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Conductivity in amorphous polyether nanocomposite materials
Solid State Ionics 126 (1999) 269–276 www.elsevier.com / locate / ssi
Conductivity in amorphous polyether nanocomposite materials a,b a b, a A.S. Best , A. Ferry , D.R. MacFarlane *, M. Forsyth a
Department of Materials Engineering, Monash University, Victoria 3168, Australia b Department of Chemistry, Monash University, Victoria 3168, Australia Received 3 June 1999; accepted 7 July 1999
Abstract Using a completely amorphous polyether we have investigated the effect of the inclusion of a nano-particulate filler on a polymer electrolyte. Nano-sized TiO 2 is shown not to significantly affect the conductivity of composite electrolytes containing 1.0 or 1.25 mol / kg LiClO 4 or 1.5 or 2.0 mol / kg LiTFSI. At 1.5 mol / kg LiClO 4 a significant increase in conductivity is observed. Raman spectroscopy experiments have been used to investigate the effect of filler on ionaggregation. Only one new vibrational mode can be assigned to the composite which is not due to the polymer electrolyte or the filler. From this work, we believe the increased conductivity observed by previous researchers as a result of filler addition may be largely attributed to the effect on the degree of crystallinity along with some disruption of ion-aggregation by the fillers in PEO based electrolytes. 1999 Elsevier Science B.V. All rights reserved. Keywords: Amorphous; Polymer nanocomposites; Conductivity; DSC; FT-Raman spectroscopy
1. Introduction Electrolyte materials for solid state lithium batteries are being intensively investigated world-wide. Of the many systems being examined, ceramic / polymer composite materials have many desirable properties for applications in lithium battery technologies [1]. By mixing two different phases together, it may be possible to produce an electrolyte that has the various properties that make them suitable in electrochemical applications [2–5]. Composite electrolytes of poly(ethylene oxide) and insulating or conducting ceramic or polymer phases have recently been at the forefront of this research. PEO is one of the most *Corresponding author. Fax: 1 61-3-9905-4597. E-mail address: [email protected] (D.R. MacFarlane)
extensively studied polyether systems due to its relatively low melting point and T g , its ability to play host to varied lithium salt systems at a range of concentrations and to act as a binder for other phases. For these reasons, PEO has been the basis of many investigations in the area based on composites of a polymer and an insulating ceramic. Studies by Krawiec et al. [3] prepared thin films of a composite containing high molecular weight PEO 8 :LiBF 4 and micron sized A1 2 0 3 particles by solvent casting. As a function of A1 2 0 3 content, a maximum in the conductivity was observed at 10 wt.% A1 2 O 3 in the polymer electrolyte. This maximum was some 3.5 orders of magnitude higher, at 308C, than the maximum conductivity exhibited by the unfilled polymer material. Croce et al. [4] have also reported similar findings, using 10 wt.% of nano-sized TiO 2 or Al 2 O 3 particles in PEO 8 :LiClO 4 .
0167-2738 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 99 )00239-8
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Again, increases in conductivity of 3 orders of magnitude were seen at temperatures up to the melting point of PEO complex. Above the melting point of the complex the effect of the filler on conductivity was only moderate. Wieczorek et al. [6] have investigated the blending of EO–PO polymers with PAAM and other methyl methacrylate systems. The addition of these polymers is thought to break up the ordered structure of the PEO and increases the conductivity by 2 orders of magnitude at 258C. Recent studies have also explored the effect of inclusions of C 60 as a filler on the morphology of PEO-based electrolytes [7]. The conductivity increases in these composite systems cannot be solely attributed to the disruption of crystallisation in PEO. Wieczorek et al. suggested that surface groups on the ceramic particles may play an active role in modifying the local structure of the polymer surrounding the filler [8]. Krawiec suggested, via 7 Li solid state NMR, that two different lithium environments exist, one within the polymer matrix and another surrounding the regions of insulating filler, which may represent a zone of disrupted ion aggregation, producing more mobile ions. One of the key features of previous work has been the use of crystalline PEO, so that it is difficult to discriminate between the effect of the filler on (i) the degree of crystallinity and thereby on conductivity and (ii) any intrinsic mechanism by which the filler might influence ion concentration or mobility in the fully amorphous state. Thus the aim of the work reported here was to test the hypothesis that insulating fillers can increase the conductivity of an amorphous polyether system. A nano-particle filler material has been composited with an exothy–propoxy polyether co-polymer which is well known to be fully amorphous at room temperature [9].
2. Experimental The base amorphous polymer material used was a polyether triol with molecular weight of 5000 g / mol, with ethylene oxide and propylene oxide units in a 3:1 ratio (3PEG). Two different salts were used in this study, LiClO 4 and LiN(CF 3 SO 2 ) 2 . Three different salt concentrations were investigated for LiClO 4 : 1.0, 1.25 and 1.5 mol / kg; for LiN(CF 3 SO 3 ) 2 (LiT-
FSI) 1.5 and 2 mol / kg were investigated. All salt concentrations are expressed as mol (salt) per kg (polymer). 1 mol / kg LiClO 4 or LiN(CF 3 SO 3 ) 2 is approximately the point of maximum conductivity at room temperature in the 3PEG system. Degussa P25 TiO 2 was used as a filler; this was dried at 2508C for 24 hours to ensure the powders were dry. All samples were prepared in a dry box by mixing 10 wt.% TiO 2 into the polymer–salt mixture plus Thorcat catalyst. The sample was then heated on a hot plate to decrease the viscosity of the polymer and remove any adsorbed nitrogen from the mixture. At this point some of the mixture was removed to measure the conductivity of the uncured system. Hexamethylene diisocyanate (Aldrich, used as received) crosslinking agent was then added stoichiometrically to the sample followed by vigorous stirring. The resulting mixture was then poured into a teflon-lined mould and compressed before curing over 3 days at 558C. Cured samples are relatively hard elastomeric materials. Differential scanning calorimetry (DSC) analysis was performed on a Perkin Elmer DSC-7 operating in temperature range of 2 1208C and 208C. The instrument was calibrated using cyclohexane. Measurements were completed at a scan rate of 208C / min, with T g determined as the onset of the transition. A.C. impedance measurements were performed using a Hewlett Packard 4192A meter at 100 mV signal amplitude between 208C and 1208C. Four different groups of samples were investigated; the uncured polymer–salt systems, the uncured filled polymer–salt systems, the cured polymer–salt systems, and the cured filled polymer–salt systems. Cured samples required silver blocking electrodes, applied using silver paint. The uncured sample measurements were completed in a specially designed cell with stainless steel blocking electrodes and calibrated using a 10 23 M demal KCl solution to determine the cell constant. All measurements were completed in a nitrogen atmosphere to prevent moisture uptake by the samples. FT-Raman spectra were recorded at room temperature using a Bruker IFS 66 with a Raman module FRA 106 and a near infrared YAG laser with wavelength 1064 nm. In order to optimize the measurement time, with respect to peak separation
A.S. Best et al. / Solid State Ionics 126 (1999) 269 – 276
and signal to noise ratio, the Raman spectra were recorded at a resolution of 3.0 cm 21 using the Happ-Genzel apodization function.
3. Results and discussion
3.1. Differential scanning calorimetry Differential scanning calorimetry (DSC) has been used to characterise the effect of the filler upon the glass transition temperature. 3PEG is a traditional polymer electrolyte in that for lithium conduction to occur, the polymer must be in the rubbery state where significant segmental motion can occur. It has been noted by previous researchers that polymer or ceramic fillers can decrease T g [6]. Table 1 shows the DSC data for the systems investigated in this work. 3PEG has a T g of approximately 2 608C. The Table 1 T g of the various polymers and nano-composites examined in this work Salt Conc.
1 mol / kg LiClO 4 1.25 mol / kg LiClO 4 1.5 mol / kg LiClO 4
T g Unfilled628C
T g Filled628C
Uncured
Cured
Uncured
Cured
2 55 2 47 2 40
2 47 2 41 2 37
2 52 2 42 2 41
2 52 2 42 2 37
271
addition of LiClO 4 showed single T g ’s between 2 408C and 2 558C. As salt concentration is increased in all cases, T g can be seen to increase. However, there is no generally significant change in T g on the addition of the filler in the cured and uncured states. This suggests that the filler does not have an effect on the dynamics of the polymer which is substantial enough to influence this bulk property.
4. Conductivity Fig. 1 shows the conductivity of the 3PEG– LiClO 4 –TiO 2 samples at 1 mol / kg LiClO 4 in the uncured and cured polyether. In both the cured and uncured cases, the filler does not produce any significant increase in conductivity. From simple considerations one might expect a slight lowering of the conductivity due to the displacing and impeding effect of the non-conducting material. The volume fraction is, however, low ( , 5%) and an effect of this magnitude would be barely discernible in Fig. 1. Fig. 2 shows that the filler also has no effect at a salt concentration of 1.25 mol / kg LiClO 4 . Fig. 3 shows the 1.5 mol / kg LiClO 4 salt concentration. In the uncured state, the filler does not improve the conductivity; in fact, there is a slight decrease in conductivity. In the cured sample, the
Fig. 1. Arrhenius plot of 3PEG 1.0 mol / kg LiClO 4 in: (s) uncured state, (h) cured state, (d) with 10 wt.% Degussa TiO 2 uncured and (j) with 10 wt.% Degussa TiO 2 cured.
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Fig. 2. Arrhenius plot of 3PEG 1.25 mol / kg LiClO 4 in: (s) uncured state, (h) cured state, (d) with 10 wt.% Degussa TiO 2 uncured and (j) with 10 wt.% Degussa TiO 2 cured.
Fig. 3. Arrhenius plot of 3PEG 1.5 mol / kg LiClO 4 in: (s) uncured state, (h) cured state, (d) with 10 wt.% Degussa TiO 2 uncured and (j) with 10 wt.% Degussa TiO 2 cured.
filler improves the high temperature conductivity (i.e. above 708C) by a factor of 3, compared to that of the unfilled sample. This conductivity increase cannot be ascribed to changes in crystallinity or crosslink density since the polymer electrolyte is completely amorphous and, if anything, the filler appears to assist the crosslinking reaction at this high salt loading. LiClO 4 was insoluble in our 3PEG polymer beyond 1.5 mol / kg, a much lower solubility limit than in the PEO polymer described in the work of Croce et al. This is most likely due to the lower
molecular weight of the pre-polymer (i.e. prior to crosslinking) involved. In all cases, the results obtained in this work for the unfilled LiClO 4 data are completely consistent with previous independent investigations in our laboratories [9,10]. It is clear from our results that, at the lowest lithium salt concentrations studied here (1.0, 1.25 mol / kg LiClO 4 and 1.5 mol / kg LiTFSI), there is no effect of filler on the conductivity. This is particularly true for the lower temperature measurements although it seems that, at higher temperatures,
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the curvature usually observed for ionic conductivity in this family of polymer electrolytes (unfilled) is not apparent when the TiO 2 nano-particles are added. The curvature has been ascribed to enhanced ion pairing and ion aggregation at high temperatures due to the entropic losses associated with ion-polymer coordination [11]. Wieczorek et al. [8,12] have investigated a similar system involving a liquid PEG (monomethoxycapped) of molecular weight 5 350 over a large range of lithium salt concentrations. In the system PEG–LiClO 4 –a-A1 2 O 3 system, an increase in conductivity was seen in the uncrosslinked state, the largest increase noted occurring at 1.5 mol / kg, where the filler caused a conductivity increase of 0.5 orders of magnitude at 208C. An effect this large was not seen in this work although a significant effect was seen above 708C. As the salt concentration was further increased, this effect became less significant; similarly at concentrations # 1 mol / kg no effect was seen by Wieczorek. The effect of the hydroxy groups on the PEG system compared to 3PEG (approximately 10 times higher concentration in the former case) is sufficient to increase the conductivity significantly at all concentrations. Fig. 4 shows the temperature dependence of conductivity of the LiTFSI based electrolytes. The 2 mol / kg salt concentration produces conductivity lower than that of 1.5 mol / kg in both the cured and
273
uncured states. This is consistent with the observations of previous workers and is attributed to the effect of ion pairing and of an increasing T g . The addition of TiO 2 to these systems produces no significant increase in conductivity. The initial rationale for the use of LiTFSI was to investigate the hypothesis that the filler was acting to decrease cation–polymer interactions thereby freeing the lithium ion. This was suggested by the data of Wiezorek et al. in the case of low molecular weight PEG hosts with LiClO 4 salts. More Li-polymer interactions were expected in the LiTFSI–polymer system, the LiTFSI being thought to be less associated in polyether electrolytes [13]. Thus, it was thought that a greater effect would be observed upon the addition of TiO 2 . As can be seen in Fig. 4, clearly this was not the case and hence the hypothesis appears to be disproved. That the enhancement of conductivity is only observed for the highest LiClO 4 salt concentration and for the highest temperatures is consistent with the idea that the filler influences the ion-aggregation interaction. For example, if the TiO 2 was able to interact with either or both the anion and cation thereby reducing ion pairing, an enhancement in conductivity could be expected due to the increase in the number (and decrease in the size) of the charge carriers. Further evidence for this mechanism of filler action comes from the LiTFSI results which show
Fig. 4. Arrhenius plot of 3PEG 2 mol / kg LiTFSI in: (s) uncured state, (d) with 10 wt.% Degussa TiO 2 uncured and (j) with 10 wt.% Degussa TiO 2 cured. Also shown is the 3PEG 1.5 mol / kg electrolytes in (앳) uncured state, (n) cured state and with (♦) 10 wt.% Degussa TiO 2 uncured.
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that, even at 2 mol / kg, insignificant changes are observed in conductivity. LiTFSI is known to aggregate to a much lesser degree than other salts and hence leads to some of the highest conductivities observed in SPE’s [14]. It is also notable that the enhancement of conductivity brought about by the addition of TiO 2 to the LiClO 4 containing SPE’s brings the conductivity of this system up to the same level as the unfilled LiTFSI samples. Comparing the overall levels of conductivity observed in this work with the work of Croce et al., in Fig. 5, it can be seen that the polymer electrolytes used in this work, before the addition of TiO 2 , are significantly more conductive than the PEO 8 :LiClO 4 system studied by Croce et al. and Krawiec et al. The effect of TiO 2 on conductivity below the melting point can thus be attributed in the latter cases, at least in part, to the lowering of the degree of crystallinity that such fillers can induce. However, conductivity increases above the melting point of PEO supports the hypothesis that the TiO 2 disrupts ion aggregation. This is further supported by Wiezorek et al’s results, whereby the largest effect of filler was observed at 1.5 mol / kg. This is to be expected since ion-aggregation is likely to be significantly greater at these salt contents. In addition, the effects observed in this work are not as substantial as those reported by Weiczorek et al. and Krawiec et al. In the former work, use of low
molecular weight polymers has a significant impact on ion-aggregation since the entropy changes on ion solvation are greater in the low molecular weight polymer solvents.
5. Raman spectroscopy Fig. 6 shows representative Raman spectra covering a spectral region including the symmetric n1 (A) ClO 4 stretching mode at | 931 cm 21 . This mode is non-degenerate, consequently, any observed splitting means that different chemical environments are present within the polymer. A component at | 932– 934 cm 21 stems from ClO 42 anions not interacting directly with Li cations. Ion-pairing is typically manifested as higher-frequency components (i.e., 937–939, 947, 958 cm 21 , respectively) [15–17] which have been observed in various liquid solvents. The studies of Schantz et al. [18–20] have observed the 938–939 cm 21 band in poly(propylene glycol) and poly(propylene oxide), the band being much stronger at high salt contents ( . 2 mol / kg) and at high molecular weights. As seen in Fig. 6, no distinct high-frequency components are found in the present work, even at a salt-content of 1.5 mol / kg in the cured sample with filler, though the n1 (A) ClO 4 stretching mode shows a possible shoulder at 939 cm 21 in this sample as compared to an uncured
Fig. 5. Arrhenius plot of 3PEG 1.5 mol / kg LiClO 4 in: (앳) uncured state, (n) cured state and with (♦) 10 wt.% Degussa TiO 2 uncured and (m) 10 wt.% Degussa TiO 2 cured. Compared to that of Croce [4] (s) PEO 8 :LiClO 4 unfilled, (h) PEO 8 :LiClO 4 with 10 wt.% TiO 2 (heating scan), and Krawiec [3] (d) PEO 8 :LiBF 4 unfilled and (j) PEO 8 :LiClO 4 with 10 wt.% A1 2 O 3 .
A.S. Best et al. / Solid State Ionics 126 (1999) 269 – 276
Fig. 6. Raman spectra of the filled cured and unfilled uncured 3PEG 1.5 mol / kg LiClO 4 electrolytes between 700–1200 wavenumbers. Circled region shows where ion aggregation is normally seen. TiO 2 spectra not shown as there is no new information in this spectral region.
electrolyte with no added filler. In this latter case, however, we expect the ion-solvation to be influenced strongly by interactions with the terminal polymer hydroxy groups [21]. As an indication of an interaction involving the ceramic filler we note a new mode appearing at | 537 cm 21 , see Fig. 7. Based on existing literature assignments and calculations for the ClO 2 4 anion, this is not a mode stemming from the anion. The dry TiO 2 powder only shows Raman bands at | 145 (strong), 398, 517, and 637 cm 21 , respectively (not shown). The 537 cm 21 mode is not present in electrolyte samples without filler nor in spectra of dry LiClO 4 salt. At present we cannot conclusively assign this spectral feature to any specific moieties or interactions of these materials, other than to note that it does indicate an effect of the nano-size filler on the vibrational signature of the composite compound. We note the change in intensity of the DLAM bands between 200 and 300 cm 21 . These bands have
275
Fig. 7. Raman spectra of the filled cured and unfilled uncured electrolytes and the TiO 2 powder between 200–700 wavenumbers. The circled region between 200–300 wavenumbers shows the suppression of the DLAM bands. The second circled region at | 530 wavenumbers shows a possible interaction between the polymer electrolyte and the filler.
been previously assigned to longitudinal backbone motions of the polymer chain [16]. As can be noted in Fig. 7, the intensity of these bands decreases significantly on the addition of the filler and curing. We are currently separately investigating the effect of curing and the nano-particulate filler effects on these bands, in order to discern which is of greater influence on these modes. The inclusion of TiO 2 does not affect any of the conformation sensitive modes in the CH 2 spectral regions ( | 800–900 cm 21 or 2800–3100 cm 21 ).
6. Conclusions We have investigated the effect of TiO 2 nanosized ceramic filler on the conductivity of lithium salt containing, fully amorphous polyether electrolytes as a function of salt type, concentration and
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temperature. It is clear from our results that, at the lowest lithium salt concentrations studied here (1, 1.25 mol / kg in the case of LiClO 4 and 1.5 in LiTFSI), there is no effect of filler on conductivity. At 1.5 mol / kg LiClO 4 , the filler influences conductivity, perhaps by lowering the degree of ionaggregation within the polymer–salt mixture, due to interactions between the cation / anion and the filler. However, the effect of filler in this instance is only to increase the conductivity to that of LiTFSI samples, which are known to have a lesser degree of ion aggregation. The several orders of magnitude increase in conductivity observed by Croce et al. compared to the factor of 2–3 observed by us in this work, can be rationalised as follows. At temperatures below the melting point of PEO, the marked increase in conductivity is primarily due to the suppression of crystallinity by the filler. Beyond the melting point of PEO, given the extremely high salt concentrations used, the filler acts to disrupt ion aggregation, probably by interaction directly with the ionic species. Evidence for this in our work is provided by Raman spectroscopy which indicates significant changes in the composite bands, probably due to TiO 2 –salt interactions. In the absence of the marked ion aggregation which is present at such high salt concentrations, significantly higher ionic conductivities are observed. Thus it appears that the effect of the nano-particulate filler is primarily to diminish the effect of ion-aggregation in electrolytes at concentrations where such aggregation is severe. In the electrolytes studied here, which are of a lower degree of aggregation, the effect of filler is much less substantial.
Acknowledgements Degussa Corporation for kindly providing the titania powder used in this work. Thank you also to
Kate Nairn for helpful discussions in this area. A.F. gratefully acknowledges support from the WennerGren foundation.
References [1] E. Quartarone, P. Mustarelli, A. Magistris, Solid State lonics 110 (1998) 1. [2] W. Wieczorek, Z. Florjanczyk, J.R. Stevens, Electrochimica Acta 40 (13 / 14) (1995) 2251. [3] L. Krawiec, L.G. Scanlon Jr., J.P. Fellner, R.A. Vaia, S. Vasudevan, E.P. Giannelis, J. Power Sources 54 (1995) 310–315. [4] F. Croce, G.B. Appetecchi, L. Persi, B. Scrosati, Nature 394 (1998) 456–458. [5] K.M. Nairn, A.S. Best, P.J. Newman, D.R. MacFarlane, M. Forsyth, Solid State Ionics, 1999 (in press) [6] W. Wieczorek, J.R. Stevens, J. Phys. Chem. B 101 (1997) 1529–1534. [7] L. Edman, A. Ferry, P. Jacobsson, Macromolecules, in press. [8] W. Wieczorek, P. Lipka, G. Zukowska, Wycislik, J. Phys. Chem. B 102 (1998) 6968–6974. [9] M. Forsyth, P. Meakin, D.R. MacFarlane, A.J. Hill, J. Phys. Condens. Matter 7 (1995) 7601–7617. [10] S.T.C. Ng, M. Forsyth, D.R. MacFarlane, M. Garcia, M.E. Smith, J.H. Strange, Polymer 39 (1998) 6261–6268. [11] S.T.C. Ng, Monash University, 1999, Ph.D. Thesis. [12] W. Weiczorek, Private communication; Fig. 1 of ref. [8] contains a y-axis error which causes the data to appear to have a conductivity 1 order of magnitude lower than actual. [13] A.G. Bishop, D.R. MacFarlane, D. McNaughton, M. Forsyth, Solid State Ionics 85 (1996) 129–135. [14] M. Armand, Solid State lonics 89 (1996) 5–7. [15] D.W. James, R.E. Mayes, Aust. J. Chem. 35 (1982) 1775– 1784. [16] S. Schantz, L.M. Torell, J.R. Stevens, J. Appl. Phys. 64 (1988) 2038–2043. [17] A. Ferry, P. Jacobsson, J.D. van Heumen, J.R. Stevens, Polymer 37 (1996) 737. [18] S. Schantz, L.M. Torell, J.R. Stevens, J. Appl. Phys. 64 (1988) 2038. [19] S. Schantz, L.M. Torell, J.R. Stevens, J. Chem. Phys. 94 (1991) 6862. [20] S. Schantz, J. Chem. Phys. 94 (1991) 6296. [21] A. Ferry, M. Tian, Macromolecules 30 (1997) 1214.
Conductivity in amorphous polyether nanocomposite materials a,b a b, a A.S. Best , A. Ferry , D.R. MacFarlane *, M. Forsyth a
Department of Materials Engineering, Monash University, Victoria 3168, Australia b Department of Chemistry, Monash University, Victoria 3168, Australia Received 3 June 1999; accepted 7 July 1999
Abstract Using a completely amorphous polyether we have investigated the effect of the inclusion of a nano-particulate filler on a polymer electrolyte. Nano-sized TiO 2 is shown not to significantly affect the conductivity of composite electrolytes containing 1.0 or 1.25 mol / kg LiClO 4 or 1.5 or 2.0 mol / kg LiTFSI. At 1.5 mol / kg LiClO 4 a significant increase in conductivity is observed. Raman spectroscopy experiments have been used to investigate the effect of filler on ionaggregation. Only one new vibrational mode can be assigned to the composite which is not due to the polymer electrolyte or the filler. From this work, we believe the increased conductivity observed by previous researchers as a result of filler addition may be largely attributed to the effect on the degree of crystallinity along with some disruption of ion-aggregation by the fillers in PEO based electrolytes. 1999 Elsevier Science B.V. All rights reserved. Keywords: Amorphous; Polymer nanocomposites; Conductivity; DSC; FT-Raman spectroscopy
1. Introduction Electrolyte materials for solid state lithium batteries are being intensively investigated world-wide. Of the many systems being examined, ceramic / polymer composite materials have many desirable properties for applications in lithium battery technologies [1]. By mixing two different phases together, it may be possible to produce an electrolyte that has the various properties that make them suitable in electrochemical applications [2–5]. Composite electrolytes of poly(ethylene oxide) and insulating or conducting ceramic or polymer phases have recently been at the forefront of this research. PEO is one of the most *Corresponding author. Fax: 1 61-3-9905-4597. E-mail address: [email protected] (D.R. MacFarlane)
extensively studied polyether systems due to its relatively low melting point and T g , its ability to play host to varied lithium salt systems at a range of concentrations and to act as a binder for other phases. For these reasons, PEO has been the basis of many investigations in the area based on composites of a polymer and an insulating ceramic. Studies by Krawiec et al. [3] prepared thin films of a composite containing high molecular weight PEO 8 :LiBF 4 and micron sized A1 2 0 3 particles by solvent casting. As a function of A1 2 0 3 content, a maximum in the conductivity was observed at 10 wt.% A1 2 O 3 in the polymer electrolyte. This maximum was some 3.5 orders of magnitude higher, at 308C, than the maximum conductivity exhibited by the unfilled polymer material. Croce et al. [4] have also reported similar findings, using 10 wt.% of nano-sized TiO 2 or Al 2 O 3 particles in PEO 8 :LiClO 4 .
0167-2738 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 99 )00239-8
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Again, increases in conductivity of 3 orders of magnitude were seen at temperatures up to the melting point of PEO complex. Above the melting point of the complex the effect of the filler on conductivity was only moderate. Wieczorek et al. [6] have investigated the blending of EO–PO polymers with PAAM and other methyl methacrylate systems. The addition of these polymers is thought to break up the ordered structure of the PEO and increases the conductivity by 2 orders of magnitude at 258C. Recent studies have also explored the effect of inclusions of C 60 as a filler on the morphology of PEO-based electrolytes [7]. The conductivity increases in these composite systems cannot be solely attributed to the disruption of crystallisation in PEO. Wieczorek et al. suggested that surface groups on the ceramic particles may play an active role in modifying the local structure of the polymer surrounding the filler [8]. Krawiec suggested, via 7 Li solid state NMR, that two different lithium environments exist, one within the polymer matrix and another surrounding the regions of insulating filler, which may represent a zone of disrupted ion aggregation, producing more mobile ions. One of the key features of previous work has been the use of crystalline PEO, so that it is difficult to discriminate between the effect of the filler on (i) the degree of crystallinity and thereby on conductivity and (ii) any intrinsic mechanism by which the filler might influence ion concentration or mobility in the fully amorphous state. Thus the aim of the work reported here was to test the hypothesis that insulating fillers can increase the conductivity of an amorphous polyether system. A nano-particle filler material has been composited with an exothy–propoxy polyether co-polymer which is well known to be fully amorphous at room temperature [9].
2. Experimental The base amorphous polymer material used was a polyether triol with molecular weight of 5000 g / mol, with ethylene oxide and propylene oxide units in a 3:1 ratio (3PEG). Two different salts were used in this study, LiClO 4 and LiN(CF 3 SO 2 ) 2 . Three different salt concentrations were investigated for LiClO 4 : 1.0, 1.25 and 1.5 mol / kg; for LiN(CF 3 SO 3 ) 2 (LiT-
FSI) 1.5 and 2 mol / kg were investigated. All salt concentrations are expressed as mol (salt) per kg (polymer). 1 mol / kg LiClO 4 or LiN(CF 3 SO 3 ) 2 is approximately the point of maximum conductivity at room temperature in the 3PEG system. Degussa P25 TiO 2 was used as a filler; this was dried at 2508C for 24 hours to ensure the powders were dry. All samples were prepared in a dry box by mixing 10 wt.% TiO 2 into the polymer–salt mixture plus Thorcat catalyst. The sample was then heated on a hot plate to decrease the viscosity of the polymer and remove any adsorbed nitrogen from the mixture. At this point some of the mixture was removed to measure the conductivity of the uncured system. Hexamethylene diisocyanate (Aldrich, used as received) crosslinking agent was then added stoichiometrically to the sample followed by vigorous stirring. The resulting mixture was then poured into a teflon-lined mould and compressed before curing over 3 days at 558C. Cured samples are relatively hard elastomeric materials. Differential scanning calorimetry (DSC) analysis was performed on a Perkin Elmer DSC-7 operating in temperature range of 2 1208C and 208C. The instrument was calibrated using cyclohexane. Measurements were completed at a scan rate of 208C / min, with T g determined as the onset of the transition. A.C. impedance measurements were performed using a Hewlett Packard 4192A meter at 100 mV signal amplitude between 208C and 1208C. Four different groups of samples were investigated; the uncured polymer–salt systems, the uncured filled polymer–salt systems, the cured polymer–salt systems, and the cured filled polymer–salt systems. Cured samples required silver blocking electrodes, applied using silver paint. The uncured sample measurements were completed in a specially designed cell with stainless steel blocking electrodes and calibrated using a 10 23 M demal KCl solution to determine the cell constant. All measurements were completed in a nitrogen atmosphere to prevent moisture uptake by the samples. FT-Raman spectra were recorded at room temperature using a Bruker IFS 66 with a Raman module FRA 106 and a near infrared YAG laser with wavelength 1064 nm. In order to optimize the measurement time, with respect to peak separation
A.S. Best et al. / Solid State Ionics 126 (1999) 269 – 276
and signal to noise ratio, the Raman spectra were recorded at a resolution of 3.0 cm 21 using the Happ-Genzel apodization function.
3. Results and discussion
3.1. Differential scanning calorimetry Differential scanning calorimetry (DSC) has been used to characterise the effect of the filler upon the glass transition temperature. 3PEG is a traditional polymer electrolyte in that for lithium conduction to occur, the polymer must be in the rubbery state where significant segmental motion can occur. It has been noted by previous researchers that polymer or ceramic fillers can decrease T g [6]. Table 1 shows the DSC data for the systems investigated in this work. 3PEG has a T g of approximately 2 608C. The Table 1 T g of the various polymers and nano-composites examined in this work Salt Conc.
1 mol / kg LiClO 4 1.25 mol / kg LiClO 4 1.5 mol / kg LiClO 4
T g Unfilled628C
T g Filled628C
Uncured
Cured
Uncured
Cured
2 55 2 47 2 40
2 47 2 41 2 37
2 52 2 42 2 41
2 52 2 42 2 37
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addition of LiClO 4 showed single T g ’s between 2 408C and 2 558C. As salt concentration is increased in all cases, T g can be seen to increase. However, there is no generally significant change in T g on the addition of the filler in the cured and uncured states. This suggests that the filler does not have an effect on the dynamics of the polymer which is substantial enough to influence this bulk property.
4. Conductivity Fig. 1 shows the conductivity of the 3PEG– LiClO 4 –TiO 2 samples at 1 mol / kg LiClO 4 in the uncured and cured polyether. In both the cured and uncured cases, the filler does not produce any significant increase in conductivity. From simple considerations one might expect a slight lowering of the conductivity due to the displacing and impeding effect of the non-conducting material. The volume fraction is, however, low ( , 5%) and an effect of this magnitude would be barely discernible in Fig. 1. Fig. 2 shows that the filler also has no effect at a salt concentration of 1.25 mol / kg LiClO 4 . Fig. 3 shows the 1.5 mol / kg LiClO 4 salt concentration. In the uncured state, the filler does not improve the conductivity; in fact, there is a slight decrease in conductivity. In the cured sample, the
Fig. 1. Arrhenius plot of 3PEG 1.0 mol / kg LiClO 4 in: (s) uncured state, (h) cured state, (d) with 10 wt.% Degussa TiO 2 uncured and (j) with 10 wt.% Degussa TiO 2 cured.
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Fig. 2. Arrhenius plot of 3PEG 1.25 mol / kg LiClO 4 in: (s) uncured state, (h) cured state, (d) with 10 wt.% Degussa TiO 2 uncured and (j) with 10 wt.% Degussa TiO 2 cured.
Fig. 3. Arrhenius plot of 3PEG 1.5 mol / kg LiClO 4 in: (s) uncured state, (h) cured state, (d) with 10 wt.% Degussa TiO 2 uncured and (j) with 10 wt.% Degussa TiO 2 cured.
filler improves the high temperature conductivity (i.e. above 708C) by a factor of 3, compared to that of the unfilled sample. This conductivity increase cannot be ascribed to changes in crystallinity or crosslink density since the polymer electrolyte is completely amorphous and, if anything, the filler appears to assist the crosslinking reaction at this high salt loading. LiClO 4 was insoluble in our 3PEG polymer beyond 1.5 mol / kg, a much lower solubility limit than in the PEO polymer described in the work of Croce et al. This is most likely due to the lower
molecular weight of the pre-polymer (i.e. prior to crosslinking) involved. In all cases, the results obtained in this work for the unfilled LiClO 4 data are completely consistent with previous independent investigations in our laboratories [9,10]. It is clear from our results that, at the lowest lithium salt concentrations studied here (1.0, 1.25 mol / kg LiClO 4 and 1.5 mol / kg LiTFSI), there is no effect of filler on the conductivity. This is particularly true for the lower temperature measurements although it seems that, at higher temperatures,
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the curvature usually observed for ionic conductivity in this family of polymer electrolytes (unfilled) is not apparent when the TiO 2 nano-particles are added. The curvature has been ascribed to enhanced ion pairing and ion aggregation at high temperatures due to the entropic losses associated with ion-polymer coordination [11]. Wieczorek et al. [8,12] have investigated a similar system involving a liquid PEG (monomethoxycapped) of molecular weight 5 350 over a large range of lithium salt concentrations. In the system PEG–LiClO 4 –a-A1 2 O 3 system, an increase in conductivity was seen in the uncrosslinked state, the largest increase noted occurring at 1.5 mol / kg, where the filler caused a conductivity increase of 0.5 orders of magnitude at 208C. An effect this large was not seen in this work although a significant effect was seen above 708C. As the salt concentration was further increased, this effect became less significant; similarly at concentrations # 1 mol / kg no effect was seen by Wieczorek. The effect of the hydroxy groups on the PEG system compared to 3PEG (approximately 10 times higher concentration in the former case) is sufficient to increase the conductivity significantly at all concentrations. Fig. 4 shows the temperature dependence of conductivity of the LiTFSI based electrolytes. The 2 mol / kg salt concentration produces conductivity lower than that of 1.5 mol / kg in both the cured and
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uncured states. This is consistent with the observations of previous workers and is attributed to the effect of ion pairing and of an increasing T g . The addition of TiO 2 to these systems produces no significant increase in conductivity. The initial rationale for the use of LiTFSI was to investigate the hypothesis that the filler was acting to decrease cation–polymer interactions thereby freeing the lithium ion. This was suggested by the data of Wiezorek et al. in the case of low molecular weight PEG hosts with LiClO 4 salts. More Li-polymer interactions were expected in the LiTFSI–polymer system, the LiTFSI being thought to be less associated in polyether electrolytes [13]. Thus, it was thought that a greater effect would be observed upon the addition of TiO 2 . As can be seen in Fig. 4, clearly this was not the case and hence the hypothesis appears to be disproved. That the enhancement of conductivity is only observed for the highest LiClO 4 salt concentration and for the highest temperatures is consistent with the idea that the filler influences the ion-aggregation interaction. For example, if the TiO 2 was able to interact with either or both the anion and cation thereby reducing ion pairing, an enhancement in conductivity could be expected due to the increase in the number (and decrease in the size) of the charge carriers. Further evidence for this mechanism of filler action comes from the LiTFSI results which show
Fig. 4. Arrhenius plot of 3PEG 2 mol / kg LiTFSI in: (s) uncured state, (d) with 10 wt.% Degussa TiO 2 uncured and (j) with 10 wt.% Degussa TiO 2 cured. Also shown is the 3PEG 1.5 mol / kg electrolytes in (앳) uncured state, (n) cured state and with (♦) 10 wt.% Degussa TiO 2 uncured.
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that, even at 2 mol / kg, insignificant changes are observed in conductivity. LiTFSI is known to aggregate to a much lesser degree than other salts and hence leads to some of the highest conductivities observed in SPE’s [14]. It is also notable that the enhancement of conductivity brought about by the addition of TiO 2 to the LiClO 4 containing SPE’s brings the conductivity of this system up to the same level as the unfilled LiTFSI samples. Comparing the overall levels of conductivity observed in this work with the work of Croce et al., in Fig. 5, it can be seen that the polymer electrolytes used in this work, before the addition of TiO 2 , are significantly more conductive than the PEO 8 :LiClO 4 system studied by Croce et al. and Krawiec et al. The effect of TiO 2 on conductivity below the melting point can thus be attributed in the latter cases, at least in part, to the lowering of the degree of crystallinity that such fillers can induce. However, conductivity increases above the melting point of PEO supports the hypothesis that the TiO 2 disrupts ion aggregation. This is further supported by Wiezorek et al’s results, whereby the largest effect of filler was observed at 1.5 mol / kg. This is to be expected since ion-aggregation is likely to be significantly greater at these salt contents. In addition, the effects observed in this work are not as substantial as those reported by Weiczorek et al. and Krawiec et al. In the former work, use of low
molecular weight polymers has a significant impact on ion-aggregation since the entropy changes on ion solvation are greater in the low molecular weight polymer solvents.
5. Raman spectroscopy Fig. 6 shows representative Raman spectra covering a spectral region including the symmetric n1 (A) ClO 4 stretching mode at | 931 cm 21 . This mode is non-degenerate, consequently, any observed splitting means that different chemical environments are present within the polymer. A component at | 932– 934 cm 21 stems from ClO 42 anions not interacting directly with Li cations. Ion-pairing is typically manifested as higher-frequency components (i.e., 937–939, 947, 958 cm 21 , respectively) [15–17] which have been observed in various liquid solvents. The studies of Schantz et al. [18–20] have observed the 938–939 cm 21 band in poly(propylene glycol) and poly(propylene oxide), the band being much stronger at high salt contents ( . 2 mol / kg) and at high molecular weights. As seen in Fig. 6, no distinct high-frequency components are found in the present work, even at a salt-content of 1.5 mol / kg in the cured sample with filler, though the n1 (A) ClO 4 stretching mode shows a possible shoulder at 939 cm 21 in this sample as compared to an uncured
Fig. 5. Arrhenius plot of 3PEG 1.5 mol / kg LiClO 4 in: (앳) uncured state, (n) cured state and with (♦) 10 wt.% Degussa TiO 2 uncured and (m) 10 wt.% Degussa TiO 2 cured. Compared to that of Croce [4] (s) PEO 8 :LiClO 4 unfilled, (h) PEO 8 :LiClO 4 with 10 wt.% TiO 2 (heating scan), and Krawiec [3] (d) PEO 8 :LiBF 4 unfilled and (j) PEO 8 :LiClO 4 with 10 wt.% A1 2 O 3 .
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Fig. 6. Raman spectra of the filled cured and unfilled uncured 3PEG 1.5 mol / kg LiClO 4 electrolytes between 700–1200 wavenumbers. Circled region shows where ion aggregation is normally seen. TiO 2 spectra not shown as there is no new information in this spectral region.
electrolyte with no added filler. In this latter case, however, we expect the ion-solvation to be influenced strongly by interactions with the terminal polymer hydroxy groups [21]. As an indication of an interaction involving the ceramic filler we note a new mode appearing at | 537 cm 21 , see Fig. 7. Based on existing literature assignments and calculations for the ClO 2 4 anion, this is not a mode stemming from the anion. The dry TiO 2 powder only shows Raman bands at | 145 (strong), 398, 517, and 637 cm 21 , respectively (not shown). The 537 cm 21 mode is not present in electrolyte samples without filler nor in spectra of dry LiClO 4 salt. At present we cannot conclusively assign this spectral feature to any specific moieties or interactions of these materials, other than to note that it does indicate an effect of the nano-size filler on the vibrational signature of the composite compound. We note the change in intensity of the DLAM bands between 200 and 300 cm 21 . These bands have
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Fig. 7. Raman spectra of the filled cured and unfilled uncured electrolytes and the TiO 2 powder between 200–700 wavenumbers. The circled region between 200–300 wavenumbers shows the suppression of the DLAM bands. The second circled region at | 530 wavenumbers shows a possible interaction between the polymer electrolyte and the filler.
been previously assigned to longitudinal backbone motions of the polymer chain [16]. As can be noted in Fig. 7, the intensity of these bands decreases significantly on the addition of the filler and curing. We are currently separately investigating the effect of curing and the nano-particulate filler effects on these bands, in order to discern which is of greater influence on these modes. The inclusion of TiO 2 does not affect any of the conformation sensitive modes in the CH 2 spectral regions ( | 800–900 cm 21 or 2800–3100 cm 21 ).
6. Conclusions We have investigated the effect of TiO 2 nanosized ceramic filler on the conductivity of lithium salt containing, fully amorphous polyether electrolytes as a function of salt type, concentration and
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temperature. It is clear from our results that, at the lowest lithium salt concentrations studied here (1, 1.25 mol / kg in the case of LiClO 4 and 1.5 in LiTFSI), there is no effect of filler on conductivity. At 1.5 mol / kg LiClO 4 , the filler influences conductivity, perhaps by lowering the degree of ionaggregation within the polymer–salt mixture, due to interactions between the cation / anion and the filler. However, the effect of filler in this instance is only to increase the conductivity to that of LiTFSI samples, which are known to have a lesser degree of ion aggregation. The several orders of magnitude increase in conductivity observed by Croce et al. compared to the factor of 2–3 observed by us in this work, can be rationalised as follows. At temperatures below the melting point of PEO, the marked increase in conductivity is primarily due to the suppression of crystallinity by the filler. Beyond the melting point of PEO, given the extremely high salt concentrations used, the filler acts to disrupt ion aggregation, probably by interaction directly with the ionic species. Evidence for this in our work is provided by Raman spectroscopy which indicates significant changes in the composite bands, probably due to TiO 2 –salt interactions. In the absence of the marked ion aggregation which is present at such high salt concentrations, significantly higher ionic conductivities are observed. Thus it appears that the effect of the nano-particulate filler is primarily to diminish the effect of ion-aggregation in electrolytes at concentrations where such aggregation is severe. In the electrolytes studied here, which are of a lower degree of aggregation, the effect of filler is much less substantial.
Acknowledgements Degussa Corporation for kindly providing the titania powder used in this work. Thank you also to
Kate Nairn for helpful discussions in this area. A.F. gratefully acknowledges support from the WennerGren foundation.
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