Electrochemistry Communications 11 (2009) 1796–1798
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UV-cured polymer electrolyte membranes for Li-cells: Improved mechanical properties by a novel cellulose reinforcement J.R. Nair a, C. Gerbaldi a,*, A. Chiappone a, E. Zeno b, R. Bongiovanni a, S. Bodoardo a, N. Penazzi a a b
Department of Materials Science and Chemical Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Turin, Italy Centre Technique du Papier (CTP), Domaine Universitaire, B.P. 251, 38044 Grenoble Cedex 9, France
a r t i c l e
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Article history: Received 19 May 2009 Received in revised form 15 July 2009 Accepted 16 July 2009 Available online 18 July 2009 Keywords: Cellulose hand-sheet UV-curing Gel-polymer electrolyte Mechanical property Li-cells
a b s t r a c t One of the main drawbacks that restricts the practical application of gel-polymer electrolytes is the inferior mechanical performance compared to other available systems. In this work, we have reinforced UV-cured methacrylic membranes with cellulose. To enhance its compatibility with the polymer matrix, cellulose is modified by UV-grafting poly(ethylene glycol) methyl ether methacrylate on it. Excellent mechanical properties are obtained and good ionic conductivity values are observed, enlightening that this kind of membrane is an interesting candidate for future applications as thin gel-polymer electrolyte in flexible lithium batteries. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Today, special attention is focused on polymer systems showing high ionic conductivity at ambient and/or sub ambient temperatures, since they find unique practical applications, such as separators in high power, versatile, rechargeable Li-based batteries. Although a commercial reality, these power sources are still the object of intense R&D aiming to improve their performances for high-end applications [1,2]. High performing innovative electrolyte materials should be capable of high ionic conductivity even at ambient temperature, with good mechanical and interfacial properties and stable performances. Moreover, they should be low cost, ecologically friendly and safe. Thermo-set membranes prepared by free radical photo-polymerisation (UV-curing) could be an interesting alternative to existing polymer electrolytes. Methacrylic-based thermo-set gel-polymer electrolytes (GPEs) obtained by free radical photo-polymerisation (UV-curing) used for lithium battery application have been already illustrated and discussed [3–5]. The preparation process is very easy, reliable and rapid; the obtained membranes show good behaviour in terms of ionic conductivity, interfacial stability with the Li-metal electrode and cyclability in lithium cells. Though the membranes are flexible, self standing and easy to handle, there is room for improving mechanical strength, in particular if application in flexible batteries is taken into consideration. In the present communication, * Corresponding author. Tel.: +39 0115644638; fax: +39 0115644699. E-mail address:
[email protected] (C. Gerbaldi). 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.07.021
we present and discuss our preliminary results regarding the improvement of the mechanical properties of GPE prepared by UV-curing using modified cellulose hand-sheets, specifically prepared and modified by UV grafting [6]. Such modification would make these hybrid organic, green, cellulose-based composite polymer systems a strong contender in the field of flexible Li-based power sources. The promising perspectives of such kind of GPEs are illustrated by the experimental data on the electrochemical response of a lithium polymer cell. 2. Experimental 2.1. Cellulose hand-sheets preparation and modification Kraft pulp, bleached and refined at 35 °SR (Schopper–Riegler degree [7]), was used as a raw material for hand-sheets preparation. A hard-wood (HW) to soft-wood (SW) combination of 60:40 w/w was re-pulped and blended using a high speed blender and made into desired concentration of 3 g L 1. It was later made to a suspension of 1.5 g L 1 which was introduced into a retention sheet-former (FRET) of 1500 RPM. The filtrate on copper wires was then dried at 90 °C under high vacuum to give a hand-sheet of 1.5 g with an average thickness of 100 lm. The hand-sheets were modified by photo-grafting technique using poly(ethylene glycol) methyl ether methacrylate (PEGMA, average Mn = 475, Aldrich), and benzophenone (Aldrich) as photoinitiator. In this grafting procedure, derived from Stachowiak et al. [6], the paper was swelled into a solution of benzophenone
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in ethanol. The swelled paper was UV irradiated for approx. 1 min under N2 flux and, then, dried at 60 °C. This pre-treated paper was swelled into a PEGMA/ethanol solution and soon after irradiated under flowing nitrogen. After washing in ethanol, the obtained modified cellulose was then dried under high vacuum at 120 °C to assure the complete removal of water and ethanol. A more detailed description of the grafting procedure and the characteristics of the modified cellulose (TGA, ATR-FTIR analyses) will be reported in a forthcoming paper. 2.2. Reinforced gel-polymer electrolyte preparation The obtained PEGMA-grafted cellulose was swelled into a reactive mixture (whose composition is described below) for 30 min and exposed to UV light for 3 min in two steps to obtain the cellulose-reinforced GPE. The photochemical curing was performed using a medium vapour pressure Hg UV lamp (Helios Italquartz, Italy), with a radiation intensity of 28 mW cm 2. The reactive mixture included: the dimethacrylic oligomer bisphenol A ethoxylate (15 EO/phenol) dimethacrylate (BEMA, Mn = 1700, Aldrich), PEGMA, a 1:1 w/w ethylene carbonate–diethyl carbonate (EC–DEC, Fluka) solution, LiTFSI (lithium bistrifluoromethane-sulfonimide, CF3SO2NLiSO2CF3, Aldrich) salt with 2-hydroxy-2-methyl-1-phenyl-propan-1-one (Darocur 1173, Ciba Specialty Chemicals) as photo-initiator. The exact weight ratio of BEMA:PEGMA:EC– DEC:LiTFSI is 15:15:40:30 with 3 wt.% of photo-initiator.
Differential scanning calorimetry measurements in the temperature range 140–120 °C under N2 were performed with a METTLER DSC-30 (Greifensee, Switzerland) instrument equipped with a low temperature probe; thermo-gravimetric analysis, by TGA/ SDTA-851 instrument from METTLER (Switzerland), were performed up to 600 °C under N2 to assess the grafting and to characterise the complete system. The methods followed for the characterization are the same as already described elsewhere [3–5]. Mechanical measurements were carried out through tensile experiments according to ASTM Standard D638, using a Sintech 10/D instrument equipped with an electromechanical extensometer (clip gauge). At least five specimens for each sample were tested; the standard deviation in Young modulus (E) was 5%. AC impedance spectroscopy was used to access the ionic conductivity of the GPE. Electrochemical stability window and galvanostatic charge/discharge cycling performances were also carried out to complete the electrochemical testing according to the procedures reported in Ref. [3–5,8].
4. Results and discussion In previous papers we described gel-polymer membranes obtained by a fast UV-curing process [3–5]. The electrochemical performances (e.g., compatibility with Li metal, specific capacity retention and cycling efficiency) were good, however their mechanical resistance was unsatisfactory. Incorporation of modified cellulose as a reinforcement is proposed to improve the mechanical behaviour. The reinforced gel-polymer electrolyte, namely CMB, is shown in Fig. 1: it is a highly translucent, freestanding, extremely flexible and non-sticky membrane. The differential scanning calorimetry analyses evidenced a Tg value of about 77.1 °C, indicating that at room temperature the polymer membrane is in a rubbery state. As clearly depicted in Fig. 1, though the Tg is very low, the membrane is self standing, extremely flexible and easy to handle.
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Fig. 1. Appearance of the CMB polymer electrolyte membrane.
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Mechanical properties obtained by means of tensile test studies (a typical force–elongation curve is reported in Fig. 2) reveal that the average Young’s modulus is 417 MPa and the average tensile strength is 2.7 MPa. If one considers that there is a liquid content of approximately 40 wt.%, these are very high values. Without cellulose reinforcement, the polymer electrolyte cannot be tested. Such an excellent mechanical behaviour is by all means a positive property: it may also improve the safety features of the GPE membranes, in which the cellulose fibres can block the growth of lithium dendrites, as the cellulose fibres are highly crystalline and very strong. The thermal stability of the polymeric film has been assessed by thermo-gravimetric analysis under nitrogen. The membrane prepared without EC–DEC solution is stable up to 300 °C. For the complete gel-polymer electrolyte there is a distinct weight loss at approx. 130 °C, as the solvents begin to evaporate: this means that the GPE can be safely used in thin flexible Li-based polymer batteries up to 100 °C. The ionic conductivity has been evaluated by impedance spectroscopy and the Arrhenius plots are shown in Fig. 3 (upper layer A). For testing, discs of 0.785 cm2 have been cut from the polymer membrane and sandwiched between two stainless-steel (SS-316) electrodes. The CMB membrane demonstrates an ionic conductivity of about 2.0 10 4 S cm 1 at room temperature. It increases
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Fig. 4. Galvanostatic charge–discharge profiles of the lithium polymer cell, assembled by sandwiching CMB between a LiFePO4/C cathode and a Li metal anode, at 1C current rate.
Voltage / V vs. Li Fig. 3. (A) Conductivity vs. temperature plot of the CMB gel-polymer electrolyte. Data obtained by impedance spectroscopy. (B) Current vs. voltage curves at room temperature for CMB membrane. Cell configuration adopted: acetylene black over Al current collector as the working electrode, Li metal as the counter electrode and CMB as the electrolyte. Anodic potential scan range from O.C.V to 5.5 V vs. Li at 0.100 mV s 1.
with increasing the temperature, resulting in a high value of 1.03 10 3 S cm 1 at 80 °C. All impedance spectra obtained in the selected temperature range are linear, with no sign of high-frequency semicircles which could indicate lack of gel homogeneity due to crystalline phase separation. In addition to high ionic conductivity, this prepared CMB membrane shows an appreciably high anodic breakdown voltage, which makes it particularly valuable in view of a practical battery application. This is shown in Fig. 3 (lower layer B) which illustrates the current–voltage response of CMB obtained in the voltage range between O.C.V. and 5.5 V vs. Li at room temperature by linear sweep voltammetry. The plateau is very flat and straight; this very low residual current level prior to breakdown voltage, with no peaks in the lower voltage range, confirms the high purity of the prepared GPE and the synthesising method adopted, because the system as a whole is sensitive to oxygen, water and other impurities. The increase of the current during anodic scan, which is related the decomposition of the electrolyte, is taken in correspondence to the onset of a low current peak at approx. 4.4 V vs. Li. If such low peak is not connected to the reaction of the electrolyte, then the stability range can be extended at least to 4.7 V vs. Li, a really interesting value. Tests are being carried out to deepen this aspect. In view of the possible practical application of the CMB gelpolymer electrolyte, a laboratory scale Li cell has been assembled by combining a lithium metal anode with a LiFePO4/C cathode and using the CMB membrane as the separator. Its electrochemical behaviour has been investigated by means of galvanostatic charge/ discharge cycling at room temperature. The cathode (electrode area: 0.785 cm2) has been prepared in the form of thin film by spreading on an Al current collector, by the so-called ‘‘doctor blade” technique, a N-methyl-2-pyrrolidone slurry of the LiFePO4/C active material (90 wt.%, about 3.5 mg cm–2) with acetylene black as electronic conducting additive (6 wt.%) and poly(vinylidene fluoride) as binder (4 wt.%). The LiFePO4/C cathode has been prepared by a slight modification of the procedure reported by Meligrana et al. [8]. The electrodes/electrolyte assembly has been housed in a Teflon-made Swagelok cell equipped with two SS316 current collector electrodes. The first results of galvanostatic
cycling seem very interesting as depicted in Fig. 4 which shows the 50th galvanostatic charge and discharge profiles at 1C current regime. Good performance at high current rate may be ascribed to the efficient ionic conduction in the polymer separator and the favourable interfacial charge transport between electrodes and electrolyte in the cell. The voltage drop in passing from charge to discharge is small, which means low resistance of the cell. The coulombic efficiency is almost 100% and the specific capacity, being preliminary results, interesting. 5. Conclusions Methacrylic-based gel-polymer electrolytes prepared by UV-curing process and reinforced with cellulose appropriately modified show excellent mechanical properties and high ionic conductivity even at ambient temperature. It is confirmed that, compared to other techniques, UV-curing is versatile due to its easiness and rapidity in processing while the use of cellulose UVgrafted with methacrylic monomer can assure good mechanical properties. Besides these, the system shows good overall electrochemical performance, and it has attractive features such as intrinsic safety, eco-compatibility, low production cost and industrialisation potentials. Therefore, these kind of GPEs present particularly promising perspectives in the field of Li-based thin flexible batteries. Such results motivate us to undertake extensive experimental investigations on this subject. Acknowledgements Financial support from the Italian Regione Piemonte Council (Research Project C116) and CTP is gratefully acknowledged. References [1] J.-M. Tarascon, M. Armand, Nature 414 (2001) 359. [2] A.S. Arico, P. Bruce, B. Scrosati, J.-M. Tarascon, W. Van Schalkwijk, Nature Mat. 4 (2005) 366. [3] J. Nair, C. Gerbaldi, G. Meligrana, R. Bongiovanni, S. Bodoardo, N. Penazzi, P. Reale, V. Gentili, J. Power Sources 178 (2008) 751. [4] C. Gerbaldi, J. Nair, C. Bonatto Minella, G. Meligrana, G. Mulas, S. Bodoardo, R. Bongiovanni, N. Penazzi, J. Appl. Electrochem. 38 (2008) 985. [5] C. Gerbaldi, J. Nair, G. Meligrana, R. Bongiovanni, S. Bodoardo, N. Penazzi, J. Appl. Electrochem., in press, doi: 10.1007/s10800-009-9805-6. [6] T.B. Stachowiak, F. Svec, J.M.J. Fréchet, Chem. Mater. 18 (2006) 5950. [7] D.U. Lima, R.C. Oliveira, M.S. Buckeridge, Carbohyd. Polym. 52 (2003) 367. [8] G. Meligrana, C. Gerbaldi, A. Tuel, S. Bodoardo, N. Penazzi, J. Power Sources 160 (2006) 516.
