Electrochimica Acta 46 (2001) 1635– 1639 www.elsevier.nl/locate/electacta
Poly(vinylidenefluoride)-based porous polymer electrolytes A. Magistris a, P. Mustarelli a,*, E. Quartarone a, P. Piaggio b, A. Bottino b a
Department of Physical Chemistry in the Uni6ersity of Pa6ia and INFM, Via Taramelli 16, 27100 Pa6ia, Italy b Department of Chemistry and Industrial Chemistry, Via Dodecaneso, Genoa, Italy Received 6 July 2000; received in revised form 15 October 2000
Abstract New multicomponent organic liquid electrolytes, based on mixtures of the commercial solution EC– DEC– LiPF6 (A) and tetraglyme/LiPF6 (G4), have been examined in terms of thermal and electrochemical behaviour. The composition with 40 vol% of G4 seems to be the best compromise among thermal stability, high conductivity and wide electrochemical window. Two porous asymmetric poly(vinylidenefluoride) membranes of porosity higher than 70%, but of different morphology (sponge-like versus finger-like), prepared by the phase inversion method, are compared in terms of physical–chemical properties, after activation with the pure solution A and the optimal mixture 60:40 v/v A:G4. Conductivities exceeding 2 mS/cm at room temperature are easily obtained, and the use of these membranes assures electrochemical stability at least up to 5 V. In spite of the cross-sectional asymmetry of these kinds of membrane and of the presence of a superficial skin, no relevant differences in the anodic limit voltages are observed on changing the orientation of the skin, as demonstrated by linear sweep voltammetry tests on both the surfaces. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Lithium batteries; Organic liquid electrolytes; Poly(vinylidene fluoride); Phase inversion; Gel electrolytes
1. Introduction The recent developments in the field of lithium batteries point towards the use of gel electrolytes (separators) based on poly(vinylidenefluoride) (PVdF) [1]. The concept of gel formation is related to the swelling of the polymer amorphous phase by a non-solvent (or a mixture of them). In the industry, however, polymer electrolytes are prepared by means of ‘dry’ approaches (e.g. the Bellcore process [2]). This allows one to minimize the problems related to the moisture-sensitive nature of the organic electrolyte solution by performing the activation in clean environments. These polymer hosts, on the other hand, can no longer be considered to be gels in the common sense of the word [3], but they are * Corresponding author. Tel.: + 39-0382-507776; fax: +390382-507575. E-mail address:
[email protected] (P. Mustarelli).
rather multiphase systems, where both pore structures filled by the solution and gel phases coexist. The ratio between the fraction of pores and gel phase is chiefly related to the polymer crystallinity and to the preparation method. We have recently shown [4] that free-standing, dry separators with porosity higher than 75% may be obtained by phase inversion by liquid immersion, which is a well-known method for preparing porous membranes with controlled properties [5] from various polymers. The use of this technique for obtaining PVdF membranes with different porosities and structures, these latter ranging from the so-called ‘finger-like’ structure to the ‘sponge-like’ one, is described elsewhere [6]. In Ref. [4] we discussed the activation processes of films characterized by different morphologies of pore structures (sponge-like versus finger-like), and were able to separate the swelling contribution from the pore-filling one. We also showed that the transport properties of
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the electrolyte solution are left substantially unaffected by the polymer, which is reasonable because the pore structure is in the majority and their typical dimensions are in the micrometre range. Because of the essentially inert nature of the host polymer, on the other hand, the thermal stability of the electrolyte solution becomes a relevant parameter of choice for the separator optimization. In this paper, first we will discuss the thermal and electrochemical stability of a new electrolyte solution obtained by adding tetraethylene glycol–dimethyl ether (TEGDME) to a commercial mixture EC–DEC–LiPF6. Then, we will report the conductivity and the electrochemical stability of polymer electrolytes prepared from spongelike and finger-like PVdF porous films.
901, were prepared by the phase inversion method starting from solutions of 20 wt% of homopolymer in N-methyl-2-pyrrolidone and triethylphosphate respectively. The solutions were first cast at room temperature (20°C) onto a glass plate to form films with 350 mm thickness; after 30 s at 20°C these were immersed in a water bath at 10°C to precipitate the polymer and form the membrane. The membranes were leached for 48 h under running water and then immersed in deionized water for the same period prior to being dried by simple exposure in air. The dried membranes, with a thickness of 250 mm, were finally activated by soaking for 18 h in a dry box in two different electrolyte solutions, pure A and 60:40 v/v A:G4.
2.3. Characterization 2. Experimental
2.1. Electrolyte solutions Several liquid electrolytes, A:G4, were prepared by mixing defined volumes of TEGDME–LiPF6 1.0 M (G4) to a solution of EC–DEC (1:1 w/w)–LiPF6 1.0 M (A), varying the volume composition of G4 from 0 to 100 vol.%. TEGDME and anhydrous LiPF6 were obtained from Aldrich (purity \99%) and Merck (Selectipur™) respectively. Solution A was also provided by Merck (Selectipur™), with a water content less than 30 ppm.
2.2. Membrane preparation Two asymmetric porous PVdF (Foraflon 1000HD, Atochem, Mv =4.5 ×105) membranes, PF-419 and PF-
The thermal properties of the electrolytes were studied using a modulated differential scanning calorimeter (MDSC 2910, TA Instruments™, USA). The measurements were carried out at 5°C/min with a modulation period of 40 s and a modulation amplitude of 0.5°C/ min. Atomic force microscopy (AFM) images were obtained with a Park system at room temperature on the non-activated membrane. The ionic conductivity was measured at room temperature by impedance spectroscopy, using a frequency response analyser (Solartron 1255) connected to an electrochemical interface (Solartron 1287), over the frequency range 10 Hz– 1 MHz at an AC voltage amplitude of 100 mV. A Solartron 1287 potentiostat was used in order to run linear sweep voltammetry experiments at room temperature on Ni as cathode at 200 mV/s scan rate. Lithium was used as an anode and reference electrode.
3. Results and discussion
3.1. Choice of electrolyte solution
Fig. 1. Behaviour of conductivity against the volume percent of G4 in A.
Fig. 1 shows the conductivity behaviour of the A:G4 solutions. A maximum at 9 mS/cm is obtained for the mixture 80:20 v/v A:G4. This non-linear behaviour is generally observed in EC-based non-aqueous solutions [7], although the maxima are often flatter than the one we observed [8]. We stress that the addition of 40 vol.% of G4 assures the same conductivity of pure A solution, which is considered to be enough for practical applications. Since thermal and electrochemical stability are at a premium against small changes in conductivity, however, our choice will be chiefly based on the results of thermal analysis and electrochemical measurements.
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Fig. 2. MDSC behaviour of the solutions A (a), 80:20 v/v A:G4 (b), and 60:40 v/v A:G4 (c). Solid line: total heat flow; dashed line: reversible component.
Fig. 2 shows the modulated differential scanning calorimetry (MDSC) behaviour of pure A (a) and of the solutions 80:20 v/v A:G4 (b) and 60:40 v/v A:G4 (c). Both the total heat flow and the reversible component are shown. Though a full study of the phase diagram of the mixture A:G4 is beyond the aim of this paper, and will be discussed elsewhere, we can note that the addition of G4 increases the overall thermal stability of the solution. Let us now focus attention on the
total curve, which contains non-reversible contributions to the heat flow. The exotherm starting at − 75°C in curve (a) is due to the crystallization of some of the solution. This feature is still present in curve (b), but almost completely disappears when 40 vol.% of G4 is added (c). The endotherms in the range − 20°C to +30°C in curve (a) are due to the melting of crystalline phases, whereas the large ones above 70°C are related to the solvent evaporation, as demonstrated by thermogravimetric analysis measurements, not shown here. Curve (c) demonstrates that the addition of G4 also stabilizes the mixture against solvent evaporation. Fig. 3 shows the electrochemical stability window of pure A and 60:40 v/v A:G4 at 20°C. Using Ni as an electrode, the addition of G4 causes an increase of the anodic limit from 3.5 to 4.6 V; this improvement is to be expected, considering the intrinsic higher resistance of the glymes towards electrochemical oxidation and reduction. Different volumes of G4 cause only minor changes (not shown here) to the limit voltage. However, a value of 4.6 V is enough to envisage the use of this solution with common intercalation anodes and cathodes. From the above-mentioned results, the 60:40 v/v A:G4 solution seems to be the ‘optimal’ liquid electrolyte for the activation of the PVdF membranes. This appears to be the best compromise in terms of conductivity, thermal and electrochemical stability. In the following, we will report on the characterization of the polymer electrolytes, activated by both pure A and the optimal solution.
3.2. The polymer electrolytes Two of the porous films described in Ref. [4] were used here to prepare polymer electrolytes. The films are characterized by different morphologies; in particular, PF-901 is sponge-like, whereas PF-419 is finger-like. The latter shows room temperature conductivity higher by a factor of 1.5 than the former, irrespective of the solution used to activate them. This result can be justified by a combination of higher uptake of electrolyte and a reduced tortuosity in the membrane structure. Fig. 4 shows the conductivity versus reciprocal temperature for the two porous films activated with pure A and the optimal 60:40 v/v A:G4 solution. In the temperature range explored ( − 70°C to + 30°C) all the curves can be described by a Vogel– Tamman– Fulcher behaviour: |(T)= A e − B/(T − T0)
Fig. 3. Electrochemical window at 20°C of the solution A (dashed line) and 60:40 v/v A:G4 (solid line) (Ni electrode, scan rate: 200 mV/s).
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The difference in conductivity between PF-901 and PF-419 appears to increase by lowering the temperature, which means that different pseudo-activation energies characterize the two samples. This can be
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tentatively explained by recalling that the activation of PVdF porous films with electrolyte solutions gives rise to multiphase systems made at least of (i) an amorphous swollen gel, (ii) crystalline strands, and (iii) cavities filled with the solution (liquid phase). Though the transport contributions from the gel phases are probably the same, the observed differences could be due to different morphologies in the pore structures. This point is presently under investigation. The PVdF porous films prepared by phase inversion are generally characterized by cross-sectional asymmetry, induced by the preparation method [6]. Fig. 5 shows AFM micrographs of the two surfaces of sample PF-901. Part (a) displays the so-called skin, i.e. the film side that is formed in contact with the glass plate during the precipitation step. The skin appears to be quite compact, and it is characterized by a pore distribution in the range 0.5–1 mm. Part (b) represents the upper surface, which displays pores larger by at least one order of magnitude. Because of this asymmetry, it is important to perform the limit voltage tests on both the surfaces.
Fig. 5. AFM images of the skin (a) and of the porous side (b) of the dried polymer PF-901.
Fig. 4. Conductivity versus reciprocal temperature of the polymer electrolytes, activated by the solution A (part 1) and the optimised one, 60:40 v/v A:G4 (part 2). Filled circles: PF-901; open circles: PF-419.
Figs. 6 and 7 show the stability windows for the samples PF-901 and PF-419, respectively, activated with both pure A and the optimal solution (see figure captions). Continuous and dashed line refer to the measurements performed with the skin in contact with Ni and Li, respectively. Generally speaking, the presence of this host polymer causes an increase of the limit voltage of 0.5– 0.8 V with respect to the solution itself, and this higher resistance is given by the resulting gel phase in the activated membranes. Finally, the different orientation of the skin in the cell causes differences of several hundred millivolts.
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Fig. 6. Electrochemical windows of polymer PF-901 activated by solution A (part 1) and the optimised one, 60:40 v/v A:G4 (part 2). Solid line: skin–Ni interface; dashed line: skin–Li interface.
Fig. 7. Electrochemical windows of polymer PF-419 activated by solution A (part 1) and the optimised one 60:40 v/v A:G4 (part 2). Solid line: skin– Ni interface; dashed line: skin– Li interface.
4. Conclusions
the case of the sponge-like membrane, where the void dimensions are smaller. In spite of the cross-sectional asymmetry of the polymer films, due to the different pore distributions and structure in the two surfaces, relevant differences in the electrochemical stability window are not observed when the orientation of the skin in the Li polymer electrolyte Ni cell is changed.
We investigated the influence of the addition of tetraglyme–LiPF6 solution (G4) on the thermal and electrochemical stability of a commercial EC–DEC–LiPF6 (A) in order to find an optimal liquid electrolyte for applications in PVdF-based lithium batteries. MDSC and impedance spectroscopy experiments demonstrate that 60:40 v/v A:G4 is an attractive composition, because it assures the same conductivity of pure A solution (7.22 mS/cm) at room temperature, with a gain in the electrochemical window of more than 1 V and a stabilization of the mixture against the solvents evaporation. This electrolyte is used for activating two asymmetric porous PVdF membranes of different morphology (sponge-like versus finger-like). Conductivities of 2 mS/cm are easily obtained at room temperature, but a higher tortuosity, induced by the different morphology of the films, causes a decrease in the conductivity values of the sponge-like membrane by a factor of 1.5 with respect to the finger-type membrane. The presence of the host polymer causes an increase of the anodic limit voltage up to 0.8 V and this increase is more evident in
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