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Effect of TiO2 nanoparticles on structural, thermal, mechanical and ionic conductivity studies of PEO12–LiTDI solid polymer electrolyte
Accepted Manuscript Title: Effect of TiO2 nanoparticles on structural, thermal, mechanical and ionic conductivity studies of PEO12 -LiTDI solid polymer electrolyte Author: Anji Reddy Polu Hee-Woo Rhee PII: DOI: Reference:
S1226-086X(16)30057-0 http://dx.doi.org/doi:10.1016/j.jiec.2016.03.042 JIEC 2889
To appear in: Received date: Revised date: Accepted date:
18-1-2016 8-3-2016 27-3-2016
Please cite this article as: A.R. Polu, Effect of TiO2 nanoparticles on structural, thermal, mechanical and ionic conductivity studies of PEO12 -LiTDI solid polymer electrolyte, Journal of Industrial and Engineering Chemistry (2016), http://dx.doi.org/10.1016/j.jiec.2016.03.042 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights PEO12–LiTDI–TiO2 nanocomposite electrolytes were synthesized for Li-ion batteries. Strong interaction among PEO, LiTDI and TiO2 was confirmed by FTIR studies.
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TiO2 gave significant enhancement in thermal stability and mechanical strength of SPEs.
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Effect of TiO2 nanofiller on conductivity increase in PEO12–LiTDI is reported.
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Effect of TiO2 nanoparticles on structural, thermal, mechanical and ionic conductivity studies of PEO12-LiTDI solid polymer electrolyte
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Anji Reddy Polu* and Hee-Woo Rhee** Polymer Materials Lab, Department of Chemical and Biomolecular Engineering,
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*Email: [email protected]
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Sogang University, 35 Baekbeom-Ro, Mapo-Gu, Seoul 121-742, South Korea
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**Corresponding Author E-mail: [email protected];
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Phone: +82 2 705 8483; Fax: +82 2 711 0439
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Abstract In the present study, poly(ethylene oxide) (PEO) complexed with lithium 2-trifluoromethyl-4, 5-dicyanoimidazole (LiTDI) nanocomposite solid polymer electrolyte membranes (NSPEMs)
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have been prepared by solution cast technique using different weight percent of nano-sized TiO2 ceramic filler. The effect of filler incorporation on the structural, thermal, mechanical and ionic
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conductivity properties of solid polymer electrolytes have analyzed. X-ray diffraction (XRD) and
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polarized optical microscopy (POM) results indicated that the crystallinity has been reduced remarkably with the incorporation of TiO2 nanofiller. The thermal stability and mechanical
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integrity of the nanocomposite polymer electrolyte system increased significantly compared to filler free electrolytes. The maximum ionic conductivity is found to be in the range of 2.11 x 10-5
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S/cm for 8 wt% TiO2 nanofiller in PEO12–LiTDI electrolyte system. These results indicated that
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the prepared TiO2 based nanocomposite membrane would be a promising alternative separator
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for rechargeable lithium-ion battery applications.
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Keywords: Solid polymer electrolyte; TiO2 nanoparticles; ionic conductivity; thermal stability; mechanical integrity.
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Introduction Solid polymer electrolyte membranes (SPEMs) have received extensive attention in the past few decades for their potential applications in solid state ionic devices such as lithium batteries,
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fuel cells, solar cells etc [1-3]. Lithium batteries have attracted a great deal of attention due to its higher energy density, improved safety hazards, and good processability [4, 5]. As it is well
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known, SPEMs have several advantages over the liquid counterpart such as desirable shape
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mouldability, light-weight, free from leakage, mechanical strength, high physical and chemical stability and flexibility of design, thereby permitting miniaturization [6]. One of the most
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important properties of high-performance polymer electrolytes is their ionic conductivity. In polymer electrolyte membranes, ionic conduction depends on the dissociation of the ionic
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conductor and the structure of the polymer matrix. High molecular weight poly(ethylene oxide)
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(PEO) based polymer electrolytes containing lithium salts have been studied extensively due to
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its compatibility with a wide range of ionic conducting salts while maintaining an acceptable chemical and electrochemical stability [7].
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Recent success of imidazole-derived lithium salts [8] and stressed as “tailor made” salts, notably LiTDI (lithium 4,5-dicyano-2-(trifluoromethyl)imidazole), has encouraged us for lithium battery applications [9]. For exploring the properties of this salt, only a few publications are available [10, 11]. In compare with many other lithium salts, LiTDI is fully stable in the presence of moisture [12]. In addition, this LiTDI electrolyte is thermally stable up to 250 °C far more than boiling point of any solvent or stability of many other organic lithium salts. The safety, stability information of LiTDI and its parameters in solvent mixtures are available elsewhere [13]. However, a major drawback of these poly(ethylene oxide)–lithium salt electrolyte
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membranes for practical applications is that they tend to crystallize at ambient temperatures resulting low ionic conductivity [14]. Generally two ways have been used to enhance the room temperature ionic conductivity of
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the polymer electrolytes; one is the incorporation of nano-sized ceramic fillers such as TiO2, CeO2, SiO2 and Al2O3 [15] in the polymer matrix and the other is the addition of plasticizers
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such as ethylene carbonate (EC), propylene carbonate (PC) and polyethylene glycol (PEG) [16].
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These plasticizers can improve the room-temperature ionic conductivity of the polymer electrolytes, but decrease the mechanical properties and the potential stability of these polymer
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membranes [17]. The incorporation of ceramic nano-fillers reduces the crystalline character, improve the ionic conductivity as well as promote the thermal behaviors, mechanical stability
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and electrochemical properties of the polymer membrane [18]. Among the several fillers in
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polymer electrolyte membranes, nano-sized titanium oxide (TiO2) supports the ionic mobility
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due to its substrate characteristics, such as shape and surface nature, which effectively disturbs the order packing tendency of the host polymer chains [19]. The effects of TiO2, Al2O3 and SiO2
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in PEO–LiClO4 polymer electrolyte has been studied by Chung et al. [20] and identified TiO2 as the filler with greatest enhancement in ionic conductivity due to the weakened interactions between the polymer chain and the Li+ ions. To the best of our knowledge, there are no reports available in the literature on the ionic conductivity, thermal stability and mechanical stability enhancement of PEO–LiTDI polymer electrolyte due to the incorporation of nano-sized TiO2 ceramic filler. In this work, we have studied the effect of TiO2 nano-filler incorporation on structural, thermal, mechanical and electrical properties of PEO12–LiTDI polymer electrolyte membrane. Xray diffraction (XRD), polarized optical microscopy (POM), thermogravimetric analysis (TGA),
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fourier transform infrared spectroscopy (FTIR) and universal testing machine (UTM) were employed to characterize the physical and chemical properties of the nanocomposite solid polymer electrolyte membranes (NSPEMs). Impedance spectroscopy was used to study the ionic
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conductivity of the NSPEMs at various temperatures. Cyclic voltammetry was used to examine
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the electrochemical potential window of the NSPEM.
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Experimental Details Materials used
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High molecular weight PEO (MW = 4 x 106 g/mole), TiO2 nanopowder (21 nm) and anhydrous acetonitrile were purchased from Sigma-Aldrich (St. Louis, USA). Lithium 2-
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trifluoromethyl-4, 5-dicyanoimidazole (LiTDI) obtained from Alfa Aesar.
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Preparation of nanocomposite solid polymer electrolyte membranes The NSPEMs have been prepared by using solution cast technique. The high molecular
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weight PEO, TiO2 nanopowder and LiTDI have been dried at 50, 100 and 100 °C under vacuum for 24 h respectively. The high molecular weight PEO and LiTDI was dissolved in anhydrous acetonitrile, followed by the addition of different weight percentages of TiO2 nanofiller. The relative amounts of PEO and LiTDI are taken at the EO/Li = 12 based only on the amount of ethylene oxide in the PEO and lithium in the LiTDI. The mixture was stirred for 24 h to obtain a homogeneous viscous solution. This solution was cast onto Teflon plates followed by the evaporation of the solvent in a dry box for about 1 day. The resulting films were further dried in vacuum oven at 40º C for 24 h. After that the films shifted to nitrogen-filled glove box and left
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undisturbed for 5 days to remove residual acetonitrile. The NSPE membrane synthesis process and the interaction scheme among PEO, LiTDI and TiO2 are displayed in Fig. 1.
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Characterization
The X-ray diffraction spectra of polymer electrolyte membranes were carried out using an X-
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ray diffractometer (D-MAX 2500, Rigaku) with Cu-Kα radiation (λ= 1.5406 Å) at ambient
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temperature in the range 2θ = 5 ~ 50º. Surface morphology and microscopic feature of the polymer films were examined using polarized optical microscope (Optiphot-2 Pol, Nikon) at
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room temperature. The thermogravimetry (TG) measurements were carried out on a TGA-2950 thermal analyzer (Hi-Res, TA instruments) by heating from 25 to 500 °C under a N2 atmosphere
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at a heating rate of 20 °C/min, with a sample about 7 mg. The formation of polymer–salt–
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nanofiller complexes and interaction among polymer, salt, and ceramic nanofiller were
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determined by fourier transform infrared spectroscopy (FTIR) using Nicolet 380 FT-IR spectrometer (Thermo Electron) in the region 4000 – 400 cm-1 at a resolution 1 cm-1. Universal
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materials testing machine (Lloyd instruments, LR5K Plus) has been used to measure the mechanical stability of the solid polymer electrolytes. Impedance and conductivity of the samples were determined using an Iviumstat (Ivium Technologies, Netherlands). The measurements were done in the frequency range from 1 Hz to 1 MHz by varying temperature from 23 to 60 ºC. The impedance studies were carried out by sandwiching the solid polymer electrolyte membrane between two gold electrodes under spring pressure. The thickness of each sample was measured using a micrometer screw gauge. The electrochemical studies were carried out by cyclic voltammetry using an electrochemical analyzer (WonA tech WBCS 3000L battery cycler system) in the potential range 0 to 5 V (vs. Li+/Li) at a scanning rate of 5 mV s-1
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Results and discussion XRD analysis The XRD patterns of pure PEO, pure LiTDI, pure TiO2 and PEO12–LiTDI–TiO2 polymer
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electrolytes at various TiO2 concentrations are shown in Fig. 2. The X-ray diffraction pattern of pristine PEO shows two high intensity diffraction peaks at 19.36º and 23.72º which are assigned
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to set of planes (120) and (112) [21]. The XRD pattern of LiTDI salt exhibits sharp peaks
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centered at 2θ angle of 10.4º, 11.2º, 15.5º, 23.4º, 24.7º, 27.2º, 31.3º, 33.5º, 40.3º due to the crystalline nature of the salt. After LiTDI inclusion, the peaks of PEO shifted to lower 2θ values
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due to increase of the interplanar crystal spacing of PEO which may have occurred from the insertion of large TDI anions into PEO lattice. In XRD pattern of PEO12-LiTDI polymer
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electrolyte (0 wt% TiO2), no diffraction peak of LiTDI is found, demonstrating that the added
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lithium salt has well dissolved into the PEO matrix. With the incorporation of LiTDI decreased
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the intensity of the peaks and broader than pure PEO. This can be attributed to a destruction effect of the lithium salt on the ordered arrangement of the polymer chains, and hence an
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enhancement in the amorphous phase and led to increase in ionic conductivity [22]. The XRD pattern of nano-sized TiO2 ceramic filler shows intensive peaks at 2θ ≈ 25°, 36° and 48° and these correspond to (1 0 1), (0 0 4) and (2 0 0) planes , which indicate its crystalline nature. After the incorporation of nano ceramic filler into PEO12-LiTDI polymer electrolyte, the intensity of the PEO diffraction peaks decreased remarkably and broadened, which implies that the PEO matrix supplemented the amorphous region. As the wt% of TiO2 nanofiller increases, the crystallinity of the polymer has been significantly reduced and this reduction is attributed to the re-organization of polymeric chains by the inert nanoparticles present in nanocomposite polymer electrolyte membranes. This reorganization facilitates higher ionic conduction [23]. A
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similar observation was reported by ElBellihi et al. and Lee et al. for XRD analysis of PEOLiClO4-ZnO and PEO/PVDF/PC-LiClO4-BaTiO3 composite polymer electrolytes [24, 25].
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POM analysis
Polarized optical microscopy (POM) was used to understand the crystalline behavior and
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their related surface morphology of the polymer films. Fig. 3 displays the typical room
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temperature POM images of pure PEO, PEO12–LiTDI and PEO12–LiTDI–8 wt% TiO2 electrolyte membranes. Fig. 3(a) shows the typical spherulitic texture along with dark boundaries of the pure
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polymer PEO, demonstrating its semi-crystalline nature. The boundaries of the spherulites were non-spherical but smooth after impingement with the adjacent spherulites. The spherulitic texture
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in the polymer film showed its lamellar crystalline nature and the dark boundaries indicate the
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amorphous content in the polymer. The lamellar was developed through regular polymer chains
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folding, leading to a long range order. The lamella radiate out from a central nucleating point. After the incorporation of LiTDI into PEO, the amount of spherulites increases and the spherulite
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size becomes small while the amorphous region (black portion) increases (Fig. 3(b)). As the 8 wt% of TiO2 is incorporated into the PEO12–LiTDI electrolyte system, the dark regions between spherulites expands and PEO spherulites cannot be observed clearly in PEO12–LiTDI–8 wt% TiO2 electrolyte membrane (Fig. 3(c)). The expansion of the dark regions suggests the amorphous region increased with the interruption of the crystallite in presence of the TiO2 nanofillers. This type of surface morphology was also observed in other PEO based systems such as PEO–LiI–Y2O3 and PEO–LiClO4–Al2O3 composite polymer electrolyte membranes [26, 27]. The dark area in PEO12–LiTDI–8 wt% TiO2 electrolyte membrane is larger than that in PEO12– LiTDI, suggesting that the ratio of amorphous PEO in 8 wt% TiO2 composite polymer
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electrolyte is higher than that in PEO12–LiTDI. It is supposed to be that the addition of TiO2 nanofillers reduces the crystalline behavior of PEO through Lewis acid-based interactions, under the steric hindrance effects of TiO2, which makes the size of the spherulites decrease without full
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growth [26].
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Thermal Stability
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Thermal stability is one of the key issues of the electrolyte membranes in practical application of lithium batteries. In order to determine the thermal stability, the prepared NSPEMs
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were subjected to TGA analysis. Fig. 4 shows the TGA curves of PEO12–LiTDI–0 wt% TiO2 and PEO12–LiTDI–8 wt% TiO2 electrolyte membranes. The weight loss % at different temperatures
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for 0 wt% and 8 wt% electrolyte membranes are displayed in Table 1. The TGA curves of both
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samples exhibited three regions; I, a plateau up to 250 ºC with a mass loss of ~ 2% for 0 wt%
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TiO2 and 0.4% for 8 wt% TiO2, this weight loss was attributed to evaporation of superficial/residual moisture in the samples [22], II, a small slope between 250 to 350 ºC with a
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mass loss of ~ 20% and 16%, and III, a large slope between 350 to 420 ºC with a mass loss of 77.7% and 74.1%. With the addition of 8 wt% of TiO2 nanofiller, the weight loss of the electrolyte membrane was decreased, because the incorporation of filler decreases the polymer relative content. This indicates that the thermal stability enhanced up on the incorporation of 8 wt% of TiO2 nanoparticles into PEO12–LiTDI. The PEO12–LiTDI–8 wt% TiO2 electrolyte was stable up to 250 °C before decomposition.
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FTIR analysis FT-IR spectroscopy is very important in the investigation of polymer structures. It is applicable in understanding the occurrence of complexation and interaction among lithium ions,
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ether oxygen of host polymer PEO and TiO2 nanoparticles. The FT-IR spectroscopy has been used to study the polyether all CH2 modes, C-O-C stretching mode and lithium salt vibrational
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modes occurring within the PEO based electrolyte membranes. It was generally believed that the
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salvation of the salt in poly(ethylene oxide) involves coordination of the cation to the ether oxygens of the polymer network.
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The FTIR spectrum of pure PEO, PEO12–LiTDI and PEO12–LiTDI–8% TiO2 (which exhibits maximum ionic conductivity) are given in Fig. 5. The most important vibrational modes and
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wave numbers exhibited by pure PEO are CH2 wagging mode at 842 cm-1, CH2 twisting mode at 963 cm-1 [28, 29], CH2 bending vibrations at 1343 and 1360 cm-1 [30], CH2 stretching broad
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peak centered at 2880 cm-1 [31] and C-O-C stretching mode divided into three peaks at 1149, 1107 and 1061 cm-1, respectively. The CH2 bending doublet and C-O-C stretching triplet modes
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confirmed the existence of PEO crystallinity [30]. The FT-IR spectrum of pure LiTDI was reported by Niedzicki et al. [32]. The peaks reported in IR spectrum were 2244 cm-1 (CN stretching), 1501 cm-1 (ring stretching), 1464 cm-1 (ring stretching), 1189 cm-1 (CF stretching), 1141 and 1002 cm-1.
After LiTDI incorporation into the PEO matrix, the intensity, shape and position of all CH2 and C-O-C modes changes, which are associated with ether oxygen-cation interactions. The intensity decreased remarkably in all CH2 and C-O-C modes of PEO12–LiTDI electrolyte membrane compared with pure PEO. The CH2 wagging mode looked like sharp peak in pure PEO, but in PEO12–LiTDI complex this peak broadened and resolved into two peaks (834 and
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849 cm-1). The CH2 twisting mode shifted towards lower wave number side in polymer-salt complex. Two well defined peaks of CH2 bending mode at 1343 cm-1 and 1360 cm-1 for pure PEO merge into a wide single peak centered at 1354 Cm-1 with decreased intensity. The CH2
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stretching mode also undergoes some variations which result in the decreased intensity and band spreading in the range 2850-3000 cm-1. With the addition of LiTDI, the C-O-C band maximum
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shifted to lower wave number side with decreased intensity which was in good agreement with
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results published by Rocco et al [33] and the peaks at 1149 and 1061 cm-1 of C-O-C triplet almost disappeared. Particularly the triplet C-O-C of PEO becomes broader and nearly combines
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into their corresponding peaks. The interactions of pure PEO with alkali metal salts have been
a similar association with PEO [34, 35].
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studied extensively and the vibrational spectroscopic results show that alkali metal salts undergo
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The peaks around 1183 and 2225 cm-1 were observed in PEO12–LiTDI electrolyte membrane,
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which were due to C-F stretching and CN stretching vibrations of LiTDI salt. The CN stretching mode associated with ion-ion interactions. The new peak observed at 978 cm-1 in polymer-salt
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complex was not observed in pure PEO, which is due to LiTDI. The small peaks around 927, 933, 948 and 2804 cm-1 of PEO were disappeared in PEO12–LiTDI electrolyte membrane. The above results confirm the complex formation between polymer and salt. The vibrational modes of the PEO–LiTDI complex have been influenced due to the addition of TiO2 nanoparticles. The characteristic peaks of PEO are shifted and changed their shape and intensity after the addition of TiO2 nanofiller. The vibrational modes of CH2 twisting, CH2 bending and C-O-C stretching were shifted to lower wave numbers. The shape of the all CH2 modes changed with decreased intensity. The changes observed in the FTIR spectrum after the TiO2 nanoparticles addition is due to the result of Lewis acid–base interaction. The TiO2
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nanoparticles can interact with the two Lewis bases, the oxygen atoms of PEO and TDI¯. After the addition of TiO2 nanoparticles, the OH groups of TiO2 form hydrogen bond with oxygen atoms of PEO which can be considered as Lewis acid–base interactions. The polymer chains thus
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become less flexible due to the cross-linking of PEO chains through TiO2 [36]. From the above analyses, it was concluded that there are significantly strong interactions among PEO, LiTDI and
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TiO2.
Mechanical Properties
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In order to quantify the mechanical strength, the stress-strain behavior of solid polymer electrolytes with and without TiO2 nanofillers was performed. Fig. 6 shows the stress-strain
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curves of PEO12–LiTDI and PEO12–LiTDI–8 wt% of TiO2 electrolyte membranes. Incorporation
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of 8 wt% TiO2 nanofillers enhances the mechanical stability of the polymer electrolytes.
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Improvement of such stability may be attributed to the structural changes induced by TiO2 nanofillers as well as their mobility [37]. For 0 wt% of TiO2 doped electrolyte the strength was
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0.462 MPa. After the incorporation of 8 wt% of TiO2 nanoparticles improved the mechanical stability of the electrolyte membranes. The stress increased from 0.462 to 0.62 MPa and elongation break also increased from 102 to 137%. This enhancement in mechanical strength is due to interaction between the ether oxygen of PEO and the hydroxyl group of TiO2, thus constructing a strong network. The Lewis-acid surfaces of TiO2 nanofillers also beneficial for the improvement of NSPEM performance [38].
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Conductivity Analysis The effect of TiO2 nanoparticles on the conductive properties of PEO12–LiTDI electrolyte was investigated by complex impedance testing. Fig. 7 shows the complex impedance plots (Z'
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vs. Z") of PEO12–LiTDI electrolyte membrane with and without TiO2 nanoparticles at room temperature (23 °C). The impedance spectrum consists of an arc in the high frequency region
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followed by a spike in the lower frequency region. The spike in the low frequency region has
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been attributed to the blocking double layer capacitance (Cdl) near the electrode-electrolyte interface formed by the ion migration and an arc in the high frequency region corresponding to
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bulk properties in their impedance plots. With the incorporation of 8 wt% of TiO2 nanoparticles, the impedance plot shifted to lower resistance side on the real axis. The size of the high
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frequency arcs also reduces with the addition of TiO2 nanoparticles. The intercept of the arc with
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the real axis (Z') in the low frequency region gives rise to the bulk resistance (Rb) of the materials.
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By knowing the bulk resistance (Rb) along with the dimensions of the sample, the conductivity of
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the sample has been calculated by using the equation: (1)
where L and A are the thickness and area of the polymer electrolyte samples, respectively. Generally, equivalent circuit is used for the analysis of impedance spectroscopy because it is simple and can provide the complete picture of the system. The equivalent circuit is shown in the inset circuit of Fig. 7. Cdl represents the pure capacitance in consideration of the double layer capacitance at the electrode/electrolyte interface. CPE is related to the bulk relaxation process of the polymer electrolyte in the equivalent circuit. In order to enhance the ionic conductivity, specific amounts of nano-sized ceramic fillers were incorporated into PEO12–LiTDI electrolyte membrane. The variation of ionic conductivity
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with temperature and TiO2 nanofiller concentration (inset) are shown in Fig. 8. The ionic conductivity of the PEO12–LiTDI system is 2.05 x 10-6 S/cm. After the incorporation of TiO2 nanofillers, the ionic conductivity increases up to 8 wt% of TiO2 nanofillers and reaches a
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maximum value of 2.11 x 10-5 S/cm. This optimum ionic conductivity is greater than the composite polymer electrolyte systems reported earlier [39-43]. When the TiO2 content was
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increased beyond 8 wt%, the ionic conductivity decreased slightly from the maximum value. It
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could be concluded that the addition of 8 wt% TiO2 content provided the most suitable environment for the ionic transportation and, thereby, achieved the highest conductivity. It was
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considered that the ion mobility of the nanocomposite electrolyte membrane was enhanced due to the decreased crystallinity and beneficial features of the TiO2 in comparison with 0 wt% of
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TiO2 electrolyte membrane. But, when the TiO2 content reaches to 12 wt%, the presence of TiO2
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aggregation reduces its miscibility with PEO matrix. The separated phase dominates the mobility
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of the conductive ions and then the conductivity of the polymer electrolytes [44]. The improvement in ionic conductivity with the incorporation of nano-sized ceramic filler
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can be further explained on the basis of Lewis acid–base interactions between OH groups at the TiO2 surface and ether oxygen atoms of PEO. The Lewis acid–base coordinates with TDI¯ and weakens the interactions between lithium ions and TDI¯, which in turn increases the dissolvability of LiTDI which increases the number of free ions [45]. The incorporation of TiO2 nanofillers could also prevents the re-crystallization of PEO12–LiTDI electrolyte membrane. From Fig. 8, the ionic conductivity increases with the increasing temperature and the ionic conductivity of 8 wt% nano-TiO2 doped PEO12–LiTDI membrane is always highest among the four PEO12–LiTDI based electrolyte membranes throughout the all temperatures. The conductivity increases linearly with temperature and follows Arrhenius relationship:
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(2) where σ0 is the pre-exponential factor, Ea is the activation energy, and k is the Boltzmann constant.
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The activation energy values calculated from linear fitting were obtained 0.449, 0.374, 0.245
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and 0.341 eV for 0 wt%, 4 wt%, 8 wt% and 12 wt% of TiO2 nanofillers respectively. The behavior of conductivity improvement with temperature can be understood in terms of the free
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volume model [46]. As the temperature increases, the polymer can expand easily and produce
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free volume. Thus, as temperature increases, the free volume increases. The resulting conductivity, represented by the overall mobility of ions and the polymer, is determined by the
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free volume around the polymer chains. Therefore, as temperature increases, ions, solvated molecules, or polymer segments can move into the free volume. This leads to an increase in ion
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hindering effect of the ion clouds.
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mobility and segmental mobility that will assist ion transport and virtually compensate for the
Electrochemical stability of the NSPEM The electrochemical stability of the solid polymer electrolytes is evaluated in terms of electrochemical potential window by cyclic voltammetry (CV). Fig. 9 shows the CV plots, recorded on the cell of different cycles comprising of the 8 wt% of TiO2 doped NSPEM sandwiched between the symmetrical lithium electrodes in the potential range of 0–5 V (vs. Li+/Li) at a scanning rate of 5 mV s-1. From Fig. 9, the working voltage in all cycles (i.e. electrochemical potential window) has been found in the range from 0.5 to 4.5 V, which appears to be acceptable working voltage for the device application point of view, particularly as electrolyte in rechargeable lithium ion battery systems [47]. The currents decrease with the
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increase of the cycle numbers, which may be due to the formation of some kind of passivation film on the electrode [48].
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Conclusions
We have successfully prepared nanocomposite solid polymer electrolyte membranes
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containing PEO12–LITDI complexed with various contents of TiO2 nanoparticles. The
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incorporation of nano-sized TiO2 ceramic fillers significantly improves the amorphous phase, thermal stability, mechanical integrity and ionic conductivity of the SPEMs. A significant
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decrease in the crystallinity phase of the polymer electrolyte with the addition of TiO2 nanoparticles has been evidenced through XRD and POM studies. XRD and FTIR studies
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confirmed the complex formation among PEO, LiTDI and TiO2. The TGA study indicated that
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the thermal stability of the PEO12–LITDI electrolyte membrane was changed with the addition of
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the 8 wt% of TiO2 content. This 8 wt% TiO2 system is thermally stable up to 250 °C. The addition of 8 wt% of TiO2 has a beneficial effect on the mechanical properties of the PEO12–
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LITDI electrolyte membrane. The ambient temperature ionic conductivity increased up to a maximum value of 2.11×10-5 S/cm with the incorporation of 8 wt% of TiO2 nanofillers. The electrochemical stability window of the nanocomposite solid polymer electrolyte membrane is ~ 4 V. The above results suggest a potential application of this kind of solid polymer electrolyte in lithium-ion batteries.
Acknowledgements This work was supported by Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT &
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Future Planning, Korea (2011-0031570) and by the Korea Center for Artificial Photosynthesis (KCAP) located in Sogang University funded by the Minister of Science, ICT and Future Planning (MSIP) through the National Research Foundation of Korea (No. 2009- 0093883) and
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also supported by the Human Resources Development program (No.20114010203090) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the
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Korea government Ministry of Trade, Industry and Energy.
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25. L. Lee, S.J. Park, S. Kim, Solid State Ionics. 234 (2013) 19. 26. L. Guijie, X.U. Jie, X.U. Weilin, S. Xiaolin, B. Zhikui, Y. Mu, J. Wuhan Uni. Tech.Mater. Sci. Ed. 27 (2012) 495. 27. J. Xi, X. Qiu, J. Wang, Y. Bai, W. Zhu, L. Chen, J. Power Sources. 158 (2006) 627.
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Armand, S. Panero, B. Scrosati, M. Marcinek, W. Wieczorek, Electrochim. Acta. 55 (2010) 1450.
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37. V. Aravindan, P. Vickraman, K. Krishnaraj, Curr. Appl. Phys. 9 (2009) 1474.
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38. K. Zhu, Y. Liu, J. Liu, RSC Adv. 4 (2014) 42278. 39. F. Croce, L. Settimi, B. Scrosati, Electrochem. Commu. 8 (2006) 364. 40. B.K. Choi, K.H. Shin, Solid State Ionics 86-88 (1996) 303. 41. W. Wieczorek, A. Zalewska, D. Raducha, Z. Florjanczyk, J.R. Stevens, A. Ferry, P. Jacobsson, Macromolecules 29 (1996) 143. 42. F. Croce, R. Curini, A. Martinelli, L. Persi, F. Ronci, B. Scrosati, R. Caminiti, J. Phys. Chem. B 103 (1999) 10632. 43. B. Kumar, L.G. Scanlon, Solid State Ionics 124 (1999) 239. 44. P.P. Chu, M.J. Reddy, J. Power Sources. 115 (2003) 288.
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45. W. Wieczorek, Z. Florjanczyk, J.R. Stevens, Electrochim. Acta. 40 (1995) 2251. 46. A.R. Polu, R. Kumar, Bull. Mater. Sci. 37 (2014) 309. 47. M. Ravi, S. Song, K. Gu, J. Tang, Z. Zhang, Mater. Sci. Eng. B 195 (2015) 74.
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48. T. Niitani, M. Shimada, K. Kawamura, K. Dokko, Y.H. Rho, K. Kanamura, Electrochem.
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Solid-State Lett. 8 (2005) A385.
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Figure Captions Fig. 1: The NSPE membrane synthesis process and the interaction scheme among PEO, LiTDI and TiO2.
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Fig. 2: XRD patterns of pure PEO, pure LiTDI, pure TiO2 and PEO–LiTDI–TiO2 SPE membranes with different amounts of TiO2.
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LiTDI–8 wt% TiO2 polymer electrolyte membranes.
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Fig. 3: Polarized optical micrographs (100 µm) of a) pure PEO, b) PEO12–LiTDI and c) PEO12–
Fig. 4: TGA heating traces of 0 wt% and 8 wt% TiO2 doped PEO12–LiTDI electrolyte
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membranes.
Fig. 5: FTIR spectra of a) pure PEO, b) PEO12–LiTDI and c) PEO12–LiTDI–8 wt% TiO2
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polymer electrolyte membranes.
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membranes.
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Fig. 6: Stress–strain curves of PEO12–LiTDI and PEO12–LiTDI–8 wt% TiO2 polymer electrolyte
Fig. 7: Nyquist impedance plots of PEO12–LiTDI and PEO12–LiTDI–8 wt% TiO2 electrolyte
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membranes at room temperature (23 ºC). The equivalent circuit is in inset. Fig. 8: Logarithm of ionic conductivity (lnσ) versus reciprocal temperature (1/T) for PEO– LiTDI–TiO2 SPE membranes with different amounts of TiO2. Conductivity versus TiO2 concentration at room temperature (23 ºC) is in inset. Fig. 9: Cyclic voltammograms of 8 wt% of TiO2 doped nanocomposite electrolyte membrane measured at 23 °C. Potential sweep rate was 5 mV s-1. Table Title Table 1: TGA data: weight loss of 0 wt% and 8 wt% TiO2 doped PEO12–LiTDI electrolyte membranes at different temperatures.
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Fig. 1
Fig. 2
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Fig. 3
Fig. 4
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Fig. 5
Fig. 6
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Fig. 7
Fig. 8
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Fig. 9
Sample
Table 1
Weight loss at different temperatures (in %)
Wt% of TiO2
100 °C
250 °C
350 °C
420 °C
0
2%
4%
20%
77.7%
8
0.4%
1.7%
16%
74.1%
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*Graphical Abstract
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S1226-086X(16)30057-0 http://dx.doi.org/doi:10.1016/j.jiec.2016.03.042 JIEC 2889
To appear in: Received date: Revised date: Accepted date:
18-1-2016 8-3-2016 27-3-2016
Please cite this article as: A.R. Polu, Effect of TiO2 nanoparticles on structural, thermal, mechanical and ionic conductivity studies of PEO12 -LiTDI solid polymer electrolyte, Journal of Industrial and Engineering Chemistry (2016), http://dx.doi.org/10.1016/j.jiec.2016.03.042 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights PEO12–LiTDI–TiO2 nanocomposite electrolytes were synthesized for Li-ion batteries. Strong interaction among PEO, LiTDI and TiO2 was confirmed by FTIR studies.
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TiO2 gave significant enhancement in thermal stability and mechanical strength of SPEs.
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Effect of TiO2 nanofiller on conductivity increase in PEO12–LiTDI is reported.
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Effect of TiO2 nanoparticles on structural, thermal, mechanical and ionic conductivity studies of PEO12-LiTDI solid polymer electrolyte
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Anji Reddy Polu* and Hee-Woo Rhee** Polymer Materials Lab, Department of Chemical and Biomolecular Engineering,
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*Email: [email protected]
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Sogang University, 35 Baekbeom-Ro, Mapo-Gu, Seoul 121-742, South Korea
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**Corresponding Author E-mail: [email protected];
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Phone: +82 2 705 8483; Fax: +82 2 711 0439
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Abstract In the present study, poly(ethylene oxide) (PEO) complexed with lithium 2-trifluoromethyl-4, 5-dicyanoimidazole (LiTDI) nanocomposite solid polymer electrolyte membranes (NSPEMs)
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have been prepared by solution cast technique using different weight percent of nano-sized TiO2 ceramic filler. The effect of filler incorporation on the structural, thermal, mechanical and ionic
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conductivity properties of solid polymer electrolytes have analyzed. X-ray diffraction (XRD) and
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polarized optical microscopy (POM) results indicated that the crystallinity has been reduced remarkably with the incorporation of TiO2 nanofiller. The thermal stability and mechanical
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integrity of the nanocomposite polymer electrolyte system increased significantly compared to filler free electrolytes. The maximum ionic conductivity is found to be in the range of 2.11 x 10-5
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S/cm for 8 wt% TiO2 nanofiller in PEO12–LiTDI electrolyte system. These results indicated that
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the prepared TiO2 based nanocomposite membrane would be a promising alternative separator
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for rechargeable lithium-ion battery applications.
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Keywords: Solid polymer electrolyte; TiO2 nanoparticles; ionic conductivity; thermal stability; mechanical integrity.
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Introduction Solid polymer electrolyte membranes (SPEMs) have received extensive attention in the past few decades for their potential applications in solid state ionic devices such as lithium batteries,
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fuel cells, solar cells etc [1-3]. Lithium batteries have attracted a great deal of attention due to its higher energy density, improved safety hazards, and good processability [4, 5]. As it is well
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known, SPEMs have several advantages over the liquid counterpart such as desirable shape
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mouldability, light-weight, free from leakage, mechanical strength, high physical and chemical stability and flexibility of design, thereby permitting miniaturization [6]. One of the most
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important properties of high-performance polymer electrolytes is their ionic conductivity. In polymer electrolyte membranes, ionic conduction depends on the dissociation of the ionic
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conductor and the structure of the polymer matrix. High molecular weight poly(ethylene oxide)
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(PEO) based polymer electrolytes containing lithium salts have been studied extensively due to
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its compatibility with a wide range of ionic conducting salts while maintaining an acceptable chemical and electrochemical stability [7].
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Recent success of imidazole-derived lithium salts [8] and stressed as “tailor made” salts, notably LiTDI (lithium 4,5-dicyano-2-(trifluoromethyl)imidazole), has encouraged us for lithium battery applications [9]. For exploring the properties of this salt, only a few publications are available [10, 11]. In compare with many other lithium salts, LiTDI is fully stable in the presence of moisture [12]. In addition, this LiTDI electrolyte is thermally stable up to 250 °C far more than boiling point of any solvent or stability of many other organic lithium salts. The safety, stability information of LiTDI and its parameters in solvent mixtures are available elsewhere [13]. However, a major drawback of these poly(ethylene oxide)–lithium salt electrolyte
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membranes for practical applications is that they tend to crystallize at ambient temperatures resulting low ionic conductivity [14]. Generally two ways have been used to enhance the room temperature ionic conductivity of
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the polymer electrolytes; one is the incorporation of nano-sized ceramic fillers such as TiO2, CeO2, SiO2 and Al2O3 [15] in the polymer matrix and the other is the addition of plasticizers
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such as ethylene carbonate (EC), propylene carbonate (PC) and polyethylene glycol (PEG) [16].
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These plasticizers can improve the room-temperature ionic conductivity of the polymer electrolytes, but decrease the mechanical properties and the potential stability of these polymer
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membranes [17]. The incorporation of ceramic nano-fillers reduces the crystalline character, improve the ionic conductivity as well as promote the thermal behaviors, mechanical stability
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and electrochemical properties of the polymer membrane [18]. Among the several fillers in
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polymer electrolyte membranes, nano-sized titanium oxide (TiO2) supports the ionic mobility
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due to its substrate characteristics, such as shape and surface nature, which effectively disturbs the order packing tendency of the host polymer chains [19]. The effects of TiO2, Al2O3 and SiO2
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in PEO–LiClO4 polymer electrolyte has been studied by Chung et al. [20] and identified TiO2 as the filler with greatest enhancement in ionic conductivity due to the weakened interactions between the polymer chain and the Li+ ions. To the best of our knowledge, there are no reports available in the literature on the ionic conductivity, thermal stability and mechanical stability enhancement of PEO–LiTDI polymer electrolyte due to the incorporation of nano-sized TiO2 ceramic filler. In this work, we have studied the effect of TiO2 nano-filler incorporation on structural, thermal, mechanical and electrical properties of PEO12–LiTDI polymer electrolyte membrane. Xray diffraction (XRD), polarized optical microscopy (POM), thermogravimetric analysis (TGA),
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fourier transform infrared spectroscopy (FTIR) and universal testing machine (UTM) were employed to characterize the physical and chemical properties of the nanocomposite solid polymer electrolyte membranes (NSPEMs). Impedance spectroscopy was used to study the ionic
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conductivity of the NSPEMs at various temperatures. Cyclic voltammetry was used to examine
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the electrochemical potential window of the NSPEM.
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Experimental Details Materials used
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High molecular weight PEO (MW = 4 x 106 g/mole), TiO2 nanopowder (21 nm) and anhydrous acetonitrile were purchased from Sigma-Aldrich (St. Louis, USA). Lithium 2-
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trifluoromethyl-4, 5-dicyanoimidazole (LiTDI) obtained from Alfa Aesar.
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Preparation of nanocomposite solid polymer electrolyte membranes The NSPEMs have been prepared by using solution cast technique. The high molecular
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weight PEO, TiO2 nanopowder and LiTDI have been dried at 50, 100 and 100 °C under vacuum for 24 h respectively. The high molecular weight PEO and LiTDI was dissolved in anhydrous acetonitrile, followed by the addition of different weight percentages of TiO2 nanofiller. The relative amounts of PEO and LiTDI are taken at the EO/Li = 12 based only on the amount of ethylene oxide in the PEO and lithium in the LiTDI. The mixture was stirred for 24 h to obtain a homogeneous viscous solution. This solution was cast onto Teflon plates followed by the evaporation of the solvent in a dry box for about 1 day. The resulting films were further dried in vacuum oven at 40º C for 24 h. After that the films shifted to nitrogen-filled glove box and left
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undisturbed for 5 days to remove residual acetonitrile. The NSPE membrane synthesis process and the interaction scheme among PEO, LiTDI and TiO2 are displayed in Fig. 1.
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Characterization
The X-ray diffraction spectra of polymer electrolyte membranes were carried out using an X-
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ray diffractometer (D-MAX 2500, Rigaku) with Cu-Kα radiation (λ= 1.5406 Å) at ambient
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temperature in the range 2θ = 5 ~ 50º. Surface morphology and microscopic feature of the polymer films were examined using polarized optical microscope (Optiphot-2 Pol, Nikon) at
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room temperature. The thermogravimetry (TG) measurements were carried out on a TGA-2950 thermal analyzer (Hi-Res, TA instruments) by heating from 25 to 500 °C under a N2 atmosphere
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at a heating rate of 20 °C/min, with a sample about 7 mg. The formation of polymer–salt–
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nanofiller complexes and interaction among polymer, salt, and ceramic nanofiller were
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determined by fourier transform infrared spectroscopy (FTIR) using Nicolet 380 FT-IR spectrometer (Thermo Electron) in the region 4000 – 400 cm-1 at a resolution 1 cm-1. Universal
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materials testing machine (Lloyd instruments, LR5K Plus) has been used to measure the mechanical stability of the solid polymer electrolytes. Impedance and conductivity of the samples were determined using an Iviumstat (Ivium Technologies, Netherlands). The measurements were done in the frequency range from 1 Hz to 1 MHz by varying temperature from 23 to 60 ºC. The impedance studies were carried out by sandwiching the solid polymer electrolyte membrane between two gold electrodes under spring pressure. The thickness of each sample was measured using a micrometer screw gauge. The electrochemical studies were carried out by cyclic voltammetry using an electrochemical analyzer (WonA tech WBCS 3000L battery cycler system) in the potential range 0 to 5 V (vs. Li+/Li) at a scanning rate of 5 mV s-1
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Results and discussion XRD analysis The XRD patterns of pure PEO, pure LiTDI, pure TiO2 and PEO12–LiTDI–TiO2 polymer
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electrolytes at various TiO2 concentrations are shown in Fig. 2. The X-ray diffraction pattern of pristine PEO shows two high intensity diffraction peaks at 19.36º and 23.72º which are assigned
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to set of planes (120) and (112) [21]. The XRD pattern of LiTDI salt exhibits sharp peaks
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centered at 2θ angle of 10.4º, 11.2º, 15.5º, 23.4º, 24.7º, 27.2º, 31.3º, 33.5º, 40.3º due to the crystalline nature of the salt. After LiTDI inclusion, the peaks of PEO shifted to lower 2θ values
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due to increase of the interplanar crystal spacing of PEO which may have occurred from the insertion of large TDI anions into PEO lattice. In XRD pattern of PEO12-LiTDI polymer
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electrolyte (0 wt% TiO2), no diffraction peak of LiTDI is found, demonstrating that the added
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lithium salt has well dissolved into the PEO matrix. With the incorporation of LiTDI decreased
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the intensity of the peaks and broader than pure PEO. This can be attributed to a destruction effect of the lithium salt on the ordered arrangement of the polymer chains, and hence an
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enhancement in the amorphous phase and led to increase in ionic conductivity [22]. The XRD pattern of nano-sized TiO2 ceramic filler shows intensive peaks at 2θ ≈ 25°, 36° and 48° and these correspond to (1 0 1), (0 0 4) and (2 0 0) planes , which indicate its crystalline nature. After the incorporation of nano ceramic filler into PEO12-LiTDI polymer electrolyte, the intensity of the PEO diffraction peaks decreased remarkably and broadened, which implies that the PEO matrix supplemented the amorphous region. As the wt% of TiO2 nanofiller increases, the crystallinity of the polymer has been significantly reduced and this reduction is attributed to the re-organization of polymeric chains by the inert nanoparticles present in nanocomposite polymer electrolyte membranes. This reorganization facilitates higher ionic conduction [23]. A
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similar observation was reported by ElBellihi et al. and Lee et al. for XRD analysis of PEOLiClO4-ZnO and PEO/PVDF/PC-LiClO4-BaTiO3 composite polymer electrolytes [24, 25].
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POM analysis
Polarized optical microscopy (POM) was used to understand the crystalline behavior and
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their related surface morphology of the polymer films. Fig. 3 displays the typical room
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temperature POM images of pure PEO, PEO12–LiTDI and PEO12–LiTDI–8 wt% TiO2 electrolyte membranes. Fig. 3(a) shows the typical spherulitic texture along with dark boundaries of the pure
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polymer PEO, demonstrating its semi-crystalline nature. The boundaries of the spherulites were non-spherical but smooth after impingement with the adjacent spherulites. The spherulitic texture
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in the polymer film showed its lamellar crystalline nature and the dark boundaries indicate the
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amorphous content in the polymer. The lamellar was developed through regular polymer chains
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folding, leading to a long range order. The lamella radiate out from a central nucleating point. After the incorporation of LiTDI into PEO, the amount of spherulites increases and the spherulite
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size becomes small while the amorphous region (black portion) increases (Fig. 3(b)). As the 8 wt% of TiO2 is incorporated into the PEO12–LiTDI electrolyte system, the dark regions between spherulites expands and PEO spherulites cannot be observed clearly in PEO12–LiTDI–8 wt% TiO2 electrolyte membrane (Fig. 3(c)). The expansion of the dark regions suggests the amorphous region increased with the interruption of the crystallite in presence of the TiO2 nanofillers. This type of surface morphology was also observed in other PEO based systems such as PEO–LiI–Y2O3 and PEO–LiClO4–Al2O3 composite polymer electrolyte membranes [26, 27]. The dark area in PEO12–LiTDI–8 wt% TiO2 electrolyte membrane is larger than that in PEO12– LiTDI, suggesting that the ratio of amorphous PEO in 8 wt% TiO2 composite polymer
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electrolyte is higher than that in PEO12–LiTDI. It is supposed to be that the addition of TiO2 nanofillers reduces the crystalline behavior of PEO through Lewis acid-based interactions, under the steric hindrance effects of TiO2, which makes the size of the spherulites decrease without full
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growth [26].
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Thermal Stability
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Thermal stability is one of the key issues of the electrolyte membranes in practical application of lithium batteries. In order to determine the thermal stability, the prepared NSPEMs
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were subjected to TGA analysis. Fig. 4 shows the TGA curves of PEO12–LiTDI–0 wt% TiO2 and PEO12–LiTDI–8 wt% TiO2 electrolyte membranes. The weight loss % at different temperatures
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for 0 wt% and 8 wt% electrolyte membranes are displayed in Table 1. The TGA curves of both
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samples exhibited three regions; I, a plateau up to 250 ºC with a mass loss of ~ 2% for 0 wt%
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TiO2 and 0.4% for 8 wt% TiO2, this weight loss was attributed to evaporation of superficial/residual moisture in the samples [22], II, a small slope between 250 to 350 ºC with a
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mass loss of ~ 20% and 16%, and III, a large slope between 350 to 420 ºC with a mass loss of 77.7% and 74.1%. With the addition of 8 wt% of TiO2 nanofiller, the weight loss of the electrolyte membrane was decreased, because the incorporation of filler decreases the polymer relative content. This indicates that the thermal stability enhanced up on the incorporation of 8 wt% of TiO2 nanoparticles into PEO12–LiTDI. The PEO12–LiTDI–8 wt% TiO2 electrolyte was stable up to 250 °C before decomposition.
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FTIR analysis FT-IR spectroscopy is very important in the investigation of polymer structures. It is applicable in understanding the occurrence of complexation and interaction among lithium ions,
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ether oxygen of host polymer PEO and TiO2 nanoparticles. The FT-IR spectroscopy has been used to study the polyether all CH2 modes, C-O-C stretching mode and lithium salt vibrational
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modes occurring within the PEO based electrolyte membranes. It was generally believed that the
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salvation of the salt in poly(ethylene oxide) involves coordination of the cation to the ether oxygens of the polymer network.
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The FTIR spectrum of pure PEO, PEO12–LiTDI and PEO12–LiTDI–8% TiO2 (which exhibits maximum ionic conductivity) are given in Fig. 5. The most important vibrational modes and
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wave numbers exhibited by pure PEO are CH2 wagging mode at 842 cm-1, CH2 twisting mode at 963 cm-1 [28, 29], CH2 bending vibrations at 1343 and 1360 cm-1 [30], CH2 stretching broad
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peak centered at 2880 cm-1 [31] and C-O-C stretching mode divided into three peaks at 1149, 1107 and 1061 cm-1, respectively. The CH2 bending doublet and C-O-C stretching triplet modes
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confirmed the existence of PEO crystallinity [30]. The FT-IR spectrum of pure LiTDI was reported by Niedzicki et al. [32]. The peaks reported in IR spectrum were 2244 cm-1 (CN stretching), 1501 cm-1 (ring stretching), 1464 cm-1 (ring stretching), 1189 cm-1 (CF stretching), 1141 and 1002 cm-1.
After LiTDI incorporation into the PEO matrix, the intensity, shape and position of all CH2 and C-O-C modes changes, which are associated with ether oxygen-cation interactions. The intensity decreased remarkably in all CH2 and C-O-C modes of PEO12–LiTDI electrolyte membrane compared with pure PEO. The CH2 wagging mode looked like sharp peak in pure PEO, but in PEO12–LiTDI complex this peak broadened and resolved into two peaks (834 and
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849 cm-1). The CH2 twisting mode shifted towards lower wave number side in polymer-salt complex. Two well defined peaks of CH2 bending mode at 1343 cm-1 and 1360 cm-1 for pure PEO merge into a wide single peak centered at 1354 Cm-1 with decreased intensity. The CH2
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stretching mode also undergoes some variations which result in the decreased intensity and band spreading in the range 2850-3000 cm-1. With the addition of LiTDI, the C-O-C band maximum
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shifted to lower wave number side with decreased intensity which was in good agreement with
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results published by Rocco et al [33] and the peaks at 1149 and 1061 cm-1 of C-O-C triplet almost disappeared. Particularly the triplet C-O-C of PEO becomes broader and nearly combines
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into their corresponding peaks. The interactions of pure PEO with alkali metal salts have been
a similar association with PEO [34, 35].
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studied extensively and the vibrational spectroscopic results show that alkali metal salts undergo
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The peaks around 1183 and 2225 cm-1 were observed in PEO12–LiTDI electrolyte membrane,
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which were due to C-F stretching and CN stretching vibrations of LiTDI salt. The CN stretching mode associated with ion-ion interactions. The new peak observed at 978 cm-1 in polymer-salt
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complex was not observed in pure PEO, which is due to LiTDI. The small peaks around 927, 933, 948 and 2804 cm-1 of PEO were disappeared in PEO12–LiTDI electrolyte membrane. The above results confirm the complex formation between polymer and salt. The vibrational modes of the PEO–LiTDI complex have been influenced due to the addition of TiO2 nanoparticles. The characteristic peaks of PEO are shifted and changed their shape and intensity after the addition of TiO2 nanofiller. The vibrational modes of CH2 twisting, CH2 bending and C-O-C stretching were shifted to lower wave numbers. The shape of the all CH2 modes changed with decreased intensity. The changes observed in the FTIR spectrum after the TiO2 nanoparticles addition is due to the result of Lewis acid–base interaction. The TiO2
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nanoparticles can interact with the two Lewis bases, the oxygen atoms of PEO and TDI¯. After the addition of TiO2 nanoparticles, the OH groups of TiO2 form hydrogen bond with oxygen atoms of PEO which can be considered as Lewis acid–base interactions. The polymer chains thus
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become less flexible due to the cross-linking of PEO chains through TiO2 [36]. From the above analyses, it was concluded that there are significantly strong interactions among PEO, LiTDI and
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TiO2.
Mechanical Properties
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In order to quantify the mechanical strength, the stress-strain behavior of solid polymer electrolytes with and without TiO2 nanofillers was performed. Fig. 6 shows the stress-strain
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curves of PEO12–LiTDI and PEO12–LiTDI–8 wt% of TiO2 electrolyte membranes. Incorporation
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of 8 wt% TiO2 nanofillers enhances the mechanical stability of the polymer electrolytes.
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Improvement of such stability may be attributed to the structural changes induced by TiO2 nanofillers as well as their mobility [37]. For 0 wt% of TiO2 doped electrolyte the strength was
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0.462 MPa. After the incorporation of 8 wt% of TiO2 nanoparticles improved the mechanical stability of the electrolyte membranes. The stress increased from 0.462 to 0.62 MPa and elongation break also increased from 102 to 137%. This enhancement in mechanical strength is due to interaction between the ether oxygen of PEO and the hydroxyl group of TiO2, thus constructing a strong network. The Lewis-acid surfaces of TiO2 nanofillers also beneficial for the improvement of NSPEM performance [38].
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Conductivity Analysis The effect of TiO2 nanoparticles on the conductive properties of PEO12–LiTDI electrolyte was investigated by complex impedance testing. Fig. 7 shows the complex impedance plots (Z'
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vs. Z") of PEO12–LiTDI electrolyte membrane with and without TiO2 nanoparticles at room temperature (23 °C). The impedance spectrum consists of an arc in the high frequency region
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followed by a spike in the lower frequency region. The spike in the low frequency region has
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been attributed to the blocking double layer capacitance (Cdl) near the electrode-electrolyte interface formed by the ion migration and an arc in the high frequency region corresponding to
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bulk properties in their impedance plots. With the incorporation of 8 wt% of TiO2 nanoparticles, the impedance plot shifted to lower resistance side on the real axis. The size of the high
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frequency arcs also reduces with the addition of TiO2 nanoparticles. The intercept of the arc with
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the real axis (Z') in the low frequency region gives rise to the bulk resistance (Rb) of the materials.
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By knowing the bulk resistance (Rb) along with the dimensions of the sample, the conductivity of
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the sample has been calculated by using the equation: (1)
where L and A are the thickness and area of the polymer electrolyte samples, respectively. Generally, equivalent circuit is used for the analysis of impedance spectroscopy because it is simple and can provide the complete picture of the system. The equivalent circuit is shown in the inset circuit of Fig. 7. Cdl represents the pure capacitance in consideration of the double layer capacitance at the electrode/electrolyte interface. CPE is related to the bulk relaxation process of the polymer electrolyte in the equivalent circuit. In order to enhance the ionic conductivity, specific amounts of nano-sized ceramic fillers were incorporated into PEO12–LiTDI electrolyte membrane. The variation of ionic conductivity
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with temperature and TiO2 nanofiller concentration (inset) are shown in Fig. 8. The ionic conductivity of the PEO12–LiTDI system is 2.05 x 10-6 S/cm. After the incorporation of TiO2 nanofillers, the ionic conductivity increases up to 8 wt% of TiO2 nanofillers and reaches a
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maximum value of 2.11 x 10-5 S/cm. This optimum ionic conductivity is greater than the composite polymer electrolyte systems reported earlier [39-43]. When the TiO2 content was
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increased beyond 8 wt%, the ionic conductivity decreased slightly from the maximum value. It
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could be concluded that the addition of 8 wt% TiO2 content provided the most suitable environment for the ionic transportation and, thereby, achieved the highest conductivity. It was
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considered that the ion mobility of the nanocomposite electrolyte membrane was enhanced due to the decreased crystallinity and beneficial features of the TiO2 in comparison with 0 wt% of
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TiO2 electrolyte membrane. But, when the TiO2 content reaches to 12 wt%, the presence of TiO2
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aggregation reduces its miscibility with PEO matrix. The separated phase dominates the mobility
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of the conductive ions and then the conductivity of the polymer electrolytes [44]. The improvement in ionic conductivity with the incorporation of nano-sized ceramic filler
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can be further explained on the basis of Lewis acid–base interactions between OH groups at the TiO2 surface and ether oxygen atoms of PEO. The Lewis acid–base coordinates with TDI¯ and weakens the interactions between lithium ions and TDI¯, which in turn increases the dissolvability of LiTDI which increases the number of free ions [45]. The incorporation of TiO2 nanofillers could also prevents the re-crystallization of PEO12–LiTDI electrolyte membrane. From Fig. 8, the ionic conductivity increases with the increasing temperature and the ionic conductivity of 8 wt% nano-TiO2 doped PEO12–LiTDI membrane is always highest among the four PEO12–LiTDI based electrolyte membranes throughout the all temperatures. The conductivity increases linearly with temperature and follows Arrhenius relationship:
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(2) where σ0 is the pre-exponential factor, Ea is the activation energy, and k is the Boltzmann constant.
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The activation energy values calculated from linear fitting were obtained 0.449, 0.374, 0.245
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and 0.341 eV for 0 wt%, 4 wt%, 8 wt% and 12 wt% of TiO2 nanofillers respectively. The behavior of conductivity improvement with temperature can be understood in terms of the free
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volume model [46]. As the temperature increases, the polymer can expand easily and produce
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free volume. Thus, as temperature increases, the free volume increases. The resulting conductivity, represented by the overall mobility of ions and the polymer, is determined by the
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free volume around the polymer chains. Therefore, as temperature increases, ions, solvated molecules, or polymer segments can move into the free volume. This leads to an increase in ion
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hindering effect of the ion clouds.
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mobility and segmental mobility that will assist ion transport and virtually compensate for the
Electrochemical stability of the NSPEM The electrochemical stability of the solid polymer electrolytes is evaluated in terms of electrochemical potential window by cyclic voltammetry (CV). Fig. 9 shows the CV plots, recorded on the cell of different cycles comprising of the 8 wt% of TiO2 doped NSPEM sandwiched between the symmetrical lithium electrodes in the potential range of 0–5 V (vs. Li+/Li) at a scanning rate of 5 mV s-1. From Fig. 9, the working voltage in all cycles (i.e. electrochemical potential window) has been found in the range from 0.5 to 4.5 V, which appears to be acceptable working voltage for the device application point of view, particularly as electrolyte in rechargeable lithium ion battery systems [47]. The currents decrease with the
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increase of the cycle numbers, which may be due to the formation of some kind of passivation film on the electrode [48].
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Conclusions
We have successfully prepared nanocomposite solid polymer electrolyte membranes
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containing PEO12–LITDI complexed with various contents of TiO2 nanoparticles. The
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incorporation of nano-sized TiO2 ceramic fillers significantly improves the amorphous phase, thermal stability, mechanical integrity and ionic conductivity of the SPEMs. A significant
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decrease in the crystallinity phase of the polymer electrolyte with the addition of TiO2 nanoparticles has been evidenced through XRD and POM studies. XRD and FTIR studies
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confirmed the complex formation among PEO, LiTDI and TiO2. The TGA study indicated that
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the thermal stability of the PEO12–LITDI electrolyte membrane was changed with the addition of
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the 8 wt% of TiO2 content. This 8 wt% TiO2 system is thermally stable up to 250 °C. The addition of 8 wt% of TiO2 has a beneficial effect on the mechanical properties of the PEO12–
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LITDI electrolyte membrane. The ambient temperature ionic conductivity increased up to a maximum value of 2.11×10-5 S/cm with the incorporation of 8 wt% of TiO2 nanofillers. The electrochemical stability window of the nanocomposite solid polymer electrolyte membrane is ~ 4 V. The above results suggest a potential application of this kind of solid polymer electrolyte in lithium-ion batteries.
Acknowledgements This work was supported by Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT &
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Future Planning, Korea (2011-0031570) and by the Korea Center for Artificial Photosynthesis (KCAP) located in Sogang University funded by the Minister of Science, ICT and Future Planning (MSIP) through the National Research Foundation of Korea (No. 2009- 0093883) and
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also supported by the Human Resources Development program (No.20114010203090) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the
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Korea government Ministry of Trade, Industry and Energy.
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Figure Captions Fig. 1: The NSPE membrane synthesis process and the interaction scheme among PEO, LiTDI and TiO2.
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Fig. 2: XRD patterns of pure PEO, pure LiTDI, pure TiO2 and PEO–LiTDI–TiO2 SPE membranes with different amounts of TiO2.
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LiTDI–8 wt% TiO2 polymer electrolyte membranes.
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Fig. 3: Polarized optical micrographs (100 µm) of a) pure PEO, b) PEO12–LiTDI and c) PEO12–
Fig. 4: TGA heating traces of 0 wt% and 8 wt% TiO2 doped PEO12–LiTDI electrolyte
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membranes.
Fig. 5: FTIR spectra of a) pure PEO, b) PEO12–LiTDI and c) PEO12–LiTDI–8 wt% TiO2
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polymer electrolyte membranes.
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membranes.
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Fig. 6: Stress–strain curves of PEO12–LiTDI and PEO12–LiTDI–8 wt% TiO2 polymer electrolyte
Fig. 7: Nyquist impedance plots of PEO12–LiTDI and PEO12–LiTDI–8 wt% TiO2 electrolyte
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membranes at room temperature (23 ºC). The equivalent circuit is in inset. Fig. 8: Logarithm of ionic conductivity (lnσ) versus reciprocal temperature (1/T) for PEO– LiTDI–TiO2 SPE membranes with different amounts of TiO2. Conductivity versus TiO2 concentration at room temperature (23 ºC) is in inset. Fig. 9: Cyclic voltammograms of 8 wt% of TiO2 doped nanocomposite electrolyte membrane measured at 23 °C. Potential sweep rate was 5 mV s-1. Table Title Table 1: TGA data: weight loss of 0 wt% and 8 wt% TiO2 doped PEO12–LiTDI electrolyte membranes at different temperatures.
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Fig. 1
Fig. 2
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Fig. 3
Fig. 4
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Fig. 5
Fig. 6
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Fig. 7
Fig. 8
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Fig. 9
Sample
Table 1
Weight loss at different temperatures (in %)
Wt% of TiO2
100 °C
250 °C
350 °C
420 °C
0
2%
4%
20%
77.7%
8
0.4%
1.7%
16%
74.1%
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ed
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*Graphical Abstract
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