Low temperature phase transitions in the PYR14TFSI ionic liquid swelling an
electrospun polymer membrane
F. M. Vitucci,1,2 O. Palumbo,1,2 F. Trequattrini,3 M. A. Navarra,2,4 S. Panero,2,4 P. Bruni,5 F.
Croce,2,5 P. Roy,6 J.-B. Brubach,6 A. Paolone1,2
1
CNR-ISC, U.O.S. La Sapienza, Piazzale A. Moro 5, 00185 Roma, Italy
Research Center Hydro-Eco, Sapienza University of Rome, Via A. Scarpa 14, 00161 Roma,
Italy
3
Physics Department, Sapienza University of Rome, Piazzale A. Moro 5, 00185 Roma, Italy
4
Chemistry Department, Sapienza University of Rome, Piazzale A. Moro 5, 00185 Roma,
Italy
5
Scienze del Farmaco Department, University ‘‘G. d’Annunzio’’, Via dei Vestini 31, 66100
Chieti, Italy
6
Synchrotron SOLEIL, F-91192 Gif Sur Yvette, France
2
Among all applications, the use of ionic liquids, ILs, in electrochemical devices, such as Liion batteries, is one of the most challenging. 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. In order to circumvent these drawbacks, electrospun polymer
membranes have been proposed as innovative separators. PVdF electrospun membranes
gelled with a a conventional electrolyte solution (e.g., 1M LiPF6 in ethylene
carbonate:dimethyl carbonate) display a good conductivity even below room temperature.[1]
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 are the ionic liquids, which
display a good conductivity even below room temperature.
At ambient temperature many ILs are in the liquid state, but 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 IL’s are not spherical and
can be arranged in various ordered or disordered geometries in the crystal structures.
We report a detailed investigation of the phase transitions of the PYR14TFSI ionic liquid,
carried out by means if differential scanning calorimetry (DSC), dynamical mechanical
analysis (DMA) and infrared vibrational spectroscopy. The behaviour of both pure ionic
liquid and of the IL swelling an electrospun PVdF membrane in the temperature range
between -100 and 20°C was investigated. 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).[2]
In order to further investigate the changes at molecular level induced by the crystallization
process and by the solid-solid phase transition, we measured the infrared vibrational spectrum
of PYR14TFSI as a function of temperature between -100 and 60°C. The molecular vibrations
of the individual ions were calculated ab-initio at the B3LYP/6-31G** level: we found good
agreement between the experimental and the computed frequencies. The infrared lines are
ascribable to molecular vibrations of the single ions, suggesting a weak interaction between
anions and cations. The anions give strong absorption lines between 150 and 800 cm-1 and
between 1050 and 1350 cm-1. The spectral lines of the cations are visible between 800 and
1050 cm-1, between 1350 and 1550 cm-1 and between 2700 and 3300 cm-1.
We will discuss the behavior of specific vibrational absorption as a function of T, in
correspondence with the phase transitions revealed by DSC and DMA measurements and we
will comment on the structural changes implied.
Acknowledgements
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).
References
[1] F. Croce, M. L. Focarete, J. Hassoun, I. Meschini, B. Scrosati, Energy & Environ. Sci. 4,
921 (2011)
[2] F. M. Vitucci, D. Manzo, M. A. Navarra, O. Palumbo, F. Trequattrini, S. Panero, P. Bruni,
F. Croce, A. Paolone, J. Phys. Chem. C 118, 5749 (2014)
[3] F. M. Vitucci, F. Trequattrini, O. Palumbo, J.-B. Brubach, P. Roy, A. Paolone, Vibrational
Spectroscopy, in press