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Structure, electrical and optical properties of (PVA/[LiAsF.sub.6]) polymer composite electrolyte films.

INTRODUCTION

Solid polymer electrolytes are currently of much interest owing to their advantageous and special mechanical properties, ease of fabrication in thin films of desirable sizes and suitability for electrode-electrolyte contacts in different electrochemical devices such as high-performance solid-state batteries, smart windows, chemical sensors, microelectronic and electrochemical devices, and variable reflectance mirrors (1), (2). Unlike inorganic compounds, intrinsically conducting polymers are more attractive electrochromic materials because of their relatively low-optical switch potentials, fast response, and colorful electrochromism (3), (4). In recent years, research on the electrical and optical properties of polymers has attracted much attention in the view of their applications in optical devices with remarkable reflection, antireflection, interference, and polarization properties (5). The optical properties of polymers can be suitably modified by the addition of dopants depending on their reactivity with the host matrix. For the past three decades, increasing attention has been paid on solid polymer electrolytes, particularly on poly(ethylene oxide) (PEO)-based electrolytes in the development of polymer Li-ion batteries (6). It has been realized that cations are solvated with polar groups on the polymer chains in polymer salt complexes whereas anions usually interact weakly with the aprotic host (7). In polymer electrolytes, fast ion transport takes place in amorphous regions, where the ionic conductivity is two or three orders of magnitude higher than that of the crystalline domain (8). Although PEO is one of the best "polymer solvents" for a variety of metal salts, there is a liquid-solid coexistence regime in the phase diagram of many PEO-metal salt systems above ambient temperatures (9) and also it has poor mechanical strength in the high-conduction region (10). Efforts have been made to improve the performance of existing electrolytes and find new polymer electrolytes with better electrochemical and mechanical properties. Vargas and coworkers (11) investigated poly(vinyl alcohol) (PVA)-based polymer electrolyte for the Vogel--Tamman--Fulcher (VTF) behavior. Bai et al. developed ionically conducting polymer electrolytes based on PVA because of its bio-compatibility and wide spread use in biomedical fields (12), (13). It has some technological advantages in electrochromic devices and fuel cells because of its superior mechanical properties and better ionic conduction. The higher film form ability inexpensive, and good electronic properties of PVA has been used in especially electrochromic devices in the composite films (3). Hydrophilicity of PVA is an advantage for its applications in the formation of composite films (14) and also a limiting factor in its characterization, since the molecules are prone to aggregate through hydrogen bonding because of its polyhydroxy groups (15). Fluorides with fluorite ([CaF.sub.2]) structure exhibit high-ionic conductivities. Fluoride content materials such as [KYF.sub.4] and [NaBiF.sub.4] are good examples of ionic conductors (16), (17). To achieve high-ionic conductivity, [LiAsF.sub.6] salt was incorporated in to the PVA polymer host.

A few attempts were made to the development of this polymer electrolytes based on lithium salt complexed with PVA films. In this investigation, an attempt has been made to characterize the polymer electrolytes based on PVA complexed with lithium hexafluoro arsenate ([LiAsF.sub.6]) at different weight percentages to evaluate their electrical and optical performance. In addition, the effect of ceramic filler [TiO.sub.2] on ionic conductivity is also investigated.

EXPERIMENTAL

Materials

The polymer composite electrolyte films under this investigation were prepared using solution casting method using PVA (Aldrich, Molecular weight 90,000), [LiAsF.sub.6] salt, ceramic fillers [TiO.sub.2] (50 nm) as the raw materials.

Preparation of Polymer Electrolyte Films

The compositional amounts of these raw materials used in the preparation of films were taken as (100-x) PVA + (x) [LiAsF.sub.6] for x = 0, 5, 10, 15, 20, 25, and 30 wt%. PVA and [TiO.sub.2] were dried at 100 and 150[degrees]C, respectively, under vacuum for at least 24 h before they were used in the process. The preparation of the samples involved first dissolving the PVA and [LiAsF.sub.6] in distilled water and the resulting solution was stirred for 8-10 h at room temperature in a stappered flask. Then 3-7 wt% amount of the filler powder was added to this solution and is stirred continuously until complete homogenization was achieved. After which, the sample is kept stable until no bubbles could be observed. The resulting slurry was cast onto polypropylene dishes and allowed to evaporate slowly at room temperature. The final product was dried thoroughly at 100[degrees]C till all the traces of solvent completely disappear. The dried composite polymer electrolyte films peeled off from the polypropylene dishes and they were stored inside the dry vacuum box. Various compositions of films were prepared using [TiO.sub.2] mixed with PVA:[LiAsF.sub.6] solution.

Characterization

The X-ray diffraction measurements were carried out using Philips vertical X-ray diffractometer (PW 3050/60 MPSS). Infrared spectra were recorded using 60-SXB spectrophotometer. Impedance measurements were carried out using TH-2818 automatic component analyzer. The optical absorption studies of PVA and PVA:[LiAsF.sub.6] films were performed by means of a JASCO spectrophotometer, model V-570 in the wave length range of 200-600 nm.

RESULTS AND DISCUSSION

XRD Analysis

The X-ray diffraction patterns of pure PVA and complexed with [LiAsF.sub.6] salt are shown in Fig. 1. From the Figure, it is clear that the pure PVA shows a remarkable peak for an orthorhombic lattice centered at 20[degrees] indicating its semi-crystalline nature (18), (19). This peak becomes less intense as the [LiAsF.sub.6] content is increased. This could be due to the disruption of the PVA crystalline structure by [LiAsF.sub.6] salt. No peaks pertaining [LiAsF.sub.6] salt appeared in the complexes, which indicate the complete dissolution of salt in the polymer matrices. The diffraction peaks are less intense in [LiAsF.sub.6] complexed PVA films when compared with that of pure PVA films. This shows a decrease in the degree of crystallinity of the polymer after the addition of [LiAsF.sub.6] salt. No sharp peaks were observed for higher contents of [LiAsF.sub.6] salt in the polymer, suggesting the dominant presence of amorphous phase (20). This amorphous nature results in greater ionic diffusivity with high-ionic conductivity, which can be obtained in amorphous polymers that have flexible backbone. Similar type of salt complexed polymer electrolytes films were observed in the literature (19-22).

[FIGURE 1 OMITTED]

FTIR Analysis

Figure 2 shows the FTIR spectra of pure PVA and com-plexed films. The prominent O--H vibrational band of hydroxyl was appeared at 3200-3400 [cm.sup.-1] (23). For pure [LiAsF.sub.6] salt band was observed at 3377 [cm.sup.-1]. This band was shifted to 3305, 3306, and 3350 [cm.sup.-1] for 10, 20, and 30 wt% salt complexed PVA films respectively. In pure PVA asymmetric [CH.sub.2] stretching and aliphatic C--H stretching occur at 2917 [cm.sup.-1] (24), and is shifted to 2850, 2851, and 2851.5 [cm.sup.-1] for 10, 20, and 30 wt% salt complexed PVA films, respectively. The C=0 stretching band appearing at 1736 [cm.sup.-1] in PVA (22). This is observed at 1725, 1722, and 1721 [cm.sup.-1] for salt complexes. The phase bending or deformation coupled to C--H wagging is observed at 1375 [cm.sup.-1] and gives rise to broad weak band in the range 1430-1275 [cm.sup.-1] in PVA (22). This is shifted to 1377, 1379, and 1380 [cm.sup.-1] in [LiAsF.sub.6] salt polymer complexes. The characteristic vibrational band appearing at 1093 [cm.sup.-1] in the region (1150-1020 [cm.sup.-1]) is assigned to C--O stretching of secondary alcohols of PVA (24). The C--C stretching frequency occurs at 1250 [cm.sup.-1] in PVA and is shifted to 1266, 1272, and 1276 [cm.sup.-1] in complexed films (25). The vibrational bands at 2945 and 1427 [cm.sup.-1] are assigned to asymmetric [CH.sub.2] stretching and aliphatic C--H stretching and weak O--H bend of PVA that are found to be absent in the complexed films (25). The C--H rocking mode of PVA was appeared at 939 [cm.sup.-1] and is shifted to 914, 919, and 917 [cm.sup.-1]. The vibrational bands 1027, 1327, 941, 700, and 686 [cm.sup.-1] of PVA are found to be absent in the complexes. The above mentioned shifts in frequency observed in salt doped systems compared with pure PVA clearly gives an insight into the specific interactions between [Li.sup.+] ion from [LiAsF.sub.6] with polar groups of the polymer. Thus, the complex formation in the polymer salt matrices has been confirmed from the analysis. Similar type of behavior was also observed by other researchers (22), (26).

[FIGURE 2 OMITTED]

Conductivity Studies

Impedance spectroscopy is used to establish the conduction mechanism, observing the participation of the polymeric chain, mobility, and carrier generation processes. The conductivities of the polymer complexes were calculated from the bulk resistance obtained by the intercepts of the typical impedance curves (Cole-Cole) for various temperatures. The real and imaginary parts were taken along the x- and y-axes, respectively. Intercept of the curve on the real axis gives the bulk resistance ([R.sub.b]) of the sample. The ionic conductivities were calculated using the relation

[sigma] = l / [R.sub.b]A (1)

where l is the thickness, [R.sub.b] is bulk resistance, and A is the contact area of the electrolyte film during the experiment. Figure 3 shows the conductivity values of the complexes in the temperature range 320-420 K. It was observed that as the temperature increases the conductivity also increases for all the complexes and this behavior is in agreement with the theory established by Armand et al. and was presented by Tareev and Croce (27), (28). This is rationalized by recognizing the free-volume model (29). When the temperature is increased, the vibrational energy of a segment is sufficient to push against the hydrostatic pressure imposed by its neighboring atoms and create a small amount of space surrounding its own volume in which vibrational motion occurred (30). Therefore, the free volume around the polymer chain causes the mobility of ions increases and also due to segmental motion of polymer causes the conductivity increases. Hence, the increment of temperature causes the increase in conductivity because of the increased free volume and their respective ionic and segmental mobility.

[FIGURE 3 OMITTED]

Figure 4a shows the effect of the ceramic filler [TiO.sub.2] on conductivity ([sigma]) of the polymer PVA:[LiAsF.sub.6] (75:25) at different weight percent of [TiO.sub.2]. The maximum effects on the conductivity enhancement were obtained for the composites containing 5 wt% [TiO.sub.2] for polymer-salt composition. The polymer salt complexed film showed ionic conductivity 3.79 X [10.sup.-4] S [cm.sup.-1] at 320 K and was increased to 3.98 X [10.sup.-2] S [cm.sup.-1] as the temperature is raised up to 420 K. The 5 wt% [TiO.sub.2] dispersed sample showed the conductivities 5.10 X [10.sup.-4] S [cm.sup.-1] and 0.11 S [cm.sup.-1] at the temperatures 320 and 420 K, respectively. This means that there is a small increment in conductivity for the ceramic fillers of the samples. Hence, the interaction of the ceramic fillers with the polymer matrix is probably weak (31). It may act as a dissociation promoter in this polymer system, but does not change any intrinsic mechanism for the ionic conduction.

[FIGURE 4 OMITTED]

According to the percolation conduction rule (32), (33), the conductivity contributed by the percolation paths would be

[sigma] = [[sigma].sub.0][(p - [p.sub.c]).sup.t] (2)

where p is the percolation probability, [p.sub.c] is the percolation threshold, and t is the value of exponent for three dimensional lattice system is equal to 2 (32). We convert the p and [p.sub.c] in Eq. 2 into the doping contents x and [x.sub.c] and modify Eq. 2 to

log[sigma] = log([[sigma].sub.0]) + tlog(x-[x.sub.c]) (3)

According to Eq. 3, the log([sigma]) ~ log(x - [x.sub.c]) relation for the percolation path should exhibit linear behavior. Figure 4b shows a straight line that indicates a proportionality between [[sigma].sub.ac] and [epsilon] = x - [[x.sub.c]/[x.sub.c]] or p - [[p.sub.c]/[p.sub.c]]. This proportionality can be compared with the following power law

[[sigma].sub.ac] [varies] [[epsilon].sup.[beta]] (4)

where [beta] is a critical component and the symbol [varies] is used to indicate the proportionality. The [beta] value 0.45 was obtained from the slope of the curve as shown in Fig. 4b. This result is consisted with percolation in three-dimensional models which predicts [beta] = 0.44 (32) and [beta] = 0.45 (34). This type of behavior is also observed by other researchers (35).

Transference Number Measurement

The lithium ion transference number (i.e., [Li.sup.+]) of the sample was measured at room temperature in the polymer electrolyte system. Under real conditions current flow is also affected by a passive layer forming, so the adequate correction for resistance changes is needed. For the Li/[Li.sup.+] [X.sup.-]/Li cell type, Bruce and coworkers (36), (37) introduced the following correction

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

where [DELTA]V is the D.C. voltage applied, [R.sub.0] is the passive layer resistance, [R.sub.s] is the steady state passive layer resistance, [I.sub.0] is the initial current, and [I.sub.s] is the steady state current.

Impedance spectroscopy measurement was taken just before and after D.C. polarization and just after it reached steady state. In this study, (PVA:[LiAsF.sub.6]) (75:25) + 5 wt% [TiO.sub.2] film showed [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] = 0.52.

Optical Properties

The study of optical absorption gives information about the band structure of solids. Insulators/semiconductors are generally classified into two types: (a) Direct band gap semiconductors and (b) Indirect band gap semiconductors. In indirect band gap semiconductor, the transition from valence band to conduction band should always be associated with a phonon of the right magnitude. Davis and Shalliday reported that near fundamental band edge both direct and indirect transitions occur and can be observed by plotting [[alpha].sup.[1/2]] and [[alpha].sup.[1/2]] as a function of energy hv (38).

The analysis of Thutupalli and Tomlin (39) is based on the following relationships for direct and indirect band semiconductors/insulators, respectively.

[(hv[alpha]n).sup.2] = [C.sub.1](hv - [E.sub.gd]) (6)

[(hv[alpha]n).sup.[1/2]] = [C.sub.2](hv - [E.sub.gi]) (7)

where hv is the photon energy, [E.sub.gd] is the direct band gap, [E.sub.gi] is the indirect band gap, n is the refractive index, [alpha] is the absorption coefficient, and [C.sub.1], [C.sub.2] are constants. The absorption coefficient ([alpha]) was calculated from the absorb-ance (A). After correction for reflection, the absorption coefficient ([alpha]) was determined using the relation

I = [I.sub.0]exp(-[alpha]x) (8)

[alpha] = [2.303/x]log[I/[I.sub.0]] = ([2.303/x])A (9)

where x is thickness of the sample. To determine the nature and width of the bandgap [alpha], [([alpha]hv).sup.2], and [([alpha]hv).sup.[1/2]] were plotted as a function of photon energy (hv). The position of the absorption edge was determined by extrapolating the linear portions of [alpha] versus hv curves (see Fig. 5) to zero-absorption value. It is observed that the absorption edge for pure PVA lies at 5.76 eV, whereas for 20 and 25 wt% LiAs[F.sub.6] doped PVA films the absorption edge lies at 4.87 and 4.70 eV, respectively. When a direct band gap exists, the absorption coefficient has the following dependence on the energy of incident photon (38), (39).

[FIGURE 5 OMITTED]

[alpha]hv = C[(hv - [E.sub.g]).sup.[1/2]] (10)

where [E.sub.g] is the band gap, C([4[pi][[sigma].sub.0]/nc[DELTA]E]) is a constant dependent on the specimen structure, [alpha] is the absorption coefficient, v is the frequency of incident light, and h is the Planck's constant.

The direct band gap values were obtained by plotting [([alpha]hv).sup.2] versus hv curves (see Fig. 6). For pure PVA, the optical band gap was observed to be 5.40 eV whereas for doped films the values were 5.12 and 4.87 eV, respectively. Similar type of results were also observed for PVA polymer blend and PVC electrolyte system (7), (10).

[FIGURE 6 OMITTED]

For indirect transitions, which require phonon assistance, the absorption coefficient has the following dependence on photon energy (38), (39).

[alpha]hv = A[(hv - [E.sub.g] + [E.sub.p]).sup.2] + B[(hv - [E.sub.g] - [E.sub.p]).sup.2] (11)

where [E.sub.p] is the energy of the photon associated with the transition, A and B are constants depending on the band structure. The indirect band gap values were obtained from the plots of [([alpha]hv).sup.[1/2]] versus hv as shown in Fig. 7. For pure PVA, the indirect band lies at 4.75 eV whereas for the doped films its value lies at 4.45 and 4.30 eV, respectively. The decrease in optical band gap/activation energy on doping may be explained on the basis of the fact that the incorporation of small amounts of dopant forms charge transfer complexes (CTCs) in the host lattice. These CTCs increase the electrical conductivity by providing additional charges in the lattice (40). This results in a decrease of activation energy. The conductivity data showed a small comparison with optical band gap energies. This is due to the fact that their nature is different. The energy required to transfer the active sites one to another in the condiction correspondingly the same energy required in the inter band transition of the optical band gap. All these values of absorption edge, direct and indirect band gap values are listed in Table 1.

[FIGURE 7 OMITTED]
TABLE 1. Absorption edge, optical band gap (both direct and indirect)
of undoped and [LiAsF.sub.6] doped PVA polymer electrolyte films.

                                                    Band gaps

Polymer electrolyte        Absorption edge  Direct (eV)  Indirect (eV)
                                 (eV)

Pure PVA                         5.76          5.40         4.75
PVA:[LiAsF.sub.6] (80:20)        4.87          5.12         4.45
PVA:[LiAsF.sub.6] (75:25)        4.70          4.87         4.30


CONCLUSIONS

The (100 - x) PVA + (x) [LiAsF.sub.6] for x = 0, 5, 10, 15, 20, 25, and 30 wt% polymer composite electrolyte films were prepared by the solution casting method along with incorporation of 0-7 wt% [TiO.sub.2] ceramic fillers. These films were characterized by XRD, FTIR, optical absorption, and conductivity measurements. The increase in conductivity with increasing concentration of [LiAsF.sub.6] is attributed to decrease in the degree of crystallinity and increase in the amorphicity. The maximum conductivity 5.10 X [10.sup.-4] S [cm.sup.-1] was observed in the 5 wt% [TiO.sub.2] dispersed PVA:[LiAsF.sub.6] (75:25) films at 320 K. The [Li.sup.+] transference number of PVA:[LiAsF.sub.6] (75:25) + 5 wt% [TiO.sub.2] film was found to be 0.52. Optical absorption edge and optical band gap (both direct and indirect) showed a decreasing trend with increased content of the dopant. The marked increase in electrical conductivity with increasing content of dopant salt makes it possible to consider this electrolyte system for electrochemical device applications.

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Madhu Mohan Varishetty, Weiliang Qiu, Yuan Gao, Wen Chen

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, People's Republic of China

Correspondence to: W. Chen; e-mail: chenw@whul.edu.cn

Contract grant sponsor: China Postdoctoral Science Foundation (CPSF); contract grant number: 20080440966; contract grant sponsor: National Nature Science Foundation of China; contract grant number: 50672071; contract grant sponsor: Program for Changjiang Scholars and Innovative Research Team in University, Ministry of Education, China (PCSIRT); contract grant number: IRT0547; contract grant sponsor: Wuhan University of Technology Management.

DOI 10.1002/pen.21437

Published online in Wiley InterScience (www.interscience.wiley.com).

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