J. Phys. Chem. B 2004, 108, 10845-10852
10845
Nanocomposite Polymer Electrolytes Based on Poly(oxyethylene) and Cellulose Nanocrystals My Ahmed Said Azizi Samir,†,‡ Fannie Alloin,*,† Wladimir Gorecki,§ Jean-Yves Sanchez,† and Alain Dufresne*,| Laboratoire d’Electrochimie et de Physico-chimie des Mate´ riaux et des Interfaces (LEPMI-INPG), BP 75, F38402 St Martin d’He` res Cedex, France, Centre de Recherches sur les Macromole´ cules Ve´ ge´ tales (CERMAV-CNRS), UniVersite´ Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France, Laboratoire de Spectrome´ trie Physique, UniVersite´ Joseph Fourier, BP 87, 38402 Saint-Martin d’He` res Cedex, France, and Ecole Franc¸ aise de Papeterie et des Industries Graphiques (EFPG-INPG), BP 65, F38402, St Martin d’He` res Cedex, France ReceiVed: February 6, 2004; In Final Form: May 24, 2004
Lithium-conducting nanocomposite polymer electrolytes based on high molecular weight poly(oxyethylene) (POE) were prepared from high aspect ratio cellulosic whiskers and lithium imide LiTFSI salt. The thermomechanical behavior of the resulting films was investigated by differential scanning calorimetry, thermogravimetric analysis, and dynamic mechanical analysis. The ionic conductivity and the electrochemical stability of the nanocomposite polymer electrolytes are quite consistent with the specifications of lithium batteries. The ionic mobilities were determined by pulsed magnetic field gradient NMR, and it was shown that the reinforcement does not affect the lithium transference number. High performance nanocomposite electrolytes based on tunicin whiskers were obtained. Indeed, the filler provides a high reinforcing effect, while a high level of ionic conductivity is retained with respect to unfilled polymer electrolytes.
Introduction Secondary lithium polymer batteries are promising highperformance energy sources well-adapted to electrical/hybrid vehicles and utility power sources (UPS). Many studies have focused on the enhancement of ionic conductivities of polymer electrolytes, most of them performed, due to their strong solvating ability, on polyoxyethylene (POE) and its copolymers. In polymer electrolytes, free of solvents, one of the main advantages is the possibility to reduce significantly the electrolyte thickness with respect to liquid electrolytes. The thickness decrease allows one, at the same conductivity level, to decrease the internal resistance of the battery. This means that both for safety and performance reasons, polymer electrolytes must exhibit, in addition to high conductivities and a wide enough electrochemical window, high thermal and mechanical performances. It was initially assumed that ionic conductivity requires an amorphous character for polymer electrolytes,1,2 and it was later confirmed by NMR investigations that conductivity predominates in amorphous phases3 where ion mobility is chain-assisted. However, the semicrystalline structure of POE results in acceptable conductivities but poor mechanical properties above its melting temperature (∼60 °C). Since the original work of Weston and Steel,4 who reported the improvement of polymer electrolyte mechanical stability by adding R-Al2O2 particles, nanocomposite polymer electrolytes have been extensively studied.5-16 * To whom correspondence should be addressed. F.A.: tel, 33 (4) 76 82 65 60; fax, 33 (4) 76 82 65 77; e-mail,
[email protected]. A.D.: tel, 33 (4) 76 82 69 95; fax, 33 (4) 76 82 69 33; e-mail,
[email protected]. † Laboratoire d’Electrochimie et de Physico-chimie des Mate ´ riaux et des Interfaces. ‡ Centre de Recherches sur les Macromole ´ cules Ve´ge´tales, Universite´ Joseph Fourier. § Laboratoire de Spectrome ´ trie Physique, Universite´ Joseph Fourier. | Ecole Franc ¸ aise de Papeterie et des Industries Graphiques.
The comprehension of inorganic filler impact on conduction, thermal, mechanical, and electrochemical properties of polymer electrolytes is still in progress. The best performances were obtained with Al2O38-14 and TiO2.10-12,15,16 The increase in conductivity was found to be more significant below the melting point of POE, after a preliminary heating of the sample. This improvement was ascribed to the highest amorphous degree of the electrolyte, due to the decrease of crystallization kinetics induced by the fillers.17 Although the improvement of conductivity at high temperature is more difficult to explain, in particular its dependence with the surface nature of the fillers, some concordant data are provided by the literature. The evolution of polymeric chain mobility after filler incorporation was evaluated using different techniques.18-22 A NMR study carried out by Chung et al.18 and Quartarone et al.19 demonstrated an invariance of the diffusion coefficient of the proton associated with the polymeric chain in the presence of TiO2 or Al2O3 in a POE-LiClO4 electrolyte. A similar result was reported by Karlsson et al.20 from quasielastic neutron scattering experiments in a fully amorphous polyether filled with TiO2 particles. Many works indicate almost no effect on Tg of the polymer matrix upon filler addition.15,11,22 However a significant decrease of both Tg and viscosity was reported for poly(ethylene glycol) monomethyl ether (Mw ) 350 g mol-1)/Al2O3 composites.8,12,13 The study of the D-LAM mode (disorder-longitudinal acoustic mode) of low molecular weight amorphous polyethers displayed a decrease of the segmental mobility of the polymeric chain in the presence of TiO2 which was not observed in the presence of Al2O3.10 The influence of particle incorporation on the concentration in charge carriers was evaluated by infrared spectroscopy (IR),8,10,12,23 by dielectric relaxation,9 and by using the formalism of Fuoss-Kraus.8,12,24 Best et al.10 observed by IR spectroscopy
10.1021/jp0494483 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/26/2004
10846 J. Phys. Chem. B, Vol. 108, No. 30, 2004 an invariance of ion pairs fraction upon filler addition. Other IR studies8,12 showed the opposite trend. An increase in cationic transference number was reported for various composite polymer electrolytes.11,14,16,21,25 It was suggested that this increase depends on the specific surface interactions between fillers and the polymer or salt.14 Two models were proposed to explain the conduction properties of polymer electrolytes in the presence of fillers. The first one is based on Lewis acid-base type interactions among ether oxygen of POE chains, filler surface groups, and salt ions.8,12,14 In the second model, filler surface effects were ignored and both filler/polymer and filler/ions electrostatic interactions were estimated using a dielectric constant approach.10,26 In a recent communication, Singh et al.27 established the role of the dielectric constant of the filler in enhancing ionic conductivity by using ferroelectric ceramic materials with thermal dielectric phase transition. Unfortunately only few studies have been performed to evaluate the mechanical properties of composite polymer electrolytes. The mechanical properties were evaluated by their dimensional stability related to the creep properties. The latter were indirectly determined by impedance spectroscopy. Improvement of the mechanical stability was reported for R-Al2O3reinforced POE8-LiClO4,4 hyperbranched polymer electrolyte filled with R- and γ-LiAlO2 particles,28 and POE8-LiClO4 filled with alumina whiskers.29 Fan et al.30 performed stress/strain tests and observed a slight decrease of the mechanical strength for POE16-LiClO4 reinforced with modified montmorillonite. Croce et al.11 reported a significant increase of both the Young modulus and yield stress for Al2O3- or TiO2-reinforced POE8-LiClO4. Unfortunately, no modulus value was reported in their paper. In our previous study,31-33 we used cellulosic rigid rods (whiskers) like a mechanical reinforcing phase in salt-free POEbased composites. Spectacular improvement of the tensile modulus was observed especially above the POE melting temperature when only 3 wt % cellulosic fillers were used. This elevated reinforcing effect was ascribed to the high aspect ratio of the fillers and to the formation of a percolating cellulosic network within the polymeric matrix above the percolation threshold (around 1 vol %). The present work reports on the use of cellulose nanocrystals as an organic filler in POE-Li[(CF3SO2)2N] polymer electrolytes. Thermal, mechanical, and transport properties and electrochemical stability were evaluated. Experimental Section Polymeric Matrix. Poly(oxyethylene) (POE) with a high molecular weight (Mw ) 5 × 106 g‚mol-1) and TiO2 powder (size < 5 µm) were purchased from Aldrich. The lithium trifluoromethanesulfonylimide (LiTFSI) salt from Fluka was dried in a vacuum for 24 h at 130 °C prior to use and then stored in a glovebox. Filler Preparation. The nanocrystalline cellulose filler (whiskers) used in this work was extracted from the mantle of a sea animal (tunicate). Its preparation consists of an acid hydrolysis of the constitutive tunicin (the cellulose extracted from the mantle of tunicate) microfibrils. Details of the preparation of aqueous suspensions of tunicin whiskers are largely described elsewhere.34-37 The resulting whiskers consist of highly polydispersed rigid rods, with an average length around 1 µm and a diameter around 15 nm. The geometrical specific surface area of whiskers is
Azizi Samir et al. around 170 m2‚g-1.37 Surface acid groups (OSO3-/H+) resulting from sulfuric acid hydrolysis induce an electrostatic stabilization of whiskers in aqueous medium. We substitute protons by lithium using a LiOH aqueous solution to prevent mobile protons in the composite polymer electrolytes. Films Processing. Unfilled polymer electrolytes were prepared in a glovebox by an acetonitrile casting technique. Composite polymer electrolytes using tunicin whiskers were prepared in two steps, the first one consisting of the preparation of POE/whiskers films and the second one the introduction of the lithium salt within the resulting tunicin whisker-reinforced POE films. The desired amount of tunicin whiskers (water suspension) was added to the aqueous POE solution. The resulting suspension was stirred for 24 h and cast into Teflon plates. Water evaporation was carried out at 40 °C for 1 week. Films were dried under vacuum for 72 h at 100 °C to eliminate the remaining water. 2,2-Dimethoxypropane was used as dehydration agent. Finally, the lithium salt was introduced by swelling the nanocomposite films with a concentrate acetonitrile salt solution. This procedure was chosen because the direct incorporation of the salt to the tunicin whiskers aqueous suspension caused its sedimentation. Composite polymer electrolytes using TiO2 were prepared by the usual casting technique. The desired amounts of TiO2 and salt were added to the POE solution in acetonitrile. The suspension was cast into a Teflon plate in a glovebox. Films were dried under vaccum for 72 h at 100 °C and then stored in a glovebox. The lithium salt content in the film was classically referred to the number n ) O/Li, which corresponds to the molar ratio oxyethylene/lithium. POE-based electrolyte will be labeled as POEn-LiTFSI. The resulting composite polymer electrolytes were dried at 60 °C under vacuum for 48 h to remove acetonitrile. The films were stored in a glovebox. Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) was performed using a TA Instrument DSC DSC2920 CE. Around 20 mg of sample was placed in a DSC cell in glovebox. Each sample was heated from -100 to 110 °C at a heating rate of 10 °C‚min-1. The glass transition temperature (Tg) was taken as the inflection point of the specific heat increment at the glass-rubber transition, while the melting temperature (Tm) was taken as the peak temperature of the melting endotherm. Dynamic Mechanical Analysis. Dynamic mechanical analysis (DMA) measurements were carried out with a spectrometer RSA2 from Rheometrics working in the tensile mode. The strain magnitude was fixed at 0.05%. This value ensures that tests were made in the linear viscoelastic domain. The samples were thin rectangular strips with dimensions about 20 × 7 × 0.2 mm3. Measurements were performed under isochronal conditions (1 Hz), and the temperature was varied between -100 and 150 °C by steps of 3 °C. Thermogravimetry. Thermogravimetric (TGA) tests were carried out using a Netzsch STA409 thermal analyzer. Samples were heated from room temperature to 500 °C at a heating rate of 10 °C min-1. All tests were carried out in a nitrogen atmosphere. Conductivity Measurement. Ionic conductivities were measured by impedance spectroscopy using a HP4192A impedance analyzer, over the frequency range 5-13 MHz. The samples were placed between two stainless steel blocking electrodes under vacuum. The temperature sweep test was conducted from 20 to 100 °C. The temperature was equilibrated for 1 h every 10 °C.
Nanocomposite Polymer Electrolytes
J. Phys. Chem. B, Vol. 108, No. 30, 2004 10847
Nuclear Magnetic Resonance. Nuclear magnetic resonance (NMR) experiments were performed by means of a homemade spectrometer at the Larmor frequency γ0 in the range 30-300 MHz and a superconducting coil to give a direct field of 6.6 T. Free induction decay (FID) [g(t)] of the proton nuclei (1H) was performed by the usual Hahn Spin-echo technique with a (π/2-τ-π) sequence. All FID were normalized to unity, i.e., g(0) ) 1. We have fitted the FID with two exponentials only to guide the eyes. The temperature dependence of the fluorine and lithium diffusion coefficients (19D and 7D) were made through the pulsed magnetic field gradient (PMFG) technique using a sequence described by Stejskal and Tanner.34 Let τ be the time interval between the initial π/2 pulse and the π pulse that is equal to the time between the π pulse and echo signal. Denoting by A(2τ) and A*(2τ) the magnitude of the echo in the absence and in the presence of the gradient pulses, respectively, it follows, for a nucleus with a gyromagnetic factor γ,
ln
( )
( )
A*(2τ) δ ) -γ2Dg2δ ∆ 3 A(2τ)
(1)
where g is the magnitude of the two gradient pulses, ∆ the time interval between these pulses, and δ their duration. The magnitude of the pulsed gradient magnetic field was varied between 0 and 900 G‚cm-1. ∆ and δ were about 40 × 10-3 s and 5 × 10-3 s, respectively. Under these conditions, the attenuation of a spin-echo magnitude was observed over a range of about 30, providing a good accuracy (5%) for the values of D. Cyclic Voltammetry. The investigation of the electrochemical stability range of tunicin whiskers was performed in an argon drybox with a three-electrode configuration using glassy carbon or platinum as working electrode, with Ag/AgNO3 as reference electrode. The reference potential of the Ag/Ag+ electrode, determined using a ferrocene/ferrocenium redox couple in propylene carbonate PC/LiClO4 solution, is 3.21 V vs Li/Li+. The study was carried out on a film of whiskers deposited on the rotating disk. The film was dried for 48 h under vacuum at 130 °C. This approach allows obtaining a high concentration of whiskers on the surface of the electrode, which should emphasize the possible reactivity of the whiskers. A more precise determination of the stability in reduction was obtained by dissolving tetrabutylammonium perchlorate in PC instead of lithium salt, which avoided lithium plating. Salt was dried under vacuum at 100 °C for 48 h. “Battery grade” solvents PC (
