- Email: [email protected]
TRANSPORT MECHANISM STUDIES OF CHITOSAN ELECTROLYTE SYSTEMS
Accepted Manuscript Title: TRANSPORT MECHANISM STUDIES OF CHITOSAN ELECTROLYTE SYSTEMS Author: S. Navaratnam K. Ramesh S. Ramesh A. Sanusi W.J. Basirun A.K. Arof PII: DOI: Reference:
S0013-4686(15)00107-3 http://dx.doi.org/doi:10.1016/j.electacta.2015.01.087 EA 24139
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
21-11-2014 19-1-2015 19-1-2015
Please cite this article as: S.Navaratnam, K.Ramesh, S.Ramesh, A.Sanusi, W.J.Basirun, A.K.Arof, TRANSPORT MECHANISM STUDIES OF CHITOSAN ELECTROLYTE SYSTEMS, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.01.087 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.
TRANSPORT MECHANISM STUDIES OF CHITOSAN ELECTROLYTE SYSTEMS
S. Navaratnam1*, K. Ramesh 2, S. Ramesh 2, A. Sanusi1, W.J.Basirun3 and A.K. Arof2 1 Faculty of Applied Sciences and Institute of Science, University Technology MARA, 40450 Shah Alam, Malaysia 2
Centre for Ionics University of Malaya, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia.
3
Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
*Email of corresponding author: [email protected]
ABSTRACT Knowledge of ion-conduction mechanisms in polymers is important for designing better polymer electrolytes for electrochemical devices. In this work, chitosan-ethylene carbonate/propylene carbonate (chitosan-EC/PC) system with lithium acetate (LiCH3COO) and lithium triflate (LiCF3SO3) as salts were prepared and characterized using electrochemical impedance spectroscopy to study the ion-conduction mechanism. It was found that the electrolyte system using LiCF3SO3 salt had a higher ionic conductivity, greater dielectric constant and dielectric loss value compared to system using LiCH3COO at room temperature. Hence, it may be inferred that the system incorporated with LiCF3SO3 dissociated more readily than LiCH3COO. Conductivity mechanism for the systems, 42 wt.% chitosan- 28wt.% LiCF3SO3-30wt.% EC/PC (CLT) and 42wt.%chitosan-28wt.% LiCH3COO30wt.% EC/PC (CLA) follows the overlapping large polaron tunneling (OLPT) model. Results show that the nature of anion size influences the ionic conduction of chitosan based polymer electrolytes. The conductivity values of the CLA system are found to be higher than that of CLT system at higher
temperatures. This may be due to the vibration of bigger triflate anions would have hindered the lithium ion movements. FTIR results show that lithium ions can form complexation with polymer host which would provide a platform for ion hopping.
KEYWORDS: chitosan, electrolytes, OLPT
1.
INTRODUCTION
Studies on polymer electrolyte are of great interest to scientist due to their potential application in various electrochemical devices. Polymer electrolyte batteries have several advantages such as, high ionic conductivity, ease of preparation, leak proof and light weight. Most research on polymer electrolytes have been conducted on polyethylene oxide (PEO) [1, 2], polyvinyl chloride (PVC) [3, 4], poly(methyl methacrylate) (PMMA) [5,6] and polyvinyl alcohol (PVA) [7]. Polymers are basically insulators. In order to enhance their conductivity the polymers are usually doped with Li salts and plasticizers. Li salts containing anions such as BF4−, ClO4− and CF3SO3− [8] are commonly used to complex the polymer electrolytes. The salt cations (for the most part) coordinate or complexes with groups along the polymer chain backbone and are transported via polymer motions through the polymer medium [9]. Propylene carbonate (PC) and ethylene carbonate (EC) are commonly used in the preparation of polymer electrolytes due to high dielectric constant and compatibility with many polymers. Plasticizers can increase the conductivity in a number of ways; by decreasing Tg (hence increasing molecular motion), increasing the number of charge carriers (decreasing ion aggregation) or providing an alternate conduction path such that the ion motions maybe decoupled from the local motions of the polymer [10]. For the last two decades, there are several research attempts have been made to improve the conductivity by incorporation of blend polymers, hybrid polymers, copolymers,
nanofillers and ionic liquids for the development of solid polymer electrolytes, gel polymer electrolyte and composite polymer electrolytes [11-15]. Chitosan a natural polymer is chosen as a host polymer as it is non–toxic and biodegradable. Earlier studies have shown that chitosan can be used as polymer matrix for ionic conduction [16-18]. The presence of lone pair on the nitrogen atom and oxygen atom in chitosan enables coordination of lithium ion from the added salt.
In this paper, we report the effect of nature of salt i.e. lithium triflate and lithium acetate on the ionic conductivity and dielectric behaviour of the chitosan polymer plasticized with ethylene carbonate (EC)/ propylene carbonate (PC). Knowledge of ion-conduction mechanisms in polymers is important for designing better polymer electrolytes for electrochemical devices.
2.
EXPERIMENTAL
2.1 Sample Preparation. In this study, 1g chitosan was obtained from Fluka was dissolved in 1% ethanoic acid solution. Lithium salts of 40 wt.% LiCH3COO and 40 wt.% LiCF3SO3 was added accordingly to the mixture. The two mixtures were stirred for several hours at room temperature. 30 wt.% of EC: PC in the ratio of 1:1 was added to the mixtures and further stirred for 24 hours. After complete dissolution, the solution was cast in petri dishes and air dried at room temperature (RT). This would result in free standing films. The residual ethanoic acid was removed from electrolyte film by rinsing with 0.1M NaOH solution followed by deionised water. The films were kept in a desiccator for one week and then dried in a vacuum oven for 2 hours prior to the characterization. 2.2 Impedance spectroscopy The ionic conductivity of the samples was measured with a HIOKI 3531-01 LCR Hi-Tester between the frequency ranges of 50 Hz to 1 MHz. The impedance
values of the samples were measured by sandwiching the samples between two stainless steel electrodes in the temperature range of 298 to 353 K. The conductivity (σ) of the samples was calculated using; σ =
t Rb A
where t is the sample thickness (cm), A the effective contact area of the electrode and the electrolyte (cm2), and Rb is the bulk resistance (Ω). Electric permittivity and electrical modulus can be determined from complex impedance [19]. The equations for the dielectric constant, εr and the dielectric loss, εi, can be calculated using Zr
εi =
ωCo (Z r2 + Z i2 )
εr =
ωCo (Z r2 + Z i2 )
Zi
where Co= εoA/t (εo is permittivity of free space),ω=2πf ( f is frequency), Zi is the imaginary part of the complex permittivity, Zr is the real part of the complex permittivity.
2.3 Fourier Transform Infrared Spectroscopy (FTIR) The FTIR studies were performed using FTIR Thermoscientific Nicolet iS10 equipped with an ATR internal reflection system was used to carry out ATR-FTIR analysis in the wave number range of 400–4000 cm−1 at a resolution of 2 cm−1. The objective of FTIR was to confirm complexation between polymer and salt.
3.
RESULTS AND DISCUSSIONS
3.1 Conductivity studies Fig.1(a) and (b) shows the complex impedance plot of chitosan – EC/PC system doped with lithium triflate salt and
lithium acetate salt respectively at room
temperature.. Both plots reveal a semicircle in the high frequency region followed by a slanted spike in
the high frequency region. The high frequency region semi-circle is characteristic of a parallel combination of bulk resistance and bulk capacitance [20], whereas the inclined straight line indicates the capacitive nature of the interface and absence of electronic conductivity [21].The intercept on the real axis gives directly the bulk resistance. The conductivity of the polymer electrolyte was calculated using the bulk resistance, area and thickness of the sample. Room temperature conductivity of the sample were found to be 6.1 x10-7 S cm-1 for chitosan system containing lithium acetate salt and 5.0 x10-6 S cm-1 for chitosan system containing lithium triflate salt. The room temperature conductivity of LiCF3SO3 electrolyte was higher than that of LiCH3COO electrolyte. The difference in conductivity is associated with the nature of the lithium salt. It may be inferred that LiCF3SO3 salt dissociated more LiCH3COO salt. Triflate anion is larger than acetate anion and therefore lithium triflate salt will have lower lattice energy than lithium acetate salt. Ionic conductivity is affected by the diffusion rate of the ions, which in turn depends on the size of each ion [22]. 3.2 Dielectric relaxation studies The variation of dielectric constant (εr) and dielectric loss (εi) as function of frequency for CLT and CLA are shown in Fig. 2 (a) and (b) respectively. Dielectric constant measures stored charges. A higher dielectric constant value for CLT system implies that more lithium ions are present. LiCF3SO3 salt has lower lattice energy than LiCH3COO salt. This allows for greater dissociation of LiCF3SO3 salt and hence greater number of free Li+ for ionic transport. Both dielectric constant and dielectric loss increase sharply at low frequencies implying that electrode polarisation and space charge effects have occurred confirming its non-Debye dependence [23]. At low frequencies, the charges align themselves along the electric field direction and thus fully contribute to the total polarisation. At high frequencies both dielectric constant and dielectric loss approaches almost constant value. As the frequency is increased, the variation in electric field becomes
too fast for the charges to follow and therefore their contribution to the polarisation becomes less with a measurable lag because of internal frictional forces [24]. It is clear from Table 1 that dielectric constant value increases with temperature. The molecular dipoles in polar materials cannot orient themselves at low temperature. When the temperature rises the dipole orientation is facilitated, and this increases the dielectric constant [24]. Table 1 also illustrates that dielectric loss exhibits strong temperature dependence. The increase of dielectric loss was explained by Stevels [25] who divided the relaxation phenomenon into three parts; conduction losses, dipole losses and vibrational losses [24]. Conduction losses is proportional to (σ/ω) and hence conduction losses rises with increase in temperature, which in turn causes the value of dielectric loss to increase. Fig. 3(a) and (b) displays the variation of dielectric loss tangent, tan δ, with frequency at different temperatures for CLT and CLA system. From the plots, it is clear that tan δ value increases with frequency at different temperatures, passes through a maximum value and thereafter decreases. The presence of loss peak suggests a dielectric relaxation process. The peak value shifts towards the higher frequency with increasing temperature. At higher temperatures the peak are out of the frequency range studied in this work. Fig. 4(a) and (b) show that the frequency for relaxation peaks increases with temperature and hence relaxation time decreases with increase in temperature. The decrease in relaxation time with increasing temperature is due to faster movement of lithium ions that synchronizes with the direction of applied field and increased motion of charge carriers in the polymer matrix. The activation energy of relaxation is 0.71eV for CLT and 0.81 eV for CLA system. Fig. 5(a) and (b) depict the plot of ln εi versus ln ω at different temperatures. The dielectric loss increases with increasing temperature for all frequencies. The values of exponent s can be determined from the slope of these plots in the high frequency region to avoid electrode polarization.
The plot s versus temperature for CLT and CLA systems is shown in Fig. 6. From the plot it can be observed that for both systems s decreases with the increase in temperature to a minimum value at a certain temperature, and then it increases with increasing temperature. Thus, the temperature dependence of s could be interpreted by the overlapping large polaron-tunnelling (OLPT) model [26].
3.3 FTIR analysis Fig.7 shows the full FTIR spectra of pure salts (lithium triflate and lithium acetate), chitosan film and electrolytes prepared using chitosan and the salts. There were important characteristic peaks observed due to their available bands in the molecular structure. A broad peak has been observed in pure chitosan film in the range of 3300 – 3500 cm-1 which could be assigned to axial stretching of O-H superimposed to N-H stretching band of chitosan [26,27]. The electrolytes prepared by the salt incorporated chitosan have also shown the same broad peak in this range with approximately same peak intensity. It can be inferred that these peaks would also be contributed by axial stretching of O-H superimposed to N-H stretching band of pure chitosan. As the samples were vacuum evaporated for 2 hours just before the experiments, the moisture absorption by the electrolyte films can be deemed negligible. The spectra have been further analysed in the range of 1000 – 1700 cm-1 depicted in Fig. 8 by considering the peaks of pure chitosan spectrum as reference. The change in peak shift and intensity will be the evidence of the complexation and incorporation of the salt in the polymer electrolyte formulation. The bands at 1227 and 1036 cm−1 are assigned to asymmetric SO3 and symmetric SO3 vibrations of LiCF3SO3, respectively. It can be noticed that the free triflate ions present in chitosan based electrolyte system shifted to 1247 and 1026 cm-1. Other important characteristic peaks of lithium triflate at 1190 and 1642 cm-1 have been observed at 1173 and 1647 cm-1 respectively of the lithium triflate salt incorporated chitosan electrolyte whereas peaks at 1166 and 1638 cm-1 appeared in pure
chitosan film [28]. The characteristic peaks of lithium acetate salt were observed at 1434 cm-1 and 1595 cm-1. Two broad peaks were observed in the lithium acetate salted chitosan film at range of 1361 –1480 cm-1 and 1490 – 1630 cm-1 whereas narrower peaks were observed at 1381, 1411 and 1543 cm-1 for pure chitosan film. This may be due to the complexation of the salt and the host polymer. From the observations made from FTIR analysis, it is understood that the free ions are responsible for conductivity. 4.
CONCLUSION
The nature of anion size influences the ionic conduction of polymer electrolyte. It was found that the chitosan system using LiCF3SO3 exhibits better conductivity than chitosan system using LiCH3COO. The dielectric studies shows the prepared materials are ionic conductors. Both systems follow OLPT regardless of the type of salt.
ACKNOWLEDGMENTS Authors would like to thank Research Management Institute (RMI), Faculty of Applied Sciences, Institute of Science, Universiti Teknologi MARA (UiTM) by supporting this study by RACE/F1/ST3/UITM/5 grant. We would also thank University of Malaya for providing the grants RACE/UM/CR003-2013 and RP025-14AFR.
References [1]
H.M.J.C. Pitawala, M.A.K.L. Dissanayake, V.A. Seneviratne, Combined effect of Al2O3 nano-fillers and EC plasticizer on ionic conductivity enhancement in the solid polymer electrolyte (PEO)9LiTf, Solid State Ionics 178 (2007) 885. [2] Jae-Won Choi, Gouri Cheruvally, Yeon-Hwa Kim, Jae-Kwang Kim, James Manuel, Prasanth Raghavan, Jou-Hyeon Ahn, Ki-Won Kim, Hyo-Jun Ahn, Doo Seong Choi, Choong Eui Song, Poly(ethylene oxide)-based polymer electrolyte incorporating room-temperature ionic liquid for lithium batteries, Solid State Ionics 178 (2007) 1235.
[3]
[4]
[5]
[6]
[7] [8]
[9]
A. Mary Sukeshini, Atsushi Nishimoto, Masayoshi Watanabe, Transport and electrochemical characterization of plasticized poly(vinyl chloride) solid electrolytes, Solid State Ionics 86–88 (1996) 385. R. H. Y. Subban, and A. K. Arof. "Experimental investigations on PVC-LiCF3SO3-SiO2 composite polymer electrolytes." Journal of New Materials for Electrochemical Systems 6 (2003) 197. Boor Singh, Rajiv Kumar, S.S. Sekhon, Conductivity and viscosity behaviour of PMMA based gels and nano dispersed gels: Role of dielectric constant of the solvent, Solid State Ionics 176–18 (2005) 1577. A.M.M. Ali, M.Z.A. Yahya, H. Bahron, R.H.Y. Subban, M.K. Harun, I. Atan, Impedance studies on plasticized PMMA-LiX [X: CF3SO3−, N(CF3SO2)2−] polymer electrolytes, Materials Letters 61 (2007) 2026. D.R MacFarlane, F Zhou, M Forsyth, Ion conductivity in amorphous polymer/salt mixtures, Solid State IonicS 113–115 (1998) 193. S. Rajendran, M. Ramesh Prabhu, M. Usha Rani, Ionic conduction in poly(vinyl chloride)/poly(ethyl methacrylate)-based polymer blend electrolytes complexed with different lithium salts, Journal of Power Sources 180 (2008) 880. R Walter, R Walkenhorst, M Smith, J.C Selser, G Piet, R Bogoslovov, The role of polymer melt viscoelastic network behavior in lithium ion transport for PEO melt/LiClO4 SPEs: the “wet gel” model, Journal of Power Sources 89 (2000) 168.
[10]
M. Forsyth, P.M. Meakin, D.R. MacFarlane, A 13C NMR study of the role of plasticizers in the conduction mechanism of solid polymer electrolytes, Electrochimica Acta, Volume 40 (1995) 2339.
[11]
V. D. Noto, S. Lavina, G. A. Giffin, E. Negro, B. Scrosati, (2011) Polymer electrolytes: Present, past and future, Electrochimica Acta, 57 (15) 4-13
[12]
F. Bertasi, K. Vezzù, E. Negro, S. Greenbaum, V. D. Noto (2014) Single-ion-conducting nanocomposite polymer electrolytes based on PEG400 and anionic nanoparticles: Part 1. Synthesis, structure and properties, International Journal of Hydrogen Energy, 39 (6),28722883
[13]
F. Bertasi, K. Vezzù, G. A. Giffin, T. Nosach, P. Sideris, S. Greenbaum, M. Vittadello, V. D. Noto (2014) Single-ion-conducting nanocomposite polymer electrolytes based on PEG400 and anionic nanoparticles: Part 2. Electrical characterization, International Journal of Hydrogen Energy, 39 (6), 2884-2895
[14]
V. D. Noto, M. Vittadello, S. Lavina, M. Fauri, S. Biscazzo, (2001) Mechanism of Ionic Conductivity in Poly(ethyleneglycol 400)/(LiCl)x Electrolytic Complexes: Studies Based on Electrical SpectroscopyJ. Phys. Chem. B 2001, 105, 4584-4595
[15]
M. J. Park, I. Choi, J. Hong, O. Kim (2013) Polymer Electrolytes Integrated with Ionic Liquids for Future Electrochemical Devices, J.Appl. Polym. Sci , 129, 2363-2376
[16]
M.Z.A. Yahya, A.K. Arof, Effect of oleic acid plasticizer on chitosan–lithium acetate solid polymer electrolytes, European Polymer Journal 39 (2003) 897.
[17]
S.R. Majid, A.K. Arof, Proton-conducting polymer electrolyte films based on chitosan acetate complexed with NH4NO3 salt, Physica B: Condensed Matter 355 (2005) 78.
[18]
S.B. Aziz, Z.H.Z. Abidin, Electrical Conduction Mechanism in Solid Polymer Electrolytes: New Concepts to Arrhenius Equation, Journal of Soft Matter (2013)
[19]
Ross J, MacDonald (1987) Impedance spectroscopy. Wiley, USA
[20]
M Venkateswarlu, K Narasimha Reddy, B Rambabu, N Satyanarayana, A.c. conductivity and dielectric studies of silver-based fast ion conducting glass system, Solid State Ionics 127 (2000) 177.
[21]
M.S Michael, M.M.E Jacob, S.R.S Prabaharan, S Radhakrishna, Enhanced lithium ion transport in PEO-based solid polymer electrolytes employing a novel class of plasticizers, Solid State Ionics 98 (1997)167.
[22]
S Rajendran, R Kannan, O Mahendran, Ionic conductivity studies in poly(methylmethacrylate)–polyethlene oxide hybrid polymer electrolytes with lithium salts, Journal of Power Sources 96 (2001) 406.
[23]
S Rajendran, O Mahendran, T Mahalingam, Thermal and ionic conductivity studies of plasticized PMMA/PVdF blend polymer electrolytes, European Polymer Journal, Volume 38 (2002) 49.
[24]
M.M. El-Nahass, A.F. El-Deeb, H.E.A. El-Sayed, A.M. Hassanien, Electrical conductivity and dielectric properties of bulk glass Se55Ge30As15 chalcogenide, Physica B: Condensed Matter 388 (2007) 26.
[25]
J. M. Stevels, in: S. Flugge (Ed.), Handbuch der Physik, Springer, Berline, (1957) pp. 350
[26]
N.H.A. Rosli, C.H. Chan, R.H.Y. Subban, Tan Winie, Studies on the Structural and Electrical Properties of Hexanoyl Chitosan/Polystyrene-based Polymer Electrolytes, Physics Procedia 25 (2012) 215
[27]
S.S. Alias, S. M. Chee and A. A. Mohamad (2014), Chitosan–ammonium acetate–ethylene carbonate membrane for proton batteries, Journal of Chemistry http://dx.doi.org/10.1016/j.arabjc.2014.05.001
[28]
Enescu, D., Hamciuc, V., Ardeleanu, R., Cristea, M., Ioanid, A., Harabagiu, V., Simionescu, B.C., 2009. Polydimethylsiloxane modified chitosan. Part III: preparation and characterization of hybrid membranes. Carbohydr. Polym. 76, 268–278.
[29]
S. Ramesh, K.H. Leen, K. Kumutha and A.K. Arof (2007). FTIR studies of PVC/PMMA blend based polymer electrolytes. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 66(4-5): 1237-1242.
(a) (b)
Fig. 1. (a) Impedance diagram of CLT at RT. (b) Impedance diagram of CLA at RT.
(a)
(b)
Fig. 2. (a) Frequency dependence of dielectric loss, εr for CLT and CLA systems at RT. (b) Frequency dependence of dielectric loss, εifor CLT and CLA systems at RT.
(a)
(b) Fig. 3. (a) Variation of loss tangent with frequency for CLT system. (b) Variation of loss tangent with frequency for CLA system
(a)
(b) Fig. 4. (a) Variations of ln τ with inverse temperatures for CLT. (b) Variations of ln τ with inverse temperatures for CLA.
(a)
(b)
Fig. 5. (a) Variation of ln εi with ln ω for CLT system. (b) Variation of ln εi with ln ω for CLA system
Fig. 6. Variation of the exponent “s” with temperature for CLT system and CLA system
Fig. 7. Complete infrared spectra of (a) lithium triflate (b) lithium acetate (c) chitosan, (d) chitosan - lithium acetate (e) chitosan lithium triflate
Fig. 8. Infrared spectra of (a) lithium triflate (b) lithium acetate (c) chitosan, (d) chitosan - lithium acetate (e) chitosan - lithium triflate in the region between 1000 to 1700 cm-1
Table 1. The dielectric constant and the dielectric loss at different temperatures at f=250Hz and 950000Hz for CLT and CLA
Dielectric constant (εεr) at Temperature 250Hz (K) CLT CLA 298 303 313 323 333 343 353
17638 21770 33488 43832 78297 90359 108351
890 1023 2343 4746 8960 36175 67726
Dielectric loss (εεi) at 250Hz
Dielectric constant Dielectric loss (εεr) at 95000Hz (εεi) at 950000Hz
CLT
CLA
CLT
CLA
CLT
CLA
26497 31392 42510 52126 84524 97682 115140
1867 2060 3722 6609 12197 45691 81868
44 46 50 52 55 55 53
11 11 13 16 20 25 12
32 40 63 87 195 255 357
3 4 6 11 25 102 251
S0013-4686(15)00107-3 http://dx.doi.org/doi:10.1016/j.electacta.2015.01.087 EA 24139
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
21-11-2014 19-1-2015 19-1-2015
Please cite this article as: S.Navaratnam, K.Ramesh, S.Ramesh, A.Sanusi, W.J.Basirun, A.K.Arof, TRANSPORT MECHANISM STUDIES OF CHITOSAN ELECTROLYTE SYSTEMS, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.01.087 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.
TRANSPORT MECHANISM STUDIES OF CHITOSAN ELECTROLYTE SYSTEMS
S. Navaratnam1*, K. Ramesh 2, S. Ramesh 2, A. Sanusi1, W.J.Basirun3 and A.K. Arof2 1 Faculty of Applied Sciences and Institute of Science, University Technology MARA, 40450 Shah Alam, Malaysia 2
Centre for Ionics University of Malaya, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia.
3
Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
*Email of corresponding author: [email protected]
ABSTRACT Knowledge of ion-conduction mechanisms in polymers is important for designing better polymer electrolytes for electrochemical devices. In this work, chitosan-ethylene carbonate/propylene carbonate (chitosan-EC/PC) system with lithium acetate (LiCH3COO) and lithium triflate (LiCF3SO3) as salts were prepared and characterized using electrochemical impedance spectroscopy to study the ion-conduction mechanism. It was found that the electrolyte system using LiCF3SO3 salt had a higher ionic conductivity, greater dielectric constant and dielectric loss value compared to system using LiCH3COO at room temperature. Hence, it may be inferred that the system incorporated with LiCF3SO3 dissociated more readily than LiCH3COO. Conductivity mechanism for the systems, 42 wt.% chitosan- 28wt.% LiCF3SO3-30wt.% EC/PC (CLT) and 42wt.%chitosan-28wt.% LiCH3COO30wt.% EC/PC (CLA) follows the overlapping large polaron tunneling (OLPT) model. Results show that the nature of anion size influences the ionic conduction of chitosan based polymer electrolytes. The conductivity values of the CLA system are found to be higher than that of CLT system at higher
temperatures. This may be due to the vibration of bigger triflate anions would have hindered the lithium ion movements. FTIR results show that lithium ions can form complexation with polymer host which would provide a platform for ion hopping.
KEYWORDS: chitosan, electrolytes, OLPT
1.
INTRODUCTION
Studies on polymer electrolyte are of great interest to scientist due to their potential application in various electrochemical devices. Polymer electrolyte batteries have several advantages such as, high ionic conductivity, ease of preparation, leak proof and light weight. Most research on polymer electrolytes have been conducted on polyethylene oxide (PEO) [1, 2], polyvinyl chloride (PVC) [3, 4], poly(methyl methacrylate) (PMMA) [5,6] and polyvinyl alcohol (PVA) [7]. Polymers are basically insulators. In order to enhance their conductivity the polymers are usually doped with Li salts and plasticizers. Li salts containing anions such as BF4−, ClO4− and CF3SO3− [8] are commonly used to complex the polymer electrolytes. The salt cations (for the most part) coordinate or complexes with groups along the polymer chain backbone and are transported via polymer motions through the polymer medium [9]. Propylene carbonate (PC) and ethylene carbonate (EC) are commonly used in the preparation of polymer electrolytes due to high dielectric constant and compatibility with many polymers. Plasticizers can increase the conductivity in a number of ways; by decreasing Tg (hence increasing molecular motion), increasing the number of charge carriers (decreasing ion aggregation) or providing an alternate conduction path such that the ion motions maybe decoupled from the local motions of the polymer [10]. For the last two decades, there are several research attempts have been made to improve the conductivity by incorporation of blend polymers, hybrid polymers, copolymers,
nanofillers and ionic liquids for the development of solid polymer electrolytes, gel polymer electrolyte and composite polymer electrolytes [11-15]. Chitosan a natural polymer is chosen as a host polymer as it is non–toxic and biodegradable. Earlier studies have shown that chitosan can be used as polymer matrix for ionic conduction [16-18]. The presence of lone pair on the nitrogen atom and oxygen atom in chitosan enables coordination of lithium ion from the added salt.
In this paper, we report the effect of nature of salt i.e. lithium triflate and lithium acetate on the ionic conductivity and dielectric behaviour of the chitosan polymer plasticized with ethylene carbonate (EC)/ propylene carbonate (PC). Knowledge of ion-conduction mechanisms in polymers is important for designing better polymer electrolytes for electrochemical devices.
2.
EXPERIMENTAL
2.1 Sample Preparation. In this study, 1g chitosan was obtained from Fluka was dissolved in 1% ethanoic acid solution. Lithium salts of 40 wt.% LiCH3COO and 40 wt.% LiCF3SO3 was added accordingly to the mixture. The two mixtures were stirred for several hours at room temperature. 30 wt.% of EC: PC in the ratio of 1:1 was added to the mixtures and further stirred for 24 hours. After complete dissolution, the solution was cast in petri dishes and air dried at room temperature (RT). This would result in free standing films. The residual ethanoic acid was removed from electrolyte film by rinsing with 0.1M NaOH solution followed by deionised water. The films were kept in a desiccator for one week and then dried in a vacuum oven for 2 hours prior to the characterization. 2.2 Impedance spectroscopy The ionic conductivity of the samples was measured with a HIOKI 3531-01 LCR Hi-Tester between the frequency ranges of 50 Hz to 1 MHz. The impedance
values of the samples were measured by sandwiching the samples between two stainless steel electrodes in the temperature range of 298 to 353 K. The conductivity (σ) of the samples was calculated using; σ =
t Rb A
where t is the sample thickness (cm), A the effective contact area of the electrode and the electrolyte (cm2), and Rb is the bulk resistance (Ω). Electric permittivity and electrical modulus can be determined from complex impedance [19]. The equations for the dielectric constant, εr and the dielectric loss, εi, can be calculated using Zr
εi =
ωCo (Z r2 + Z i2 )
εr =
ωCo (Z r2 + Z i2 )
Zi
where Co= εoA/t (εo is permittivity of free space),ω=2πf ( f is frequency), Zi is the imaginary part of the complex permittivity, Zr is the real part of the complex permittivity.
2.3 Fourier Transform Infrared Spectroscopy (FTIR) The FTIR studies were performed using FTIR Thermoscientific Nicolet iS10 equipped with an ATR internal reflection system was used to carry out ATR-FTIR analysis in the wave number range of 400–4000 cm−1 at a resolution of 2 cm−1. The objective of FTIR was to confirm complexation between polymer and salt.
3.
RESULTS AND DISCUSSIONS
3.1 Conductivity studies Fig.1(a) and (b) shows the complex impedance plot of chitosan – EC/PC system doped with lithium triflate salt and
lithium acetate salt respectively at room
temperature.. Both plots reveal a semicircle in the high frequency region followed by a slanted spike in
the high frequency region. The high frequency region semi-circle is characteristic of a parallel combination of bulk resistance and bulk capacitance [20], whereas the inclined straight line indicates the capacitive nature of the interface and absence of electronic conductivity [21].The intercept on the real axis gives directly the bulk resistance. The conductivity of the polymer electrolyte was calculated using the bulk resistance, area and thickness of the sample. Room temperature conductivity of the sample were found to be 6.1 x10-7 S cm-1 for chitosan system containing lithium acetate salt and 5.0 x10-6 S cm-1 for chitosan system containing lithium triflate salt. The room temperature conductivity of LiCF3SO3 electrolyte was higher than that of LiCH3COO electrolyte. The difference in conductivity is associated with the nature of the lithium salt. It may be inferred that LiCF3SO3 salt dissociated more LiCH3COO salt. Triflate anion is larger than acetate anion and therefore lithium triflate salt will have lower lattice energy than lithium acetate salt. Ionic conductivity is affected by the diffusion rate of the ions, which in turn depends on the size of each ion [22]. 3.2 Dielectric relaxation studies The variation of dielectric constant (εr) and dielectric loss (εi) as function of frequency for CLT and CLA are shown in Fig. 2 (a) and (b) respectively. Dielectric constant measures stored charges. A higher dielectric constant value for CLT system implies that more lithium ions are present. LiCF3SO3 salt has lower lattice energy than LiCH3COO salt. This allows for greater dissociation of LiCF3SO3 salt and hence greater number of free Li+ for ionic transport. Both dielectric constant and dielectric loss increase sharply at low frequencies implying that electrode polarisation and space charge effects have occurred confirming its non-Debye dependence [23]. At low frequencies, the charges align themselves along the electric field direction and thus fully contribute to the total polarisation. At high frequencies both dielectric constant and dielectric loss approaches almost constant value. As the frequency is increased, the variation in electric field becomes
too fast for the charges to follow and therefore their contribution to the polarisation becomes less with a measurable lag because of internal frictional forces [24]. It is clear from Table 1 that dielectric constant value increases with temperature. The molecular dipoles in polar materials cannot orient themselves at low temperature. When the temperature rises the dipole orientation is facilitated, and this increases the dielectric constant [24]. Table 1 also illustrates that dielectric loss exhibits strong temperature dependence. The increase of dielectric loss was explained by Stevels [25] who divided the relaxation phenomenon into three parts; conduction losses, dipole losses and vibrational losses [24]. Conduction losses is proportional to (σ/ω) and hence conduction losses rises with increase in temperature, which in turn causes the value of dielectric loss to increase. Fig. 3(a) and (b) displays the variation of dielectric loss tangent, tan δ, with frequency at different temperatures for CLT and CLA system. From the plots, it is clear that tan δ value increases with frequency at different temperatures, passes through a maximum value and thereafter decreases. The presence of loss peak suggests a dielectric relaxation process. The peak value shifts towards the higher frequency with increasing temperature. At higher temperatures the peak are out of the frequency range studied in this work. Fig. 4(a) and (b) show that the frequency for relaxation peaks increases with temperature and hence relaxation time decreases with increase in temperature. The decrease in relaxation time with increasing temperature is due to faster movement of lithium ions that synchronizes with the direction of applied field and increased motion of charge carriers in the polymer matrix. The activation energy of relaxation is 0.71eV for CLT and 0.81 eV for CLA system. Fig. 5(a) and (b) depict the plot of ln εi versus ln ω at different temperatures. The dielectric loss increases with increasing temperature for all frequencies. The values of exponent s can be determined from the slope of these plots in the high frequency region to avoid electrode polarization.
The plot s versus temperature for CLT and CLA systems is shown in Fig. 6. From the plot it can be observed that for both systems s decreases with the increase in temperature to a minimum value at a certain temperature, and then it increases with increasing temperature. Thus, the temperature dependence of s could be interpreted by the overlapping large polaron-tunnelling (OLPT) model [26].
3.3 FTIR analysis Fig.7 shows the full FTIR spectra of pure salts (lithium triflate and lithium acetate), chitosan film and electrolytes prepared using chitosan and the salts. There were important characteristic peaks observed due to their available bands in the molecular structure. A broad peak has been observed in pure chitosan film in the range of 3300 – 3500 cm-1 which could be assigned to axial stretching of O-H superimposed to N-H stretching band of chitosan [26,27]. The electrolytes prepared by the salt incorporated chitosan have also shown the same broad peak in this range with approximately same peak intensity. It can be inferred that these peaks would also be contributed by axial stretching of O-H superimposed to N-H stretching band of pure chitosan. As the samples were vacuum evaporated for 2 hours just before the experiments, the moisture absorption by the electrolyte films can be deemed negligible. The spectra have been further analysed in the range of 1000 – 1700 cm-1 depicted in Fig. 8 by considering the peaks of pure chitosan spectrum as reference. The change in peak shift and intensity will be the evidence of the complexation and incorporation of the salt in the polymer electrolyte formulation. The bands at 1227 and 1036 cm−1 are assigned to asymmetric SO3 and symmetric SO3 vibrations of LiCF3SO3, respectively. It can be noticed that the free triflate ions present in chitosan based electrolyte system shifted to 1247 and 1026 cm-1. Other important characteristic peaks of lithium triflate at 1190 and 1642 cm-1 have been observed at 1173 and 1647 cm-1 respectively of the lithium triflate salt incorporated chitosan electrolyte whereas peaks at 1166 and 1638 cm-1 appeared in pure
chitosan film [28]. The characteristic peaks of lithium acetate salt were observed at 1434 cm-1 and 1595 cm-1. Two broad peaks were observed in the lithium acetate salted chitosan film at range of 1361 –1480 cm-1 and 1490 – 1630 cm-1 whereas narrower peaks were observed at 1381, 1411 and 1543 cm-1 for pure chitosan film. This may be due to the complexation of the salt and the host polymer. From the observations made from FTIR analysis, it is understood that the free ions are responsible for conductivity. 4.
CONCLUSION
The nature of anion size influences the ionic conduction of polymer electrolyte. It was found that the chitosan system using LiCF3SO3 exhibits better conductivity than chitosan system using LiCH3COO. The dielectric studies shows the prepared materials are ionic conductors. Both systems follow OLPT regardless of the type of salt.
ACKNOWLEDGMENTS Authors would like to thank Research Management Institute (RMI), Faculty of Applied Sciences, Institute of Science, Universiti Teknologi MARA (UiTM) by supporting this study by RACE/F1/ST3/UITM/5 grant. We would also thank University of Malaya for providing the grants RACE/UM/CR003-2013 and RP025-14AFR.
References [1]
H.M.J.C. Pitawala, M.A.K.L. Dissanayake, V.A. Seneviratne, Combined effect of Al2O3 nano-fillers and EC plasticizer on ionic conductivity enhancement in the solid polymer electrolyte (PEO)9LiTf, Solid State Ionics 178 (2007) 885. [2] Jae-Won Choi, Gouri Cheruvally, Yeon-Hwa Kim, Jae-Kwang Kim, James Manuel, Prasanth Raghavan, Jou-Hyeon Ahn, Ki-Won Kim, Hyo-Jun Ahn, Doo Seong Choi, Choong Eui Song, Poly(ethylene oxide)-based polymer electrolyte incorporating room-temperature ionic liquid for lithium batteries, Solid State Ionics 178 (2007) 1235.
[3]
[4]
[5]
[6]
[7] [8]
[9]
A. Mary Sukeshini, Atsushi Nishimoto, Masayoshi Watanabe, Transport and electrochemical characterization of plasticized poly(vinyl chloride) solid electrolytes, Solid State Ionics 86–88 (1996) 385. R. H. Y. Subban, and A. K. Arof. "Experimental investigations on PVC-LiCF3SO3-SiO2 composite polymer electrolytes." Journal of New Materials for Electrochemical Systems 6 (2003) 197. Boor Singh, Rajiv Kumar, S.S. Sekhon, Conductivity and viscosity behaviour of PMMA based gels and nano dispersed gels: Role of dielectric constant of the solvent, Solid State Ionics 176–18 (2005) 1577. A.M.M. Ali, M.Z.A. Yahya, H. Bahron, R.H.Y. Subban, M.K. Harun, I. Atan, Impedance studies on plasticized PMMA-LiX [X: CF3SO3−, N(CF3SO2)2−] polymer electrolytes, Materials Letters 61 (2007) 2026. D.R MacFarlane, F Zhou, M Forsyth, Ion conductivity in amorphous polymer/salt mixtures, Solid State IonicS 113–115 (1998) 193. S. Rajendran, M. Ramesh Prabhu, M. Usha Rani, Ionic conduction in poly(vinyl chloride)/poly(ethyl methacrylate)-based polymer blend electrolytes complexed with different lithium salts, Journal of Power Sources 180 (2008) 880. R Walter, R Walkenhorst, M Smith, J.C Selser, G Piet, R Bogoslovov, The role of polymer melt viscoelastic network behavior in lithium ion transport for PEO melt/LiClO4 SPEs: the “wet gel” model, Journal of Power Sources 89 (2000) 168.
[10]
M. Forsyth, P.M. Meakin, D.R. MacFarlane, A 13C NMR study of the role of plasticizers in the conduction mechanism of solid polymer electrolytes, Electrochimica Acta, Volume 40 (1995) 2339.
[11]
V. D. Noto, S. Lavina, G. A. Giffin, E. Negro, B. Scrosati, (2011) Polymer electrolytes: Present, past and future, Electrochimica Acta, 57 (15) 4-13
[12]
F. Bertasi, K. Vezzù, E. Negro, S. Greenbaum, V. D. Noto (2014) Single-ion-conducting nanocomposite polymer electrolytes based on PEG400 and anionic nanoparticles: Part 1. Synthesis, structure and properties, International Journal of Hydrogen Energy, 39 (6),28722883
[13]
F. Bertasi, K. Vezzù, G. A. Giffin, T. Nosach, P. Sideris, S. Greenbaum, M. Vittadello, V. D. Noto (2014) Single-ion-conducting nanocomposite polymer electrolytes based on PEG400 and anionic nanoparticles: Part 2. Electrical characterization, International Journal of Hydrogen Energy, 39 (6), 2884-2895
[14]
V. D. Noto, M. Vittadello, S. Lavina, M. Fauri, S. Biscazzo, (2001) Mechanism of Ionic Conductivity in Poly(ethyleneglycol 400)/(LiCl)x Electrolytic Complexes: Studies Based on Electrical SpectroscopyJ. Phys. Chem. B 2001, 105, 4584-4595
[15]
M. J. Park, I. Choi, J. Hong, O. Kim (2013) Polymer Electrolytes Integrated with Ionic Liquids for Future Electrochemical Devices, J.Appl. Polym. Sci , 129, 2363-2376
[16]
M.Z.A. Yahya, A.K. Arof, Effect of oleic acid plasticizer on chitosan–lithium acetate solid polymer electrolytes, European Polymer Journal 39 (2003) 897.
[17]
S.R. Majid, A.K. Arof, Proton-conducting polymer electrolyte films based on chitosan acetate complexed with NH4NO3 salt, Physica B: Condensed Matter 355 (2005) 78.
[18]
S.B. Aziz, Z.H.Z. Abidin, Electrical Conduction Mechanism in Solid Polymer Electrolytes: New Concepts to Arrhenius Equation, Journal of Soft Matter (2013)
[19]
Ross J, MacDonald (1987) Impedance spectroscopy. Wiley, USA
[20]
M Venkateswarlu, K Narasimha Reddy, B Rambabu, N Satyanarayana, A.c. conductivity and dielectric studies of silver-based fast ion conducting glass system, Solid State Ionics 127 (2000) 177.
[21]
M.S Michael, M.M.E Jacob, S.R.S Prabaharan, S Radhakrishna, Enhanced lithium ion transport in PEO-based solid polymer electrolytes employing a novel class of plasticizers, Solid State Ionics 98 (1997)167.
[22]
S Rajendran, R Kannan, O Mahendran, Ionic conductivity studies in poly(methylmethacrylate)–polyethlene oxide hybrid polymer electrolytes with lithium salts, Journal of Power Sources 96 (2001) 406.
[23]
S Rajendran, O Mahendran, T Mahalingam, Thermal and ionic conductivity studies of plasticized PMMA/PVdF blend polymer electrolytes, European Polymer Journal, Volume 38 (2002) 49.
[24]
M.M. El-Nahass, A.F. El-Deeb, H.E.A. El-Sayed, A.M. Hassanien, Electrical conductivity and dielectric properties of bulk glass Se55Ge30As15 chalcogenide, Physica B: Condensed Matter 388 (2007) 26.
[25]
J. M. Stevels, in: S. Flugge (Ed.), Handbuch der Physik, Springer, Berline, (1957) pp. 350
[26]
N.H.A. Rosli, C.H. Chan, R.H.Y. Subban, Tan Winie, Studies on the Structural and Electrical Properties of Hexanoyl Chitosan/Polystyrene-based Polymer Electrolytes, Physics Procedia 25 (2012) 215
[27]
S.S. Alias, S. M. Chee and A. A. Mohamad (2014), Chitosan–ammonium acetate–ethylene carbonate membrane for proton batteries, Journal of Chemistry http://dx.doi.org/10.1016/j.arabjc.2014.05.001
[28]
Enescu, D., Hamciuc, V., Ardeleanu, R., Cristea, M., Ioanid, A., Harabagiu, V., Simionescu, B.C., 2009. Polydimethylsiloxane modified chitosan. Part III: preparation and characterization of hybrid membranes. Carbohydr. Polym. 76, 268–278.
[29]
S. Ramesh, K.H. Leen, K. Kumutha and A.K. Arof (2007). FTIR studies of PVC/PMMA blend based polymer electrolytes. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 66(4-5): 1237-1242.
(a) (b)
Fig. 1. (a) Impedance diagram of CLT at RT. (b) Impedance diagram of CLA at RT.
(a)
(b)
Fig. 2. (a) Frequency dependence of dielectric loss, εr for CLT and CLA systems at RT. (b) Frequency dependence of dielectric loss, εifor CLT and CLA systems at RT.
(a)
(b) Fig. 3. (a) Variation of loss tangent with frequency for CLT system. (b) Variation of loss tangent with frequency for CLA system
(a)
(b) Fig. 4. (a) Variations of ln τ with inverse temperatures for CLT. (b) Variations of ln τ with inverse temperatures for CLA.
(a)
(b)
Fig. 5. (a) Variation of ln εi with ln ω for CLT system. (b) Variation of ln εi with ln ω for CLA system
Fig. 6. Variation of the exponent “s” with temperature for CLT system and CLA system
Fig. 7. Complete infrared spectra of (a) lithium triflate (b) lithium acetate (c) chitosan, (d) chitosan - lithium acetate (e) chitosan lithium triflate
Fig. 8. Infrared spectra of (a) lithium triflate (b) lithium acetate (c) chitosan, (d) chitosan - lithium acetate (e) chitosan - lithium triflate in the region between 1000 to 1700 cm-1
Table 1. The dielectric constant and the dielectric loss at different temperatures at f=250Hz and 950000Hz for CLT and CLA
Dielectric constant (εεr) at Temperature 250Hz (K) CLT CLA 298 303 313 323 333 343 353
17638 21770 33488 43832 78297 90359 108351
890 1023 2343 4746 8960 36175 67726
Dielectric loss (εεi) at 250Hz
Dielectric constant Dielectric loss (εεr) at 95000Hz (εεi) at 950000Hz
CLT
CLA
CLT
CLA
CLT
CLA
26497 31392 42510 52126 84524 97682 115140
1867 2060 3722 6609 12197 45691 81868
44 46 50 52 55 55 53
11 11 13 16 20 25 12
32 40 63 87 195 255 357
3 4 6 11 25 102 251