J . Phys. Chem. 1994,98, 9047-9055
9047
Comparison of Properties of Composite Polymeric Electrolytes Based on the Oxymethylene-Linked Poly(ethylene oxide) NaC104 Electrolyte with Polyacrylamide or cu-AlzO3 Additives W. Wieczorek, K. Such, S. H. Chung, and J. R. Stevens’ Department of Physics, University of Guelph, N1 G 2Wl Guelph, Ontario, Canada Received: May 27, 1994; In Final Form: July I , 1994”
A comparison is made between two, amorphous, composite polyether electrolytes; one with an inorganic filler a-A1203and the other with an organic filler, poly(acry1amide) (PAAM). Oxymethylene-linked poly(ethy1ene oxide) (OMPEO) is used as the polyether and NaC104 as the salt. Differential scanning calorimetry (DSC), impedance spectroscopy, FT-IR, 23Na N M R , and energy-dispersive X-ray (EDX) were used. Unlike the OMPEO-PAAM-LiC104 electrolytes which had ionic conductivities considerably enhanced over the basic OMPEO-LiC104 system, these composite electrolytes show only slightly enhanced conductivity below 300 K. The increase in conductivity in the OMPEO-PAAM-LiC104 is due to enhanced flexibility in the amorphous phase as indicated by the detection of two Tg’s, one a t 205 K associated with a clearly detectable amorphous uncomplexed O M P E O phase. The OMPEO-NaClO4-based composites discussed herein exhibit only one Tg, so that there is no highly flexible amorphous phase to promote a higher conductivity than the OMPEO-NaC104 electrolyte except a t temperatures below 300 K. The ionic conductivities for the OMPEO-PAAM-NaC104 system are slightly lower than the OMPEO-a-Al203-NaC104 system a t equal mass percent of filler. EDX spectra show that (r-Al2O3 is homogeneously distributed whereas PAAM is nonuniformly distributed; the sodium cations follow the filler particles. A discussion of the complexes formed is given based upon ET-IR results. In the case of the P A A M filler the competition between ion-dipole interactions and hydrogen bonding is discussed where the N a + cation and the ether oxygens and N-H moieties are involved. It is found that bonding due to the N a + cation competes favorably with hydrogen bonding but does not dominate as was the case for Li+ in the OMPEO-PAAM-LiC104 system. In the a-Al203 case a Lewis acid (Na+)-base (a-Al203) discussion is used which relates to the active surface centers of a-Al203. Here strong interaction between surface oxygens and the N a + cation are evident; these reduce the number of transient cross-links between N a + cations and ether oxygens. Assuming that the increase in conductivity of these composite polymer electrolytes is associated with interphase phenomena, the conductivity results were analyzed in terms of a model based on effective medium theory.
Introduction Polymer solid electrolytes based on complexes of various metal salts with polyether matrices have recently received considerable attention due to the possibility of their application in various electrochemical devices working at ambient temperatures.I4 It has been recognized that in such electrolyte ionic transport occurs in the amorphous polymer regions and is very often governed by the segmental motions of polymer chainsS5 Therefore flexible, amorphous macromolecules containing heteroatoms (usually 0, N, S) with lone electron pairs of a donor power sufficient to complex cations are the most suitable candidates for matrices for polymer ionic conductors. Poly(ethy1ene oxide) (PEO), which is so far the most widely studied polymer matrix, is semicrystalline under ambient conditions and forms crystalline complexes with most of the salts used as ionic dopants. Several different methods have been proposed to inhibit the crystallization of PEO based electrolytes.1-4 Ambient temperature conductivities as high as 10-4 S/cm have been reported for several modified systems. Unfortunately, most highly amorphous polymeric electrolytes behave like “soft solids” and thus generally have poor mechanical properties. The direct use of such polymers may then give rise to those problems commonly met in conventional liquid electrolyte systems, such as electrolyte leakeage and the loss of electrode to electrolyte contact. The concept of composite polymeric electrolytes, based on polyether matrices with dispersed inorganic or organic fillers, has been proposed to overcome the difficulties of partial Abstract published in Advunce ACS Abstracts, August 1, 1994.
0022-3654/94/2098-9047%04.50/0
crystallinity. This idea has been widely explored for composite systems based on PEO containing various conducting or nonconducting inorganic a d d i t i v e ~ . ” ~The ~ results of these studies have been briefly summarized in a recent report.25 Studies have shown that the mixing of inorganic fillers into a polymer matrix improves the mechanical stability of the host polymer and extends its temperature stability range.10324 Moreover, the addition of fine inorganic fillers (grain sizes 1-2 pm) has led to an increase in the ambient temperature conductivities of the electrolytes studied.15J6 These fillers have large effective surface areas and concentrations of 10-20 mass %. The increase in conductivity obtained for composite electrolytes in comparison to the PEOalkali metal systems results from a decrease in crystallinity as has been shown by X-ray,’6 NMR,” DSC,ZOJ and Raman spectroscopic26investigations. Unfortunately the addition of small inorganic particles stiffens the electrolyte and limits the conductivity to a maximum of l t 5 S/cm at ambient temperatures. Scrosati and co-workers have also shown the effect of inorganic fillers on the improvement of the electrochemical stability of polymeric electrolytes23.24 and on the properties of the lithium electrode-composite polymeric electrolyte i n t e r f a ~ e . ~ 3 ? ~ ~ Studies in our laboratory have shown that the mixing of PEO with softer organic fillers (such as polyacrylamide (PAAM))25 can inhibit polyether crystallization without impeding segmental chain motion and ionic carrier mobility. Ambient temperature conductivities as high as 7 X 10-5 S/cm have been measured for the PEO-PAAM-LiC104 electrolyte.25 The conductivity of amorphous oxymethylene-linked PEO (OMPEO) copolymer (which displays the structure of an ideal amorphous PEO phase) 0 1994 American Chemical Society
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The Journal of Physical Chemistry, Vol. 98, No. 36, 195’4
can be improved by the addition of PAAM.Z5 Conclusions based on this work suggest that the increase in conductivity results from processesoccurringat the polymer host-filler interface where there is a change in the polymer ultrastructure. The presence of a highly flexible amorphous phase characterized by a low glass transition temperature ( Tg)has been reported on the basis of the DSC studie~.~5 The aim of the present paper is to compare composite electrolytes based on an amorphous OMPEO-NaC104 matrix with on one hand a dispersed inorganic a-Alz03filler and on the other hand a dispersed organic PAAM filler. As mentioned above, preliminary indications are that the properties of PEO-based composite electrolytes containing inorganicz0and organic fillers2s are different. This comparison will concentrate on an examination of composite polymeric electrolytes based on the amorphous OMPEO matrix. NaC104 was used as the dopant salt due to the possibility of energy-dispersive X-ray (EDX) investigations which cannot be performed for lithium salt systems. The structure of the electrolytes studied is also analyzed by N M R and FT-IR. The thermal properties of these composite electrolytes are determined by DSC and ionic conductivities are calculated on the basis of impedance spectroscopy data. The conductivity of composite electrolytes is analyzed in terms of a model based on effective medium theory (EMT) which assumes that an increase in the conductivity in composite polymeric electrolytes in comparison to pure polyether systems is associated with interphase phenomena.
Experimental Section Sample Preparation. PAAM ( M w = 1.6 X lo5 g/mol) has been prepared by the free-radical polymerization of acrylamide (Aldrich, reagent grade) in an acetonitrile solution using benzoyl peroxide (Aldrich, reagent grade) as initiator. The product was dried under vacuum at 100 OC for 48 h. a-A1203grain sizes less than 5 pm were prepared according to a procedure described elsewhere16J8 and dried at 100 OC for 48 h prior to use. The synthesis of OMPEO followed that of Nicholas et al.2’ The resulting transparent elastomers were dried under vacuum for 48 h before synthesizing the polymer electrolyte. Acetonitrile (Aldrich, reagent grade) was distilled twice under vacuum over type 4A molecular sieves before use. All of the steps in the preparation procedure were performed in an argon-filled drybox (moisturecontent lower than 20 ppm). NaC104 (Aldrich, reagent grade) was dried under vacuum a t 120 O C prior to incorporation. The concentration of NaC104 was equal to 10 mol %with respect to the ether oxygen concentration. The solid components were mixed in stoichiometric amounts in a small glass reactor and then acetonitrile was added to form an approximately 5 mass % suspension with respect to all solid components. The mixture was magnetically stirred until an homogeneous suspension was obtained. Excess acetonitrile was removed by vacuum distillation. The composite electrolytes obtained were dried under vacuum for 48-72 h at 60 OC. The concentration of PAAM in the composite electrolytes varied between 5 and 50 mass % which roughly corresponds to 5 and 50 vol % concentration. The concentration of a-Alz03 in the composite electrolytes varied between 5 and 50 mass % which approximately corresponds to the volume concentration range from 2 to 24 vol %. DSC Studies. DSC data were obtained between -1 10 and 150 OC using a DuPont TA 2910 scanning calorimeter with a lowtemperature measuring head and liquid nitrogen cooled heating element. In run 1 15 mgsamples in aluminumpans were stabilized by slow cooling to-1 10 O C and then heated at 10 OC/min to 150 OC. Run 2 was performed after annealing the same samples used in run 1 at 150 OC (for approximately 10-15 min) and then following the same procedure as for run 1. Conductivity Measurements. Ionic conductivity was determined using the complex impedance method in the temperature range -20 to 100 OC. The samples were sandwiched between two
Wieczorek et al.
TABLE 1: Comparison of DSC, FT-IR, and Ambient Temperature Conductivity Data Obtained for OMPEO-NaClOd Composite Polymeric Electrolytes Containing a-AlzO, and PAAM kindof filler a
b
(Y-Al203 a-A1203 a-Al203 (T-Al203 a-AlzO3 a-AlzO:, ~Y-Alzoo a-&03
filler filler TdK C-O-C content, content, run run mode, mass % vol% 1 2 cm-1 0 0 5 10 15 20 25 30 40 50 5 10 15 20 25 30 40 50
0 0 1.7 3.5 5.4 7.5 9.7 12.1 17.7 24.4 5 10 15 20 25 30 40 50
215 258 246 247 249 249 245 245 248 247 253 251 254 253 249 253 252 257
216 257 247 246 248 249 245 245 248 246 252 251 255 254 251 253 248 248
1114 1085 1116 1116 1113 1110 1094 1094 1095 1094 1087 1083 1082 1076 1089 1090 1090 1092
uR, S cm-’ 8.0X 6.3 X 3.2X 1.4X 1.OX 5.3 X 4.1 X 2.6X 1.5 X 5.5X 4.0X 1.3X 2.5X 5.1 X 1.2X 2.7X 1.1 X
10-6 10-6 10-6
lo” 10-6 10-6 10-6 10-6 10-6 10-6
PAAM PAAM 10-6 PAAM 10-6 PAAM 10-6 PAAM 10-6 PAAM 10-6 PAAM 10-7 PAAM 10-7 a Undoped OMPEO sample. OMPEO-NaC104 electrolyte.
Tc/Ta
ratio 1.o 1.3
1.2 1.2 1.2 1.3 1.2 1.2 1.1 1.2 1.2 1.2 1.2 1.2
stainless steel blocking electrodes and placed in a temperaturecontrolled furnace. The impedance measurements were carried out on a computer-interfaced HP 4192A impedance analyzer over the frequency range 5 H z to 13 MHz. Fourier Transform Infrared Spectroscopy. Infrared spectra were recorded on a computer-interfaced Nicolet FT-IR system 4.4 instrument with a wavenumber resolution of 2 cm-1. Thinfilm electrolyte foils were sandwiched between two NaCl plates. NMR. Spin-lattice relaxation times (TI) and line-shape measurements were performed over the temperature range 240370 K using a home-built spectrometer with a Bruker 6.8T superconducting magnet. In this field the 23Na resonance oo/2n occurs at 77.0 MHz. Relaxation times were determined using an inversion recovery pulse sequence. The error associated with each T I measurement is estimated to be between 5 and 10%. At low enough temperatures (- 1.2TJ the sample produced a dipolar echo following a (T/z)X-T-(T)X pulse sequence which suggests that the 23Naline shape is primarily determined by dipoledipole interactions for T < 1.2Tg. Precautions such as using quartz sample tubes instead of Pyrex were taken to eliminate any extraneous sources of 23Na signal which may present problems with the interpretation of the data. EDX. A Voyager I1 X-ray Quantitative Microanalysis 1100/ 11 10 system with digital imaging (Noran Instruments) was used for EDX experiments. Spectra were recorded as digital dot maps or line scans obtained on the basis of an image transfer from an Hitachi S-570 scanning electron microscope (SEM). A line scan maps a particular element’s spectral intensity as a function of position along a SEM image. A set of X-ray maps shows different views of the same sample area. Each view maps the location of a particular element and is composed of a matrix of pixels. To develop a set of X-ray maps, a spectrum is acquired for each pixel. Regions corresponding to the mapped elements are located in the spectrum. The X-ray counts in these regions are converted to pixel intensities for the maps. The areas in an image that have high concentrations of a particular element appear bright. The map is then inverted during the analysis of data so that the region of the highest concentration of an element appears as black dots.
Results Table 1 presents the DSC results obtained for composite polymeric electrolytes containing a-AlzOs and PAAM. All the samples studied are amorphous. Unlike the OMPEO-PAAMLiC104 electrolytes25 only one DSC Tgis observed in the OMPEO-
Properties of Composite Polymeric Electrolytes PAAM-NaC104 and OMPEO-NaC104-a-Al203 systems. This suggests that the flexible amorphous phase with high segmental mobility which was responsible for the good conductivity in electrolytes doped with LiC104 is not present in the systems containing NaC104. The same T, values (f3 “C) are measured in both runs with the exception of those values for samples containing 40 and 50 vol% of PAAM for which T,values measured in run 2 (performed after short annealing of the samples at 150 “C) were respectively4 and 9 OC lower than in run 1. Considering this observation it was decided not to perform conductivity studies for annealed samples as was done for the OMPEO-PAAMLiC104 system.25 It should be noted that T, values measured for both composite systems are approximately 2-13 O C lower than for the pristine OMPEO-NaC104 electrolyte. T, values measured for electrolytes containing inorganic fillers are roughly the same over the entire filler concentration range. For the system containing PAAM T, varied in the narrow temperature range 248-257 K. Table 1 also shows the changes in the position of the maximum of the C-0-C IR band for OMPEO-PAAM-NaC104 and OMPEO-NaC104 a-A1203electrolytes. The changes found for the composite electrolyte containing PAAM are almost the same as previously described for OMPEO-PAAM-LiC104 electrol y t e ~ .The ~ ~ C-0-C band, centered at 1114 cm-1 for pure OMPEO, shifts to 1085 cm-1 after the addition of NaC104; this indicates changes in the ether oxygen environment due to the formation of polyether complexes involving Na+ transient crosslinks (“crown ether” type complexes). For concentrations of PAAM >5 vol % the maximum of the C-0-C band shifts to lower frequencies and reaches a minimum at 1076 cm-1; this is observed for the sample containing 20 vol % of PAAM. Such behavior has been attributed to the formation of mixed complexes in which an interaction between PAAM segments and polyether oxygens takes place through Na+ cations. A formation of these mixed complexes further weakens theC-0-C bands. For PAAM concentrations exceeding 20 vol % the maximum of the C-0-C band shifts again to higher frequencies reaching 1092 cm-’ for the sample of the highest PAAM concentration. It has been suggestedz5that this is due to the formation of complexes between PAAM segments through the Na+ cation which decreases the concentration of “crown ether” type and mixed PAAM-Na+polyether complexes. The hydrogen bonded N-H band has been found for all of the composite OMPEO-PAAM-NaC104 electrolytes and decreases from 3207 cm-’ for the sample containing 5 vol % of PAAM to 3 191 cm-I for the sample of the highest PAAM concentrations. The position of the C=O band is observed at 1652 cm-’ and does not change for all of the systems studied. A different behavior of the C-0-C mode can be observed for composite electrolytes containing a-A1203. The position of the maximum of the C-0-C band found for the samples containing up to 5.4 vol % of a-A1203is approximately the same as for the undoped OMPEO sample. This is evidence of the strong interactions occurring between the polymer host and the filler leading to a decrease in “crown ether” type complexes involving cation transient cross-links. The maximum of the symmetric stretch C104- band shifts to 938 cm-1, which is characteristic of contact ion pairs rather than “free” anions for which the maximum in this band should appear at 931 cm-1.28 The latter band was not found for the OMPEO-a-A1203-NaC104 electrolytes. This shows that with increase in a-A1203concentration cations are not involved in the formation of complexes involving polyether segments but rather interact with anions leading to the formation of ion pairs and possibly multiplets. This conclusion is supported by the fact that the C-0-C symmetric stretch frequency is at 1 1 16 cm-I which is characteristic of uncomplexed ether oxygens. Complexing reduces this frequency to around 1080 cm-1.28 For the higher concentrations of a-A1203the position of the maximum of C-0-C band shifts to 11 10 cm-1 (for the sample containing 7.5 vol % of a-Al2O3) and than drops down to 1094 cm-1 for the
The Journal of Physical Chemistry, Vol. 98, No. 36, 1994 9049
..
. . =
I
.
.
e.
21 pm
Figure 1. Dot maps of the distribution of Na (a) and 0 (b) obtained for the OMPEO-PAAM-NaC104 electrolyte (30 vol % of PAAM) on the basis of EDX experiments.
other systems studied. This may indicate that for higher concentrations of a-Al203 the filler interacts with the polymer chains and possibly plays a steric hinderance role which weakens the C-0-C band. Table 1 also presents values of the room-temperature ionic conductivity measured for all of the composite systems studied. As can be seen all conductivities obtained for the composite systems are lower than the conductivity measured for the pristine OMPEO-NaC104 electrolyte. For both composite systems a similar dependence of the conductivity on the concentration of filler added is observed. An initial decrease in conductivity is followed by an increase with a maximum obtained for the sample containing 25 wt % of the filler. For higher filler concentrations conductivity decreases. Conductivities measured for the samples containing PAAM are higher than for the samples containing the same volume concentration of a-Alz03 (see Table 1). EDX. EDX measurements have been performed in order to find the distribution of particular elements in composite polymeric electrolytes. For the OMPEO-PAAM-NaC104 system the compositional maps of N, 0, Na, and Cl were determined. For the OMPEO-NaC104 a-A1203system the distribution of Al, Na, and C1 has been investigated. The main difficulty in EDX analysis arises in the investigation of the distribution of N due to the presence of the much more intensive C and 0 peaks in the vicinity of the N peak. So the distribution of N has been mainly
9050
Wieczorek et al.
The Journal of Physical Chemistry. Vol. 98, No. 36, 1994
. ..-.
. . . ...... .
,
&
~~10.4 pm
~~
~
.* .' .
.. a.
. .,. . .
. . .. . . .
..
Figure 2. Concentration profiles of the distribution of 0, Na, and CI obtained for OMPE&PAAM-NaCI04 electrolyte (30 voI % of PAAM) on the basis of EDX experiments (line scan technique).
.. . . .. ....
5
. . i .s pm. . . . . .. .. . : , . 8% .., .
I
F i p 4. Dot map of the distribution of AI obtained for the O M P E C NaCIOra-AI20, electrolyte (9.7 vol %of a AbOd on the basis of EDX experiments.
I
-
10.4 pm
Figure 3. Concentration profiles of the distribution of N. Na. and CI obtained forOMPEO-PAAM-NaCI04electrolytesonthe basisof EDX
experiments (line scan technique). Sample containing 5 vol % PAAM. analyzed by a line scan map which shows the distribution of elements along a line drawn between two particular points of the EDX image. Figure 1 presentsdot mapsofOandNa forOMPECLPAAMNaC104 composite electrolytes containing 30 vol % of PAAM. It can be observed that the 0 and Na are inhomogeneously distributed within the electrolyte. It can also be noticed that the dot map of Na is roughly the same as the dot map obtained for 0. Similar observations have been made for the other systems studied. Since the background. which is mainly due to the OMPEO host, has been subtracted, this observation suggests that the highest concentrationofsodium corresponds to the fillerrich parts of thecompositesystems for which t h e 0 concentration should increase. This is confirmed by a comparison with line scans measured for the sample containing 30 vol % ' of PAAM (see Figure 2). Concentration profiles of 0 and Na are analyzed along a line drawn between points A and B in Figure 1. The profile of the oxygen distribution is almost the same as that for Na, whereas the CI map shows a homogeneous distribution. Separate dot maps for N, Na, and CI obtained on the basis of different images indicate the inhomogeneous distribution of the filler due to the inhomogeneous distribution of N. The N line scans obtained for the samples containing 5.30,and 40 vol %of PAAM (see Figure 3 for the 30 vol % case) confirm the above suggestion. The concentration profiles for Na are almost the same as for N; particularly noticable for samples containing 30 and 40 vol Sb of PAAM. The CI concentration profile is fairly uniform, similar to the scan presented in Figure 2. It can be concluded that PAAM is inhomogeneously distributed in the composite system. The sodium ions are preferentially situated in the vicinity of the filler which is confirmed by a comparison oftheNandNalinescans(seeFigure3). Thissupportsprevious suggestionsconcerningtheformationofthePAAM-alkali metal cation-polyether and PAAM-alkali metal cation-PAAM complexes in OMPE@PAAM-NaCI04 composite electrolytes as
