented in Fig. 3. General tendency is the same as in the Fig. 2. Also this sample is characterized by a monotonic decrease in the conductivity with decrease in temperature. However upon the addition of BPh3 (1:1) to the sample the decrease of the conductivity is more visible. That is be-

cause the lower dissolution of the BPh3 and formation of the white complex. The dissolution is facilitated in higher temperatures. Fig. 4 presents the studies of the conductivity for the samples contained 1 mol of the lithium triflate and different amount of the triphenylborane compound. Up to the 0.5 mol concentration the boron compound is perfectly dissolved and we observe the maximum decrease of the overall conductivity. No visible complexes are formed. Two factors affect the conductivity behavior in PEO– salts–BPh3 . One is the complexation ability (dependent on the amount) of the boron compound and the second is its dissolution in given electrolyte. For the more diluted samples (0.2 mol/kg of the salt) the triphenylborane is well dissolved in the samples and thus we are able to eliminate the role of the second factor. For higher salt concentration of the salt and (in the case of 1:1 ratio) triphenylborane the boron compound is poorly dissolved and the decrease in the conductivity is partially caused by this fact. Due to the fact that the conductivity process in PEODME based lithium salt electrolytes occurs via anion and cation the decrease of the overall ionic conductivity for sample containing BPh3 suggests the more selective cationic transport in the temperature range above the room temperature. The lithium salt boron compound ratio plays the crucial role in the conductivity mechanism. 3.2. FT-IR As is known, various interactions in polymer electrolytes are reflected by changes in the IR spectrum. In the presented work we mainly focused on analysis of IR bands of BPh3 and doping salts. Because the strongest conductivity enhancement was obtained for LiCF3 SO3 doped systems, we decided to confine more detailed analysis to the PEODMELiCF3 SO3 -BPh3 electrolytes. For comparison, we analysed

3938

M. Marcinek et al. / Electrochimica Acta 50 (2005) 3934–3941

also spectra of systems doped with LiI, for which no conductivity improvement was found at the temperatures higher than 5 ◦ C. In IR spectrum of triphenylborane-PEODME mixtures several bands of BPh3 appear to be sensitive to the temperature and salt presence. For example, strong peak ascribed to the ␯C=C ring vibration can be resolved into two contributes with maxima at 1596 and 1592 cm−1 . The intensity of the former increases with an increase in temperature, while the intensity of the latter decreases. Similar splitting was found for peaks with maxima at 638 and 602 cm−1 . With rise in temperature intensities of these bands decreases and new bands at 644 and 605 cm−1 appear to exist. On the other hand, bands attributed to C–H out of plane bending vibration at 685 cm−1 moves only slightly as the temperature increases. The analysis of IR spectra of PEODME-LiCF3 SO3 solution reveals the presence of free anions as well as ion pairs and aggregates (see Fig. 5). The peak ascribed to ␦s CF3 can be easily split into two contributes with maxima at 757 (ion pairs) and 751 cm−1 (free anions); intensity of the latter decreases with increase in temperature. A strong band of ␦s SO3 vibration mode at room temperature is clearly split into two bands peaking at 638 and 640 cm−1 ; the former slowly disappears as the temperature increases from 25 to 70 ◦ C. Such behaviour allows us to conclude that these two bands might originate from free ions and ion pairs, respectively. Fig. 6a and b shows exemplary spectra of PEODMELiCF3 SO3 -BPh3 and PEODME-LiCF3 SO3 electrolytes recorded in −20, +25 and +50 ◦ C. In PEODME solutions containing BPh3 and lithium triflate (see Figs. 5–7) the characteristic vibrations of both components are affected by formation of various types of complexes. The most pronounced change was observed for the ␯C=C ring vibration. At room temperature, in the presence of salt the maximum of this peak is found at 1596 cm−1 compared to 1592 cm−1 for the PEODME-BPh3 solution. As the temperature increases one may observe only slight increase in the intensity of the shoulder at 1592 cm−1 . In the spectral range between 760–730 cm−1 and 670–620 cm−1 bands of the BPh3 are overlapped with bands of the salt, however, the overall shape of the ␦s SO3 peak (at 638 cm−1 ) is similar for the PEODME-LiCF3 SO3 -BPh3 and PEODME-LiCF3 SO3 samples. The peak at 602 cm−1 is also influenced by the salt–BPh3 interactions and the observed trend is similar to that for PEODME-BPh3 samples. It has to be noticed that in the LiCF3 SO3 containing systems the influence of the temperature is much smaller than in the PEODME-BPh3 solutions. The intensity of the peak at 605 cm−1 is higher at room temperature for the LiCF3 SO3 containing systems than in the PEODME-BPh3 samples, while at 70 ◦ C the trend is opposite. In spectra of samples with high (3 m kg−1 ) (see Fig. 6a and b) salt concentration the most pronounced difference between electrolytes with or without BPh3 is the shift of

Fig. 5. Spectra of PEODME-LiCF3 SO3 electrolytes recorded at +25 ◦ C (dashed line) and +70 ◦ C (solid line). Salt content equal 0.2 mol/kg salt content.

the ␯C–O–C band towards lower wavenumbers. At 25 ◦ C the maximum of this peak appears at 1107 and 1110 cm−1 for PEODME-LiCF3 SO3 -BPh3 and PEODME-LiCF3 SO3 , respectively. This allows us to assume that complexation of CF3 SO3 − anions by BPh3 molecules results in stronger coordination of Li+ cations by polyether oxygens. The position of the maximum of the strong peak attributed to ␯s SO3 vibration (1039 cm−1 ) indicates that in both samples most of the salt exists as ion pairs. The band of the ␯as SO3 vibration mode, however, seems to be dependent on the the presence of BPh3 . In sample without BPh3 the maximum of this band is found at 1273 cm−1 whereas in that containing LiCF3 SO3 BPh3 complex it appears at 1276 cm−1 . According to studies of Bishop at al. [14] this peak corresponds to the “spectroscopically free” anions. These authors observed several bands in this spectral region and attributed them to ion pairs (1259 cm−1 ) and various types of charged ionic aggregates (1270, 1288 cm−1 ). In spectra of the systems obtained in this

M. Marcinek et al. / Electrochimica Acta 50 (2005) 3934–3941

3939

study the deconvolution of this spectral region is doubtful because of the overlapping of the bands of CH2 twisting vibration (1249 and 1299 cm−1 ) with the characteristic bands of the salt. In the studied systems we observed a band peaking at 1257 cm−1 which is characteristic for ion pairs. The comparison of spectra recorded in various temperatures revealed that an increase in temperature involves an increase of the relative intensity of the peak ascribed to ion pairs (1257 and 1039 cm−1 ), which is in agreement with studies of other authors. Note that in the spectra of the BPh3 containing sample at each temperature the intensity ratio of the peak of “free” ions (1031 cm−1 ) to that of “ion pairs” (1039 cm−1 ) is higher than in the non-modified sample. It is also worth to notice that the temperature-dependent splitting of the ␦s SO3 observed for the PEODME-LiCF3 SO3 electrolytes was not found in the PEODME-LiCF3 SO3 -BPh3 samples. In our opinion, the described changes give evidence of strong interactions between CF3 SO3 − anion and hence better dissociation of LiCF3 SO3 in the BPh3 containing systems. The behaviour described above is in agreement with the trend observed for conductivity. For the systems where the addition of BPh3 results in the increase in conductivity we observe the differences in the spectra of non-modified and BPh3 containing systems. Let’s compare PEODME-LiI-BPh3 and PEODME-LiCF3 SO3 -BPh3 electrolytes (see Fig. 7a and b). In the first case, peaks attributed to C–B bending vibrations (␦ BC3 and ␦BC3 , with maxima at 641 and 600 cm−1 ) exhibit similar changes with the temperatures as in PEODMEBPh3 solutions. For PEODME-LiCF3 SO3 -BPh3 , as mentioned above, a profile of these peaks is almost independent on the temperature. For systems doped with LiClO4 , LiBF4 and LiTFSI we noticed intermediate behaviour, i.e. the shift of the maximum of the BPh3 peaks or appearance of shoulders, but these changes were smaller than in PEODMEBPh3 or PEODME-LiI-BPh3 systems. These results indicate also the BPh3 -LiX 1:1 coordination, at least in the case of LiCF3 SO3 and LiTFSI. In spectra of PEODME-LiTFSIBPh3 and PEODME-LiClO4 -BPh3 samples with higher (2:1) BPh3 -LiX ratio we found similar changes as in the PEODMEBPh3 solutions which is an evidence of presence of “free” BPh3 in these systems. 3.3. DSC

Fig. 6. Exemplary spectra of PEODME-LiCF3 SO3 -BPh3 (a) and PEODMELiCF3 SO3 (b) electrolytes recorded in −20, +25 and +50 ◦ C. Salt content equal to 3 mol/kg, salt/BPh3 molar ratio equal to 1.

Sample thermograms for PEODME based systems are shown in Fig. 8. DSC traces display an endothermic phase transition occurring in the range 260–265 K for all samples. This transition corresponds to the melting of the crystalline phase. Sets of data obtained from DSC studies are labeled in Table 1. The interesting feature is observed when the area of the endothermic peak is normalized with the respect to constant mass of the polymer. In all cases for the samples with presence of triphenylborane the enthalpy of the melting of crystalline phase is lower.

3940

M. Marcinek et al. / Electrochimica Acta 50 (2005) 3934–3941

Table 1 The differences between the thermograms of the systems Sample

Salt concentration (mol/kg PEODME)

Mass of the sample taken to DSC (mg)

Tm onset (K)

Enthalpy of the melting peak (mJ)

Normalized enthalpy with respect to 100 mg PEODME

PEODME PEODME-BPh3 a PEODME-LiCF3 SO3 PEODME-LiCF3 SO3 -BPh3 b PEODME-LiCF3 SO3 PEODME-LiCF3 SO3 -BPh3 b

– – 0.2 0.2 3 3

113.00 124.11 103.83 159.88 144.36 125.25

264.1 264.5 258.6 261.2 268.7 262.1

−79.342 −74.016 −106.2 −111.71 −102.18 −54.628

−70.21 −62.73 −109.48 −75.47 −74.04 −68.29

a b

BPh3 concentration was equal to 0.2 mol/kg of polymer. Salt:BPh3 molar ratio was equal 1:1.

Fig. 8. DSC curves for PEODME-LiCF3 SO3 and PEODME-LiCF3 SO3 BPh3 electrolytes.

This feature is a result of the changes of the dilution of the boron compound during the experiment.

4. Discussion The interaction observed in the system studied, for anion traps, can be explained in terms of competition of electron pairs donors (DN) between the polymer DN of ether oxygen ≈22 and that of the anion. Based on the extensive studies of Armand1 of properties of various anions the sequence of increasing donor properties is as follows: Fig. 7. FT-IR spectra of PEODME-LiI-BPh3 (a) and PEODME-LiCF3 SO3 BPh3 (b) electrolytes recorded at −15 ◦ C (dotted line), +35 ◦ C (dashed line) and +70 ◦ C (solid line) with equimolar content of salt and BPh3 . Salt content equal to 0.2 mol/kg.

3.4. Rheology In polymer electrolytes viscosity can be analysed as a physical variable, which can be connected with the mobility of charge carriers present in the electrolyte. The viscosity measured at 25 ◦ C is more or less similar for all the samples contained 0.2 mol/kg of the salts and relatively invariant upon addition of the BPh3 . The measurement of viscosity is impossible for this sample in a lower shear rate. The viscosity of the sample with the 3 mol/kg exhibits the dramatic increase of the viscosity (from 45 to 196 cP with BPh3 ). However this value is affected by the high tixotropy.

I− < CF3 SO3 − < ClO4 − < BF4 − < (CF3 SO2 )N− Boron is known as a very “hard” Lewis centre. The CF3 SO3 − (DN = 16.90 and “hard”) should, as found in the combined conductivity and FT-IR data, exhibit the strongest interaction and this explains the influence in the conductivity. In opposite I− anion (DN = 28.9) is not expected to give much interactions because it is very “soft”. Also both BF4 − (DN = 6.03) and ClO4 − (DN = 8.44) are effectively the less prone to compete with the polyether.

1 (a) W. Linert, R.F. Jameson, A. Taha,“Donor numbers of anions in solution: the use of solvatochromic Lewis acid-base indicators”, J. Chem. Soc. Dalton Trans. (2002) 3181–3186. (b) W. Linert, A. Camard, M. Armand, C. Michot, “Anions of low Lewis basicity for ionic solid state electrolytes”, Coord. Chem. Rev. 226 (2002) 137–141.

M. Marcinek et al. / Electrochimica Acta 50 (2005) 3934–3941

TFSI anion (DN = 5.40), the weakest donor of all the series. We may hypothesize that in this case the either specific boron nitrogene interactions are favoured or that the plasticizing effect of TFSI is thwarted by the rigid BPh3 molecules. However in this particular polymer matrix the former seems to play the more important role. We also have to remember that DNs are measured in a given solvent (here ClCH2 CH2 Cl), but probably the relative order is maintained in other solvents (polyethers) or that the hard/soft concept predominates.

5. Conclusions The new model composite electrolytes have been synthesized and analysed in order to verify the use of triphenylborane as an anion receptor in polymer ionic conductive system. This paper describes the first attempt to study composites of this sort as single ion conductors in the systems with different anions. For the BPh3 containing samples we have observed the increase in the conductivity below the melting point of polyether. This fact can be correlated with the reduction of the crystallinity. Also on the basis of FT-IR experiments the reduction of the ionic pairs (and increase of the free ions and triplets) has been estimated. The influence of the triphenylborane on the conductivity in higher temperature range (higher than 5 ◦ C) has been noticed only for the lithium triflate and LiTFSI containing electrolytes. A lot of effort concerned to the composite model system has been done previously with ceramic powders. In comparison with previously studied ceramic powders doped electrolyte the better reproducibility of

3941

the conductivity results has been reached due to the homogeneity (e.g. no sedimentation of the powder no agglomeration of ceramic particles) of the samples.

References [1] Y. Matsuda, H. Hayashida, M. Morita, J. Electrochem. Soc. 134 (1987) 2107. [2] A.F. Danil de Namor, M.A. Llosa Tanco, M. Salomon, J.C.Y. Ng, J. Phys. Chem. 98 (1994) 11796. [3] A.F. Danil de Namor, M.A. Llosa Tanco, M. Salomon, J.C.Y. Ng, J. Phys. Chem. 100 (1996) 14485. [4] A.F. Danil de Namor, L.E. Pulcha Salazar, M.A. Llosa Tanco, D. Kowalska, J.V. Salas, R.A. Schulz, J. Chem. Soc. Faraday Trans. 94 (1998) 3111. [5] P. Johansson, Electrochim. Acta 48 (2003) 2291. [6] J. McBreen, H.S. Lee, X.Q. Yang, X. Sun, J. Power Sources 89 (2000) 163. [7] H.S. Lee, X.Q. Yang, X. Sun, J. McBreen, J. Power Sources 97-98 (2001) 566. [8] H.S. Lee, X.Q. Yang, J. McBreen, L.S. Choi, Y. Okamoto, Electrochim. Acta 40 (1995) 2353. [9] H.S. Lee, X.Q. Yang, J. McBreen, L.S. Choi, Y. Okamoto, J. Electrochem. Soc. 143 (1996) 3825. [10] A.F. Danil de Namor, R.G. Hutcherson, F.J.S. Verlade, M.L.Z. Ormachera, L.E.P. Salazar, I.A. Jammaz, N.A. Rawi, Pure Appl. Chem. 70 (1998) 769. [11] H.S. Lee, X.Q. Yang, C.L. Xiang, J. McBreen, L.S. Choi, J. Electrochem. Soc. 145 (1998) 2813. [12] X. Sun, H.S. Lee, S. Lee, X.Q. Yang, J. McBreen, J. Electrochem. Solid State Lett. 1 (1998) 239. [13] F. Zhou, D.R. MacFarlane, M. Forsyth, Electrochim. Acta 48 (2003) 1749. [14] A.G. Bishop, D.R. MacFarlane, D. McNaughton, M. Forsyth, J. Phys. Chem. 100 (1996) 2237.