Hot Pressing of Electrospun PVdF-CTFE Membranes as Separators for Lithium Batteries: a Delicate Balance Between Mechanical Properties and Retention

A systematic investigation of the effect of hot-pressing on polyvinylidene fluoride (PVdF) chlorotrifluoroethylene (CTFE) electrospun membranes, intended as separators and substrate for the electrolyte of lithium batteries, has been carried out by means of dynamic mechanical analysis. Absorption and desorption gravimetric measurements and infrared spectroscopy have been also performed. In particular, the mechanical properties of the membranes have been measured as a function of hot pressing temperature and pressure in the range 20 120 °C and 0 4 kbar, finding an increase of about 30 times of the tensile modulus after pressing at the highest temperature and pressure. Moreover, the conductivity of hot pressed membranes swelled by an ionic liquid measured as a function of temperature between 60 and + 70 °C practically superimposes on that of the unpressed membrane.


Introduction
Lithium ion batteries are the 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 1 . Despite their large availability at commercial level, many aspects of lithium batteries could be implemented, for example with the use of electrodes operating at higher voltage or avoiding the presence of liquid and flammable electrolytes.
One of the improvable parts of Li-ion cells is the microporous polyolefin separator which is extremely 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 [2][3][4][5][6][7][8][9][10][11] . Among the few polymers that meet the requirements for obtaining of a good polymer electrolytes, poly (vinylidene fluoride) (PVdF) and its copolymers, have been found to be very promising as they have good electrochemical stability and affinity to electrolyte solutions. However, as prepared electrospun membranes are very soft and difficult to handle because of their extremely low thickness (in the range of 20 µm) and low mechanical modulus. In order to avoid such problems, hot pressing of the polymer electrospun membranes has been proposed, but only some sporadic studies are available. He et al. 12 studied poly (vinylidene fluoride-trifluoroethylene) electrospun membranes. These authors reported the formation of the β phase of PVdF during the electrospinning process and an increase of the storage modulus by a factor 2 -3 after hot pressing. However, the details of the hot pressing procedure are not given. A broader investigation was reported by Na et al. 13 on PVdF electrospun membranes, subject to a hot rolling process. Na et al. observed a retention of ~ 400% of the electrospun membrane, that reduces to a value of 100 % when rolling is performed at 100°C and becomes a vanishing low value for higher rolling temperatures 13 . Conversely, the tensile modulus increases as the rolling temperature is increased. These authors were mainly concerned about the mechanical properties of the membranes, because they were intended as filtration systems. The same authors extended the investigation to some blends of PVdF with other polymers, such as polycarbonate and poly(methyl acrilate), confirming the better mechanical properties of hot rolled membranes 14 .
Choi et al. 8 reported an investigation of the physical properties of PVdF fibers before and after a thermal treatment at 150 °C. The tensile strength and elongation at break as well as the tensile modulus were notably improved by the thermal treatment.
Lee et al. 5 sandwiched two poly(vinylidenefluoridehexafluoropropylene) and PVdFelectrospun membranes between the anode and cathode of a lithium battery and performed hot pressing at 70 °C (at an unknown pressure). Then, dried cells were immersed in 1M LiPF 6 in an ethylene carbonate/dimethyl carbonate/diethyl carbonate/1:1:1, w/w/w) solution for 12 h. The ionic conductivity of the cell after hot pressing was the same of the cell before hot pressing (~ 6·10 -2 Scm -1 ) when measured at room temperature and above, while it was slightly lower for T< 290 K 14 The aim of the present paper is to perform a systematic investigation of the effect of hot-pressing mainly on the mechanical properties of electrospun membranes, intended as separators and substrate for the electrolyte of lithium batteries.
To the best of our knowledge such a detailed investigation as a function of hot pressing temperature and pressure has never been reported.
The electrospun membranes were prepared following a well established procedure by means of a homemade apparatus described in Ref. 7 using a solution prepared at room temperature by dissolving PVdF-CTFE in 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.
Pressing was performed in a rectangular shaped die with dimensions of 6 x 40 mm 2 . The die can be heated by an electric resistance. Each membrane was pressed for 10 minutes at selected temperatures (20, 50, 80 or 120 °C) and pressures (0, 0.4 or 4 kbar).
Infrared spectroscopy measurements were performed by means of an Agilent Cary 660 spectrometer, equipped with a ceramic source, a KBr beamsplitter and a MCT detector. The spectral resolution was 0.5 cm -1 . 15 Absorption and desorption of LP30 by the membranes was measured by means of a Mettler Toledo AT261 balance placed in a glove bag with a flowing argon atmosphere in order to prevent contamination of the liquid by water. The absorption was obtained by dropping LP30 on the membrane and removing all the excess liquid by means of optical paper.
The mechanical properties of the membranes were measured by means a Perkin Elmer DMA 8000 in order to perform dynamic mechanical analysis on small membrane pieces 4-6 mm wide and 10-12 mm long in the so-called "tension configuration" [16][17][18][19] . The storage modulus, M, and the elastic energy dissipation, tan δ, were measured at 10 Hz, between -100 and 100 °C with a temperature rate of 4° C/min.
As a preliminary check of the hot-pressing effect on their electrochemical properties, the membranes have been swelled by 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (PYR 14 -TFSI), which is one of the most promising ionic liquid studied as electrolytes in lithium batteries 20 . The conductivity of the samples has been determined by acquiring impedance spectra at each temperature with a sinusoidal voltage of 10 mV rms in the frequency range 200 kHz -1 Hz The samples were positioned in a sample holder between stainless steel disk electrodes 100 micrometers apart and having a diameter of 0.8 cm. The resistance of the sample was determined by extrapolating the high frequency tail of the spectra toward the real axis. The temperature of the samples has been controlled by a Tenney (USA) Environmental Chamber Mod. TJR in the temperature range +70 / -60 °C.

Infrared spectroscopy
The infrared spectrum of PVdF-CTFE is dominated from the absorptions due to PVdF 21 and indeed all the membranes of PVdF-CTFE 10 % pressed at different temperatures and pressures display clear absorptions around 490,510, 532, 795, 840, 880, 907, 975, 1026, 1068, 1190 and 1278 cm -1 (see Figure 1). The lines centered around 510, 840 and 1275 cm -1 are the fingerprints of the crystalline β phase of PVdF, while the absorptions around 490, 532 and 975 cm -1 are the signature of the presence of the α phase 22 .The other lines are common to both structures of PVdF 22 .Therefore, one can conclude that in all membranes both crystalline phases of PVdF are present. Moreover it seems that the relative concentration of the α and β phase does not drastically change as a function of heating temperature or pressure, as the relative intensity of the lines is only slightly affected by the hot pressing procedure.  δ, of the PVdF-CTFE 10 % electrospun membrane before any hot pressing procedure (red points). For comparison, also the anelastic spectra of a PVdF-CTFE 20 % membrane (olive points) and of a pure PVdF membrane (blue points) are reported. The glass transitions of PVdF and CTFE polymers, are clearly detected from the peaks in the tan δ data, around -47 and 6 °C, respectively.

Mechanical properties
For the PVdF-CTFE 10 % membrane (red points), at T= 25°C, M is ~ 9·10 6 Pa andthe modulus displays a typical temperature dependence with a decrease as temperature increases 23 . In particular, M reaches a value of 1.6·10 8 Pa at -100 °C and 1.1 10 6 Pa at 100 °C.
In order to study the modulus variation as a function of the pressure, p hp , and the temperature, T hp , of hot pressing, we performed DMA measurements on all the PVdF-CTFE 10 % membranes, obtaining curves of M with the same qualitative dependence on T of to those reported in Figure 2. However, the absolute value of the modulus strongly depends on the hot pressing conditions (see Figure 3). Indeed, even after pressing at T hp = 20°C and p hp = 400 bar, M increases by an order of magnitude. The elastic modulus further increases when the temperature and pressure of hot pressing are increased, reaching a value ~ 30 times the value of the as prepared membrane for T hp = 120°C and p hp = 4 kbar. Figure  3 reports the values of the modulus measured at selected temperatures for membranes subjected to hot pressing at different temperature and pressure conditions, in particular it shows that the higher the temperature and pressure of hot pressing, the higher modulus is obtained for the electrospun membranes. Figure 4 reports the retention of all PVdF-CTFE 10 % membranes. Before hot pressing the PVdF-CTFE membrane has a retention of ~ 410 %. Heating the polymer without pressing does not alter retention, while pressing without heating slightly decreases retention to ~ 370 %. However the combination of pressure and temperature further decreases the retention reaching a value of ~ 300 % for T hp = 80 °C and p hp = 400 bar and ~ 250 % for T hp = 120°C and p hp = 4 kbar. It must be noted that in an absolute sense the measured values of the retention are extremely high. Indeed membranes obtained by casting polymers 24 can have retention of the order of 400 %, but must be sealed in a closed environment to avoid release of the liquid.

Retention and release of LP30
In the case of the electrospun membrane the release of LP30 occurs in very long times. Figure 5 displays the mass variation of wet samples as a function of time. The pristine membrane has a mass variation of only 19 % after 24 hours of exposure to flowing argon, and even after 72 hours Δm/m is 57 %, so that there is still some LP30 left in the sample. In the case of hot pressed membranes the mass variation is slightly faster, but however the membrane treated at T hp = 80 °C and p hp = 400 bar displays a mass variation of 32 % after 24 h and of 75 % after 72 h.

Conductivity
In Figure 6 the comparison of the temperature dependence of the conductivity of a PVdF membrane swelled by 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR 14 -TFSI), which is one of the most promising ionic liquid studied as electrolytes in lithium batteries, and of a hot pressed membrane (T = 50 °C, p = 4 kbar) of the same polymer is reported. It is noted that the two lines, practically, superimpose one on the other, showing that the hot pressing procedure does not dramatically decrease the conductivity of the system.

Conclusions
The effect of hot-pressing on the tensile modulus measured at various temperatures of polyvinylidene fluoride (PVdF)-10% chlorotrifluoroethylene (CTFE) electrospun membranes, has been investigated by means of Dynamic Mechanical Analysis, as a function of the pressure, p hp , and temperature, T hp , used during hot pressing. An increase of the tensile modulus with increasing p hp , and T hp is obtained, reaching a value of ~ 30 times after pressing at the highest temperature (120 °C) and pressure (4 kbar). Concomitantly the retention of the LP30 electrolyte decreases from ~ 410 % at ambient p and T, towards ~ 250 % for T hp = 120°C and p hp = 4 kbar, a value which is still compatible with the application in lithium batteries. Moreover, even when exposed to a argon atmosphere, the release of LP30 occurs in very long times (of the order of 30-70 hours). The conductivity of the system polymer membrane/ionic liquid is only slightly affected by the hot pressing procedure.