Low-Temperature Phase Transitions of 1-Butyl-1 ... - ACS Publications

F. M. Vitucci , O. Palumbo , F. Trequattrini , J.-B. Brubach , P. Roy , I. Meschini , F. Croce , A. Paolone. The Journal of Chemical Physics 2015 143 ...
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Low-Temperature Phase Transitions of 1‑Butyl-1methylpyrrolidinium Bis(trifluoromethanesulfonyl)imide Swelling a Polyvinylidenefluoride Electrospun Membrane F. M. Vitucci,†,‡ D. Manzo,§ M. A. Navarra,†,∥ O. Palumbo,†,‡ F. Trequattrini,§ S. Panero,†,∥ P. Bruni,⊥ F. Croce,†,⊥ and A. Paolone*,†,‡ †

Research Center Hydro-Eco, Sapienza University of Rome, Via A. Scarpa 14, 00161 Roma, Italy CNR-ISC, U.O.S. La Sapienza, Piazzale A. Moro 5, 00185 Roma, Italy § Physics Department, Sapienza University of Rome, Piazzale A. Moro 5, 00185 Roma, Italy ∥ Chemistry Department, Sapienza University of Rome, Piazzale A. Moro 5, 00185 Roma, Italy ⊥ Scienze del Farmaco Department, University ‘‘G. d’Annunzio’’, Via dei Vestini 31, 66100 Chieti, Italy ‡

ABSTRACT: We studied the temperature behavior of 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI) swelling a polyvinylidenefluoride (PVdF) electrospun membrane by means of differential scanning calorimetry and dynamic mechanical analysis. The pure ionic liquid undergoes a glass transition around −85 °C independently of the cooling rate (4 or 0.5 °C/min). However, when PYR14TFSI swells the polymer membrane crystallization can be induced, either by cooling with a slow temperature rate (0.5 °C/min) around −52 °C or by prolonged isothermal treatments at −40 °C. The kinetics of the crystallization can be described by the usual Johnson−Mehl−Avrami−Kolmogorov equation, for both the isothermal and nonisothermal process. An Avrami index n = 3.78 ± 0.09 and an activation energy for the crystallization on cooling of the ionic liquid (IL) in the PVdF membrane of 16.0 ± 0.4 kJ/mol (37.9 ± 0.1 J/g) are obtained from our experimental data.

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INTRODUCTION Ionic liquids (ILs) are presently considered as new materials with great potentialities in a wide range of applications,1−4 such as green solvents for catalysis, synthesis, and separation or as heat carriers in solar-thermal energy generators. Among all applications, the use of ILs in electrochemical devices, such as Li-ion batteries, is one of the most challenging.5,6 Indeed, lithium-ion batteries are state-of-the-art devices for the storage of energy, as they possess an energy density (210 Wh kg−1, 650 Wh l−1) which exceeds at least by a factor of 2.5 any rivalling technology.7 Ionic liquids show large values of ion conductivity (10−4−10−2 S cm−1). At ambient temperature many ILs are in the liquid state; however, they can crystallize at lower temperatures, and they usually display plastic solid phases which originate in the rotational disorder present in the crystals. Indeed, the ions which compose the ILs are not spherical and can be arranged in various ordered or disordered geometries in the crystal structures.5 Some diffraction studies even identified the low-temperature crystal structure of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide,8 N-ethyl-Nmethylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR12TFSI),91,3-dimethylimidazolium, 1,2,3-triethylimidazolium bis(trifluoromethanesulfonyl)imide,10 and 1,1,3,3-tetramethylguanidinium bis(trifluoromethylsulfonyl)amide 11 and © 2014 American Chemical Society

showed that on heating phase transitions to rotationally disordered phases occur. The low-temperature phase transitions were also studied by means of differential scanning calorimetry (DSC), as in the case of the ionic liquid considered in the present study (1-butyl-1methylpyrrolidinium bis(trifluoromethanesulfonyl)imide) (PYR14TFSI).12−14 The DSC measurements in the PYR14TFSI reported in refs 12 and 14 showed that the traces measured on heating starting around −100 °C strongly depend on the thermal history of the samples. A glass transition around −80 °C was reported,14 and various peaks in the DSC heat exchanges were observed between −80 °C and the melting point (−13 °C). The phase diagrams of the mixtures of PYR1RTFSI where R = 2 −5 and Li bis(trifluoromethanesulfonyl)imide (LiTFSI) are even more complex and display many metastable phases,13,14 whose occurrence is strongly dependent on the cooling/heating rate and on the procedure adopted during the DSC experiments. Moreover, the ionic conductivity and the thermal phase stability of PYR1RTFSI with LiTFSI were examined, in view of the application of ionic liquids as electrolytes in lithium batteries.14 Received: January 20, 2014 Revised: February 25, 2014 Published: February 26, 2014 5749

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One of the improvable parts of Li-ion cells is the microporous polyolefin separator which is expensive and has poor wetting capability due to its small pore size and porosity. To circumvent these drawbacks, electrospun polymer membranes have been proposed as innovative separators.15−20 PVdF electrospun membranes gelled with a LP30 solution (1 M LiPF6 − EC:DMC) display a good conductivity even below room temperature. However, at present, much research is devoted to avoid the use of flammable solutions and of the toxic salt LiPF6. One valid alternative to alkyl carbonates is the ionic liquids, which display a good conductivity even below room temperature. In this paper we investigated the thermal properties of a representative PVdF electrospun separator, gelled with PYR14TFSI in the temperature range between −100 and 30 °C by means of differential scanning calorimetry (DSC) and dynamic mechanic analysis (DMA), with a special emphasis on the occurrence of phase transitions, vitrification, crystallization, cold crystallization, and changes between rotationally ordered and disordered crystalline phases of the pure ionic liquid and of the IL swelling the polymer membrane. Both thermodynamic and mechanical properties are important for applications, and therefore we investigated the swelled membrane by means of both experimental techniques. In particular, the aim of the present study is to investigate the influence of the presence of the polymer membrane on the crystallization process of the ionic liquid. This issue is important both from a fundamental and from an applicative point of view, as we are studying a composite system, which is also a prototype for the separator of a lithium cell. In the present study, however, we did not add also the Li salts, such as LiTFSI, to study a simpler system.

were performed after cooling with a temperature rate of 0.5 °C/min.

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RESULTS In Figure 1 we report the DSC traces measured on the pure PVdF membrane, on the pure ionic liquid, and on the

Figure 1. DSC measurements of the PVdF electrospun membrane, of pure PYR14TFSI, and of the swollen membrane, measured on cooling and on heating with the two temperature rates (slow = 0.5 °C/min, fast = 4 °C/min). The insets display the temperature regions of the glass transition.

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membrane swelled by the ionic liquid, with the two temperature rates of 4 °C/min (fast rate) or of 0.5 °C/min (slow rate). Both on cooling and on heating, the PVdF membranes display a smooth background, and even the glass transition of the polymer, which is expected around −40 °C, is hardly detectable at both rates, likely due to the low values of dT/dt used in the present experiments. For the pure ionic liquid, when the cooling process is conducted with the faster rate, an exothermic peak can be observed around −87 °C. On heating at 4 °C/min, the pure PYR14TFSI shows an endothermic peak around −87 °C (peak A), two exothermic peaks around −63 and −30 °C (peak B and C, respectively), and an intense endothermic peak around −7 °C (peak D). The DSC traces measured at 4 °C/min are quite similar to those previously reported in refs 12 and 14. According to the literature, process D corresponds to the melting of the ionic liquid,14 while peak A was assigned to the glass transition of IL.14 Finally peaks B and C should be due to rotational order−disorder transition in the crystalline phase, i.e., to the phase transition between crystalline phases with different rotational disorder.12,14 When the measurements on the IL are conducted with the lower temperature rate, one observes only a smooth background on cooling, probably beacuse the glass transition signal is too weak to be detected at such a slow rate. On heating the ionic liquid with the slow temperature rate, the endothermic peak (A) around −89 °C is quite weak; the two exothermic peaks (B and C) are displaced toward lower temperatures; while the temperature position of the intense endothermic peak (D) is almost unchanged. The enthalphy changes calculated for the four peaks of the pure PYR14TFSI sample measured with a

EXPERIMENTAL SECTION Ultrahigh molecular weight (UHMW 5130) polyvinylidenefluoride (PVdF) polymer was provided by Solvay Fluoropolymers. Acetone and 1-butyl-bis(trifluoromethanesulfonyl)imide 1-methylpyrrolidinium (PYR14TFSI) were purchased from Sigma-Aldrich Co. and Solvionic, respectively, and were used without further purification. The electrospun membranes were prepared following a wellestablished procedure by means of a homemade apparatus described in ref 20 using a solution prepared at room temperature by dissolving UHMW PVdF in an acetone: N,Ndimethylacetamide (DMAc) 70:30 v/v at a concentration of 13% w/v. The typical thickness of the electrospun membranes is in the range 20−50 μm. Differential scanning calorimetry measurements were performed by a Mettler-Toledo DSC 821, under an inert nitrogen flux, cooling from room temperature down to −100 °C, and then heating back to 20 °C with a temperature rate of 4 (fast rate) or 0.5 °C/min (slow rate). Dynamic mechanical analysis was carried out on small membrane pieces 4−6 mm wide and 10−12 mm long by using a Perkin-Elmer DMA 8000 in the socalled “tension configuration”. The storage modulus, M, and the elastic energy dissipation, tan δ, were measured with the same temperature scan used for DSC measurements in an inert argon atmosphere. Swelling of the PVdF membrane by the ionic liquid is obtained by dropping the IL on the electrospun PVdF and removing the excess of liquid with optic paper. Typically, the w/w ratio between IL and PVdF ranges from 8:1 to 10:1. On the swelled membranes, some isothermal DMA measurements 5750

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temperature rate of 0.5 °C/min, after the background subtraction, are 0.48, 31.2, 13.1, and 55.8 J/g, respectively, for peaks A, B, C, and D. The DSC trace of the membrane swollen in the IL measured with the faster rate is quite similar to the traces of the pure ionic liquid, except that peak C is displaced from −30 °C to −36 °C. The main differences can be observed when measurements are performed at a temperature rate of 0.5 °C/min: indeed, on cooling an exothermic peak centered around −60 °C (peak E) is clearly visible. On the subsequent heating of the swollen membrane, peak A, due to the IL glass transition, is absent, while peaks B and D are practically unchanged in position with respect to the faster temperature rate. Peak C is displaced toward lower temperatures by about 5 °C, as in the case of the faster rate. The enthalpy of the process corresponding to peak E is 13.2 J/g. It is notable that peak E is present only in the swollen membrane when it is cooled with the slow temperature rate. To further investigate the physical process giving rise to peak E, we performed also DMA measurements, on the pure electrospun PVdF membrane and on the membrane swollen by PYR14TFSI. Indeed, the elastic properties of samples are strongly dependent on the microscopic arrangement of atoms and can even detect the occurrence of phase transitions which are not easily evidenced by DSCs, such as second-order phase transition.21,22 The DMA data of the pure unswollen membrane, acquired with a temperature rate of 4 °C/min, are reported in Figure 2

Figure 3 reports the temperature dependence of the elastic modulus, M, and of the elastic energy dissipation, tan δ, for the swollen membranes, measured with the slow (0.5 °C/min) and the fast (4 °C/min) temperature rate. At room temperature, the modulus is around 107 Pa and tends to increase on cooling. With the slow rate, the modulus displays a decrease around −45 °C and an abrupt increase by 2 orders of magnitude around −55 °C, with a concomitant peak in tan δ. Those features occur in the same temperature range of peak E in the DSC measurements. The modulus increases on further cooling, reaching a value of 1010 Pa at −100 °C. On the subsequent heating at 0.5 °C/min, the modulus maintains high values up to −40 °C, smoothly decreases up to −10 °C, and then has a sudden decrease around −8 °C, when the IL is known to become a liquid. One can note small slope deflections of the modulus around −70 and −40 °C. When measurements are conducted with the fast rate (4 °C/ min), the phenomenology is more complex: on cooling the modulus smoothly increases down to −70 °C, and then it shows a sharp increase by 2 orders of magnitude between −70 and −90 °C. On subsequent heating, the modulus reproduces the sharp variation between −90 and −70 °C, but on further temperature increase, M increases by a factor ∼100, at the same temperature where peak B is present in DSC measurements. Around −40 °C, in correspondence with peak C of calorimetric measurements, a further small increase of the modulus can be observed. Finally, a strong decrease of M is shown between −10 and −0 °C, in the same temperature range of peak D in the DSC measurements. Both on fast and on slow heating one can observe broad peaks around −40 °C in tan δ and concomitant small variations of M, which could be possibly attributed to occurrence of the glass transition of PVdF.

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DISCUSSION We suggest that the phenomenology of the DSC and DMA measurements described in the previous section can be interpreted straightforward considering that, upon cooling PYR14TFSI, one can obtain at low temperatures a glassy or a crystalline phase depending on the thermal history of the sample and the environment in which cooling occurs. For the bulk IL, DSC measurements indicate that both for the slow (0.5 °C/min) and the high (4 °C/min) temperature rate a glass transition can be clearly observed on heating around −90/−85 °C (peak A). Therefore, below those temperatures a glassy state has been reached. On heating, in the temperature range between peaks A and B the pure IL is a supercooled liquid, as DSC measurements indicate that no crystallization takes place. However, on further heating the pure PYR14TFSI, one observes the exothermic peak B, which can be ascribed to the cold crystallization of the IL (14 kJ/mol). Probably the rearrangement of heavy ions into an ordered structure needs an amount of energy given by temperature, which is not available below −60 °C. Only above −60 °C the energy provided by the higher T allows the small structural changes which transform the liquid into an ordered crystalline state (peak B). On further heating the crystal structure is eventually modified, and correspondingly one observes peak C in the DSC measurements. To the best of our knowledge, diffraction data as a function of temperature are not available for the ionic liquid considered here; however, a diffraction study at low temperatures was performed on a similar compound (N-ethyl-N-methylpyrrolidinium bis[trifluoromethanesulfonyl]imide, PYR12TFSI).9 According to

Figure 2. Storage modulus and elastic energy dissipation of the pure electrospun PVdF membrane measured on cooling and on heating at 4 °C/min.

both on cooling and on the subsequent heating. The elastic modulus increases as temperature decreases, as expected in polymers. In the temperature range between −50 and −30 °C a sudden small change in the modulus and a peak in tan δ are clearly visible. Those features are due to the glass transition of the PVdF, which is known to occur around −40 °C. Even at such a low temperature rate, DMA experiments can easily detect the occurrence of glass transitions. It is worth noting that the temperature dependence of the modulus and of tan δ of our electrospun membrane strongly resembles that of bulk PVdF reported in ref 23. 5751

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Figure 3. Temperature dependence of the storage modulus and tan δ of the swollen membrane measured on cooling (red symbols) and on heating (blue symbols) with the slow (0.5 °C/min) and fast (4 °C/min) temperature rate. For comparison also the DSC traces measured in the same conditions are reported.

Henderson et al., below −85 °C PYR12TFSI adopts a monoclinic structure; the crystallographic cell contains two PYR12 cations and two TFSI anions; and all ions are ordered. However, between −80 and 0 °C, PYR12TFSI displays a triclinic structure; the unit cell contains one PYR12 cation and two half TFSI anions; and all ions are disordered. The crystal structure of higher temperatures was not investigated. Finally, peak D of the presently reported DSC data, detected around −8 °C, is due to the melting of PYR14TFSI. A similar behavior is observable also in the membrane swelled by the ionic liquid and measured with the hightemperature rate (4 °C/min). Indeed, the DSC traces of the swelled membrane are similar to those of the pure IL. Moreover, also the DMA measurements conducted with the fast temperature rate are easily interpreted in this framework: in fact on cooling no abrupt change of the modulus is observed before −65 °C, when an important change of the slope of M signals the transformation of the undercooled liquid to a glassy state. On heating, the decrease of the modulus between −90 and −65 °C indicates that the glass tends to transform again into the liquid, but around −65 °C the liquid crystallizes, as signaled by the increase of the modulus by 2 orders of magnitude. On the contrary, when the DMA measurements are conducted with the slow temperature rate, the large increase of the modulus observed on cooling around −60 °C and the lack of the minimum around −65 °C of the M values recorded on heating confirm that crystallization has already happened during the cooling process (peak E in DSC measurements). DMA measurements are extremely sensitive to the formation of a solid phase.24 A remark concerns the values of the variation of enthalpy associated to the melting of the pure IL. One can note that the value of ΔH for the melting process of pure PYR14TFSI obtained in the present work (∼56 J/g) is much higher than the value reported in ref 14 (18 J/g). This difference can be due to the different experimental procedure adopted: in fact,

Martinelli et al. quenched the sample at a relatively faster rate (i.e., 10 °C/min) to avoid crystallization and to reach a glass state at low temperature. However, it seems plausible that most of the sample used by Martinelli et al. remained in a supercooled state and did not participate in the melting process when heated back to room temperature. Coming back to the swelled membranes, in view of all the previous discussion, peak E can be ascribed to the crystallization on cooling of the IL which swells the PVdF membrane. This kind of transition is here reported for the first time and should be further studied. To gain further insight into such a transition, we investigated also the kinetics of the transition to verify whether the usual theory of crystallization kinetics is valid also for this system and to obtain the parameters of the transformation. Crystallization has been investigated in a large number of systems.25−30 In most cases crystallization involves a nucleation and a growth process, and in such cases the Johnson−Mehl−Avrami−Kolmogorov equation is applied.31,32,25,26 In that framework, for isothermal processes, the fraction of the sample transformed into the second phase (α) is supposed to have an exponential dependence on time, t α = 1 − exp( −kt n)

(1)

where n, the Avrami exponent, is a constant and k is a rate constant depending on temperature, usually with an Arrhenius type function, k = k0 exp(Ea/RT) (Ea is the activation energy of the crystallization process). For isothermal measurements, eq 1 implies that a plot of ln(−ln(1 − α)) vs ln(t) should be linear, and the slope of the line is the Avrami exponent, n. The Avrami exponent depends both on the dimensionality of the growing system (D), on the mechanism leading to growth (c = 1 for interface controlled and c = 1/2 for diffusion controlled), and on the nucleation index, a (a = 1 for no nucleation, 0 < a < 1 for decreasing nucleation rate, and a < 1 for increasing nucleation rate) 5752

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(2)

n = a + Dc 33

For some archetypal ionic liquids it was shown that crystallization is a thermal or mass diffusion controlled process. This fact implies that in formula 2 c = 1/2. The values of the Avrami exponent (n = 1.8 ± 2.2) for the bulk liquids indicate that growth occurs with a decreasing nucleation rate (0 < a < 1).33 Devitrification of 1-ethyl-3-methylimidazolium tetrafluoroborate (i.e., the transition from the glass state to the crystalline state) displays an Avrami exponent n ≈ 4.1.33 Moreover, Pas et al. proved that the Avrami exponent can be obtained for ionic liquids also from nonisothermal DSC measurements,33 as proposed by Henderson for different systems.34 Indeed, assuming that the dependence of k is of an Arrhenius type, differentiating eq 1 with respect to time and integrating with nonisothermal conditions one obtains ⎤n ⎡k T nE ln(− ln(1 − α)) = ln⎢ 0 π (x)⎥ − a ⎦ ⎣ β RT

Figure 5. Time dependence of the relative variation of the storage modulus of the membrane swollen by PYR14TFSI at various temperatures. The isotherms are measured after cooling from room temperature at a constant rate of 0.5 °C/min.

(3)

the transition to the crystalline state. At −30 °C, even after 40 000 s, the modulus stays constant at the low value, typical of the membrane swelled in the liquid. These observations suggest that the nucleation time decreases as temperature is decreased between −30 and −50 °C. The fraction of sample transformed into the second phase, α(T), was obtained by the modulus values measured for increasing time, M(T), as follows

where k0 is the frequency factor of the rate constant k; Ea is the activation energy; β is the temperature rate (in °C/s); and π(x) is an approximation of the temperature integral. If the logarithm in the right side of eq 3 is constant, a plot of ln(−ln(1 − α)) vs 1/T must be linear, and the slope gives nEa/ R.33,35 Figure 4 displays this kind of plot in nonisothermal

α (t ) =

M(t ) − M min M max − M min

where Mmin and Mmax are the modulus values at t = 0 s (membrane swelled by the undercooled liquid) and t → ∞ (membrane with the crystallized IL). In Figure 6 we report the

Figure 4. Avrami plot of the crystallization process of the IL swelling the electrospun membrane obtained from the nonisothermal DSC measurements of Figure 1 (peak E): blue symbols are the experimental data, while the red line is the best fit curve.

conditions based on the DSC measurements performed at the slow scan rate and reported in Figure 1. As usual, α(T) is calculated by the ratio between the integral of peak E between the onset temperature (about −52 °C) and temperature T, S(T), and the integral between the onset and the ending temperature of the peak, S0. In summary α = S(T)/S0. The data of Figure 5 can be fairly fitted by the Avrami model for nucleation and growth and the value of nEa/R = 7280 ± 9 K mol. To obtain a direct measure of the Avrami index, we performed isothermal measurements of the elastic modulus, M, of the IL swelling the PVdF membrane, following a previously reported procedure.36 Figure 5 displays the relative variation of M as a function of time in isothermal conditions at −50, −40, and −30 °C, after cooling from room temperature with a rate of 0.5 °C/min. At −50 and −40 °C, after an incubation time of about 1000 and 2200 s, respectively, one can observe an increase of the modulus by 2 orders of magnitude, which signals

Figure 6. Isothermal Avrami plot of the membrane swollen by PYR14TFSI obtained from the storage modulus measurements at −40 °C: symbols are the experimental data, and the line is the best fit curve.

dependence of ln(−ln(1 − α)) vs ln(t), and as expected from the Avrami theory, we obtained a linear dependence. The slope of the line, which is the Avrami index, is n = 3.78 ± 0.09. Taking into account the value of nEa/R obtained by the analysis of the nonisothermal DSC measurements, we find an activation energy for the crystallization of the IL in the PVdF membrane of 16.0 ± 0.4 kJ/mol (37.9 ± 0.1 J/g) All the previous discussion indicates that the IL swelling a PVdF membrane can crystallize according to the Avrami kinetic theory, as witnessed by the good fitting results of Figures 4 and 5753

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6. The main difference between pure PYR14TFSI and the IL swelling an electrospun PVdF membrane is the fact that, cooled in the same conditions (at 0.5 °C/min), the first one transforms into a glass, while the latter exhibits a crystalline state below −60 °C. The origin of such a big difference can be found in the interaction between the IL and the polymer, which acts as a nucleation site for the development of the crystal structure. Indeed, it is known for many systems25 that the impurities present in the liquid or the placement of a liquid in contact with the surface, even the walls of the container, can strongly affect the kinetics of a crystallization process. In fact even the maximum supercooling of classical liquids is affected by the substrate on which the experiments are conducted.25 For this reason a number of experimental techniques which allow us to minimize the concentration of active heterogeneous nucleation sites have been used: droplet emulsion, drop towers or tubes, levitation by different means.25 The present work suggests that the ionic liquid used for the present measurements is extremely pure, due to the high values of undercooling which were obtained in the absence of the polymer membrane. Moreover, it must be noted that the presence of the polymer membrane alters the physical properties of the ionic liquid, and this fact should be considered also for applications. Finally, the fact that the kinetics of the crystallization process can be fitted by a single crystallizing component (Figures 4 and 6) indicates that the system composed by the ionic liquid and the polymer membrane is extremely homogeneous, probably thanks to the microstructure of the electrospun membrane.

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CONCLUSIONS We reported a detailed investigation of the phase transitions of the pure ionic liquid PYR14TFSI and of the IL swelling an electrospun PVdF membrane in the temperature range between −100 and 20 °C. DSC measurements indicate that the pure ionic liquid transforms into a glass around −85 °C. On heating, the glass transforms into a supercooled liquid, and around −60 °C it crystallizes. This thermal behavior is displayed also by PYR14TFSI swelling an electrospun PVdF membrane, when the cooling process is conducted with a faster temperature rate (4 °C/min). However, we report for the first time that when the temperature rate is slower (0.5 °C/min) the IL crystallizes during the cooling process around −60 °C, due to the interaction with the electrospun polymer membrane. The kinetics of this last crystallization process can be described by the usual Johnson−Mehl−Avrami−Kolmogorov equation. We find an Avrami index n = 3.78 ± 0.09 and an activation energy for the crystallization of the IL in the PVdF membrane of 16.0 ± 0.4 kJ/mol (37.9 ± 0.1 J/g).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +39-064957697. Tel.: +39-06-49914400. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The results of this work have been obtained by the financial support of the European Community within the Seventh Framework Program APPLES (Advanced, High Performance, Polymer Lithium Batteries for Electrochemical Storage) Project (contract number 265644). 5754

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