LiClO4

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ScienceDirect Materials Today: Proceedings 3 (2016) 1451–1459

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Recent Advances In Nano Science And Technology 2015 (RAINSAT2015)

Studies on the morphology and conductivity of PEO/LiClO4 J. Gurusiddappaa, W. Madhurib, R. Padma Suvarnaa, K. Priya Dasanb,* a

Department of Physics, JNTU College of Engineering (Autonomous), Anantapuram, Andhrapradesh 515002, India b School of Advanced Sciences, VIT University, Vellore 632 014, TN, India

Abstract Polymer based ion conducting materials have potential applications as an electrolyte and separator in the field of lithium batteries. The present work is aimed at the optimization of LiClO4 loading in PEO/LiClO4 films for solid polymer electrolyte (SPE) applications. PEO was complexed at different wt% ratios (95:5), (92.5:7.5), (90:10), (87.5:12.5) and (85:15) and then made into films by solution casting. The PEO/LiClO4 films thus fabricated are characterized by X-ray diffraction (XRD), Differential Scanning Calorimetry (DSC) and Scanning Electron Microscopy (SEM). The above studies have indicated enhancement in amorphous phase of polymer with increase of salt concentration. Further, a reduction in melting temperature (Tm) of PEO was observed from DSC results indicating increase in the flexibility of the polymer chains. Ionic conductivity studies were carried out as a function of frequency and temperature using Hioki LCR Hitester. The PEO/LiClO4 films with maximum salt concentration (85:15) have exhibited an enhancement in conductivity of about four orders of magnitude compared to host polymer electrolyte. © 2015 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015.]. Keywords: Solid polymer electrolyte; Poly(ethyleneoxide)(PEO); Lithium perchlorate(LiClO4); Ionic conductivity; Activation energy

1. Introduction Polymer materials in combination with suitable metallic salts have gained technological importance as electrolyte materials for the solid state electrochemical devices such as batteries, smart windows and photo-electrochemical

* Corresponding author. Tel.: +91-9500359563. E-mail address: [email protected] 2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015. ].

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cells [1]. The advantages of using solid polymer electrolyte (SPE) over their counterpart of liquid electrolytes are flexibility, no leakage, ease of processing as thin films of large surface area, electrochemical stability and volumetric stability over repeated charge-discharge cycles in a given device. A large number of Li+, Na+ and H+ conducting polymer electrolytes, formed by the dissolution of alkali metal salts in various polymer hosts have been reported in literature [2]. In order to achieve a good complexed polymer-metal-salt system, the choice of polymer host and the metal salt plays a key role. The choice of polymer host depends mainly on factors such as atoms or group of atoms with sufficient electron donor capacity to form coordination bond with cations, low barrier to bond rotations, so that segmental motion of the polymer chain can take place easily, suitable distance between coordinating centers, to facilitate the formation of multiple intrapolymer ion bonds. In this context, a variety of polymers such as poly (ethylene oxide) (PEO), poly (ethylenimine), poly (propylene oxide), poly (ethylene succinate), have been examined as polymer hosts [3]. The high molecular weight polyethylene oxide (PEO) has widely been investigated because of its ability to dissolve high concentrations of a wide variety of a metal salts. Such electrolytes have mainly been confined to alkali metal salt systems, with particular attention being focused on lithium. However, this polymer showed low ionic conductivity in the range of 10-8 ohm-1 cm-1 to 10-7 ohm-1 cm-1 which restricted the mobility of ions [4]. Attempts have been made to increase the ionic conductivity of PEO by adding metallic salts. Certain Li+ conducting electrolyte systems have been explored in order to understand the structure, conduction mechanism and its application in solid state lithium batteries. The conductivities of PEO with Lithium trifluoromethanesulfonate(LiCF3SO3) LiTf bis(trifluoromethanesulforonimidate) (Li(CF3SO2)2N) (LiTFSI),Lithium perchlorate (LiClO4) and Lithium bis(oxalato)borate (LiB(C2O4)2)) (LiBOB) salts are reviewed in the literature; the highest conductivity was recorded for LiTFSI and the lowest for LiBETI.[5]. The aim of present work is to optimize the LiClO4 loading in PEO with respect to film stability and conductivity. The SPE films of pure PEO and various compositions of complexed PEO with LiClO4 salt of wt% ratios (95:5), (92.5:7.5), (90:10), (87.5:12.5) and (85:15) were synthesized and characterized by means of X-ray diffraction (XRD), differential scanning calorimetry (DSC) followed by scanning electron microscopy (SEM). The temperature and frequency dependence of conductivity studies were carried out using Hioki LCR Hitester. Nomenclature PEO poly (ethylene oxide) LiClO4 lithium perchlorate SPE solid polymer electrolyte 2. Experimental Materials: Pure reagent materials of Poly (ethylene oxide) PEO (with average molecular weight of 8×106, Sigma Aldrich, USA), LiClO4 (ACS reagent, ≥95.0%, Sigma Aldrich, Japan), methanol (HPLC grade, SDfine) were used as starting materials to prepare solid polymer electrolytes. 2.1. Preparation of solid polymer electrolyte: The SPE films of pure PEO and PEO/LiClO4 films at various compositions were prepared in the wt% ratios (95:5), (92.5:7.5), (90:10), (87.5:12.5) and (85:15) by solution cast technique using methanol as a solvent. The solutions were magnetically stirred for 30-35 h at room temperature to get viscous solution with high homogeneity. The viscous solution was poured into petri dish and left to dry at room temperature for 2-3days to allow the slow solvent evaporation. The prepared SPE films with different salt concentrations were then kept in desiccators. Bubble free regions of the films were employed for the characterization techniques.

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2.2. Characterization techniques The X-ray diffraction spectra of these SPE films were carried out using (SHIMDZU XRD 7000) X-ray diffractrometer in 2θ range 10-80°using CuKα radiation. The surface morphology of the solid polymer electrolytes were studied by Scanning electron microscopy (SEM) using (SEM, MODEL:Hitachi S-4700). Differential Scanning Calorimetry (DSC) studies were performed using NETZSCH DSC204 over a temperature range of -100 to +100° C and at a heating rate of 10° C min-1. The samples of about 10–15mg were sealed in an aluminum pans and all experiments were carried out under nitrogen gas atmosphere. The complex impedance measurements were performed using computer controlled (HIOKI-3532-50) LCR HiTESTER in the frequency range 100Hz to 1MHz and in the temperature range 308K- 353K. 3. RESULTS AND DISCUSSION 3.1. X-ray diffraction Generally polymer electrolytes are composed of amorphous phase and crystalline phase. XRD technique is widely used to study the phase compositions of polymer electrolytes. Inherent relations between the crystal structure and the diffraction pattern can be solved by analyzing the diffraction peak characteristics. The XRD patterns of SPE films of pure PEO, (PEO+LiClO4) SPE films of wt% ratios (95:5), (92.5:7.5), (90:10), (87.5:12.5) and (85:15) are shown in Fig 1. The XRD pattern reveals that there is decline in characteristic crystalline diffraction peaks of SPE with increase of LiClO4 concentration. As to pure PEO, there are two high intensity characteristic peaks appearing at 2θ=19.19° and 23.33°. XRD pattern indicates that the pure PEO polymer electrolyte has strong ability of crystallization. The high degree of crystallinity is not desirable for ion transfer as it mainly happens in the amorphous regions. Addition of LiClO4 has considerably decreased the crystallinity of the polymer electrolyte and XRD pattern of complexes showed broadening and reduction of PEO peak intensity. This can be attributed to distribution of the ordered arrangement of polymer side chain and homogeneous dispersion of LiClO4 in the PEO host.

Fig.1. XRD patterns of the pure PEO and PEO-LiClO4 SPE films.

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3.2. Surface morphology The surface morphology of SPE films are show in Fig 2. Micrograph of pure PEO (Fig 2.a) showed a rough surface which had several crystalline domains [6]. On the addition of LiClO4, the SPE adopted spherolitic structure, diameter of the spherolites and its encroachment to adjacent spherolites increases with increase in LiClO4 concentration. This may be due to LiClO4 acting as a nucleating centre during film formation.

Fig. 2. (a) - (f) SEM images of pure PEO and (PEO + LiClO4) SPE films with wt% ratios (95:5), (92.5:7.5), (90:10), (87.5:12.5) and (85:15)

3.3. Thermal analysis: Fig. 3 shows typical DSC traces of pure PEO film and the complexed (PEO+LiClO4) SPE films with various wt % ratios (95:5), (92.5:7.5), (90:10), (87.5:12.5) and (85:15). The DSC curve of pure PEO showed a melting temperature around 341 K. In SPE films the melting temperature (Tm) and melting enthalpy (ΔHm) gradually decreased with increase in LiClO4 loading. The observed low melting enthalpy in SPE film of wt% ratio (85:15) as compared to PEO indicates that the presence of Li+ ions reduces the crystallinity considerably. DSC provides a rapid method for determining degree of crystallinity based on the amount of heat required to melt the crystalline sample. In other words, the melting enthalpy is proportional to the crystallinity of sample [7]. The melting enthalpy (ΔHm) of the crystalline phase of SPE films can be evaluated by considering the area under the melting peak. The degree of crystallinity (χc) has been calculated by taking into account pure PEO as 100% crystallinity [8] and using equation χc

=

∆

∆

(1)

Where, ΔHp equals to 203 J/g which is melting enthalpy of pure PEO of 100% crystallinity and ΔHc is melting enthalpy of composites. The calculated relative crystallinity (χc), melting temperature (Tm), change in melting

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enthalpy (ΔHm) of crystalline phase of polymer evaluated during the heating process from -100 oC to + 100 oC are summarized in Table 1. From Table 1, it is observed that the endothermic peak of PEO is broadened and peak height is decreased by doping of LiClO4 into polymer host. The melting temperature of polymer host is found to be decreasing from ~ 341 K to ~ 327 K due to the addition of LiClO4 salt.

Fig. 3. DSC curves of pure PEO and (PEO+ LiClO4) polymer electrolyte system

The ΔHm decreases from 153 to 54.95 J/g in the entire composite range. The percentage of degree of crystallinity decreases remarkably from 75.7 to 27.0. This indicates increase of amorphous nature of SPE films. These results substantiate the Braggs peak broadening and smooth surface morphology observed in the XRD pattern and SEM micrographs. Table 1. Comparison of thermal parameters of PEO: LiClO4 system

Sample

χc

Tm

ΔHm

o

PEO: LiClO4

(%)

( C)

(J/g)

100 : 0

75.7

68.0

153.7

95 :5

68.3

67.05

138.6

92.5 : 7.5

50.6

66.5

118.4

90 : 10

58.3

64.1

102.7

87.5 : 12.5

31.9

58.1

64.84

85 : 15

27.0

54.1

54.95

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3.4. Conductivity studies

Fig. 4. Variation of conductivity with LiClO4 composition of (PEO+LiClO4) SPE films

The variation of dc conductivity (σ) as a function of the concentration of LiClO4 in PEO at room temperature is shown in Fig.4. The conductivity data for these SPE films at room temperature are represented in the Table 2. The results indicates that the conductivity of pure PEO is about 10-9 S cm-1 at room temperature and its value increases sharply to 10-5 S cm-1 on complexing with 5 wt% of LiClO4. The increase in conductivity is slower on further addition of LiClO4 salt to the pure PEO. This behaviour may be due to the charge transport in PEO solid polymer complexes involving dissociation of cation Li+ from its coordinating oxygen to an adjacent site. The high ionic conductivity in an electrolyte is attributed to increased ionic charged carrier concentration. The motion of ions in SPE is a liquid like mechanism, by which the movement of ions through polymer matrix is associated by large amplitude of the segmental motion [9]. The maximum conductivity is obtained for 15 wt% of LiClO4 (85:15). In general, conductivity increases as the degree of crystallinity decreases. The increase in conductivity of SPE with increasing salt concentration is attributed to a decrease in degree of crystallinity, as confirmed by XRD studies [10]. The temperature dependence of conductivity of pure PEO and its composites over a temperature range 308- 353 K is shown in Fig 5. In this temperature range of study, the conductivity is found to be increasing with increase of temperature in pure PEO as well as in all the compositions of (PEO+LiClO4). From Fig.5 it is observed that the graphs obey Arrhenius equation of conductivity throughout. The plots are fitted to the relation, (2) σ = σ0 exp

Using linear fit program, where σ0 is the pre-exponential factor, Ea, the activation energy, KB the Boltzmann constant and T is the absolute temperature. The overall feature of the plots is almost similar for all investigated SPE films. The conductivities increases slowly up to their melting points and above that the conductivities increases fast, above which the polymer metal salt complex starts to soften. The activation energy is a combination of charge carrier creation (defect formation) and the energy of ion migration that can be evaluated by linear fitting to the log σ with 1/T plots. Activation energy values of different SPE films were calculated from the slopes of linear fit of Arrhenius plots and listed in Table 2. Inspection of Table. 2. shows that the activation energy (Ea) of PEO: LiClO4 SPE films are found to be decreased from 0.67 to 0.34 eV with an increase in salt content. The low activation energy for Li+ ion transport is due to the amorphous nature of polymer electrolytes which facilitates the fast Li+ motion in polymer matrix. The amorphous nature also provides a more free volume in the polymer electrolyte system with increase in the temperature [11]. It can be observed that sample PEO: LiClO4 ratio (85:15) has a higher ionic conductivity (3.35×10-4 S cm-1 at 358 K) and low activation energy 0.34 eV compared with the other compositions.

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Fig. 5. Temperature dependence of conductivity of (PEO+LiClO4) SPE films Table. 2. DC conductivity (σ), activation energies of pure PEO and PEO complexed with LiClO4 salt at room temperature (308 K).

SPE films (PEO+LiClO4 )

σ (S cm-1)

Ea (eV)

Pure PEO

3.66×10-9

0.67

95:5

1.11×10-5

0.49

92.5:7.5

9.97×10-6

0.48

90:10

1.13×10-5

0.41

87.5:12.5

2.00×10-5

0.38

85:15

3.35×10-5

0.34

3.5. AC-conductivity The Ac-conductivity of all the (PEO+LiClO4) SPE films were analysed at temperatures between 308 and 353 K and in the frequency range of 100Hz –1MHz. The results are shown in Fig.6. It can be observed that the conductivity increased with increasing frequency and temperature. The low frequency dispersion may be due to space charge polarization. As the frequency decreases, more and more charge accumulation occurs at the electrolyte and electrode interface, which leads to a decrease in the number of mobile ions and eventually to drop in conductivity at low frequency. In high frequency region, the mobility of charge carriers is high and hence conductivity increases with frequency [12].

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Fig. 6. Variation of AC-conductivity with frequency at different temperature of (PEO+LiClO4) with wt% ratio (85:15), (87.5:12.5), (90:10), (92.5:7.5) and (100:0) SPE films

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4. Conclusions The SPE films of pure PEO and various compositions of complexed PEO with LiClO4 salt with different wt% ratios (95:5), (92.5:7.5), (90:10), (87.5:12.5) and (85:15) have been prepared by solution casting method and characterized by XRD, SEM and DSC studies. The broadening and reduction in intensity of the Bragg peaks confirms the dissolution of LiClO4 salt in the polymer host. The DSC results have further confirmed a reduction in the crystallinity. The conductivity of PEO complexed with LiClO4 is found to be 10-4 S cm-1, which is of four orders greater compared to pure PEO. The SPE film (PEO+LiClO4) with wt% ratio (85:15) showed a conductivity of 3.35×10-5 S cm-1 at room temperature (308K), which has highest conductivity among the investigated films. The conductivity increases with temperature in pure PEO and all the compositions of the (PEO+LiClO4) polymer electrolyte system. The conductivity-temperature plots follow the Arrhenius behaviour throughout with one activation energy ranging between 308 and 353K. The ionic conductivity increases and activation energy decreases with the increase in salt concentration.

Acknowledgements Authors express their sincere thanks to University Grants Commission (UGC), New Delhi for providing financial support through the major research project No.42-796/2013(SR). Authors specially thank Prof. S. Kalainathan, SAS, VITU, Vellore for his support in electrical measurements.

References [1] [2] [3] [4] [5] [6] [7] [8]

J. Siva Kumar, A.R. Subrahmanyam, M. Jaipal Reddy, U.V. Subba Rao, Materials Letters 60 (2006) 3346–3349 Emad M. Masoud, A.-A. El-Bellihi, W.A. Bayoumy, M.A. Mousa, Journal of Alloys and Compounds 575, (2013) 223–228 A. Bhide, K. Hariharan., Journal of Power Sources 159 (2006) 1450–1457 E.M. Masoud, A.-A. El-Bellihi, W.A. Bayoumy, M.A. Mousa, Materials Research Bulletin 48 (2013), 1148–1154 J.W.Fergus, Jounal of Power Sources 195(2010) 4554-4569 A. Bhide, K. Hariharan, European Polymer Journal 43 (2007) 4253–4270 Benson K. Money, K.Hariharan, J. Swenson Solid State Ionics 262 (2014) 785–789 Abdelhameed Ahmed ElBellihi, Wafaa Abdallah Bayoumy, Emad Mohamed Masoud and Mahmoud Ahmed, Mousa Bull. Korean Chem. Soc. 2012, Vol. 33, No. 9 [9] M. Jaipal Reddy, Peter P. Chu, Journal of Power Sources 109 (2002) 340–346 [10]Chang Hyun Park, Dong Won Kim, Jai Prakash, Yang-Kook Sun, Solid State Ionics 159 (2003) 111–119 [11]R. Baskaran, S. Selvasekarapandian, N. Kuwata, J. Kawamura, T. Hattori, Journal of Physics and Chemistry of Solids 68 (2007) 407–412 [12]S. Ramesh, Tai Fung Yuen, Chia Jun Shen, Spectrochimica Acta Part A 69 (2008) 670– 675