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nanocrystalline cellulose porous membranes as separators for lithium-ion batteries
Electrochimica Acta 214 (2016) 38–48
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Nanocomposite poly(vynilidene fluoride)/nanocrystalline cellulose porous membranes as separators for lithium-ion batteries Marco Bollolia,b,* , Claire Antonellic, Yannick Molméreta,b , Fannie Alloina,b,d, Cristina Iojoiua,b,d, Jean-Yves Sancheza,b,c,* a
Univ. Grenoble Alpes, LEPMI, F-38000 Grenoble, France CNRS, LEPMI, F-38000 Grenoble, France Universidad Carlos III de Madrid, Materials Sciences Leganés, Spain d Réseau sur le Stockage Electrochimique de l'Energie (RS2E), CNRS, FR3459, 33 Rue Saint Leu, 80039 Amiens Cedex, France b c
A R T I C L E I N F O
Article history: Received 18 April 2016 Received in revised form 5 August 2016 Accepted 5 August 2016 Available online 6 August 2016 Keywords: PVdF nanocomposite phase inversion nanocrystalline cellulose NCC separator
A B S T R A C T
Nanocomposite materials were obtained from poly(vinylidene fluoride) (PVdF) as matrix polymer and a stable DMF suspension of nanocrystalline cellulose (NCC) as the reinforcing phase. Porous and dense nanocomposite membranes were prepared by non-solvent induced phase separation (NIPS) and film casting methods, respectively. The resulting films were characterized regarding their structuration, i.e., the content of crystalline phases, as well as their transport and thermo-mechanical properties. The presence of the fillers led to a mechanical reinforcement, associated with a lower strain at break. For dense nanocomposites, a thermal stabilization at temperatures higher than the melting temperature was highlighted and ascribed to the formation of a rigid cellulosic network within the matrix. The superior electrochemical performances together with the observed reinforcement effect render these porous nanocomposites membranes as interesting candidates for the replacement of commercial polyolefinbased microporous separators in lithium-ion batteries. ã 2016 Elsevier Ltd. All rights reserved.
1. INTRODUCTION Lithium-ion and lithium metal-polymer batteries are at present the most competitive power sources for high capacity energy storage because of their high energy density and power capability. In both technologies, the electrolyte does not only serve as ion conductore but moreover as separator to prevent the direct contact between the electrodes. One approach, largely predominant in the latter technology, consists in using a macromolecular solvent, generally poly(oxyethylene) POE based, in order to guarantee a sufficient dissociation and solvation of the ionic species and, at the same time, to ensure the mechanical separation between electrodes to prevent short-circuits. In this case, an improvement of the mechanical properties of the polymer film has been shown to be crucial. Different strategies were explored such as the use of (i) cross-linked polyether networks [1,2], (ii) block copolymers [3], or different fillers blended in the membranes [4,5]. Although these
* Corresponding authors at: Univ. Grenoble Alpes, LEPMI, F-38000 Grenoble, France. E-mail addresses: [email protected] (M. Bolloli), [email protected] (J.-Y. Sanchez). http://dx.doi.org/10.1016/j.electacta.2016.08.020 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
polyethers can be effectively swelled by liquid electrolytes and substantially reinforced, they are nevertheless unstable in the presence of current positive electrodes operating above 4 V vs Li+/Li. Separators based on polyolefins or fluorinated polymers, compatible with high-voltage cathodes, are therefore more widely used in lithium-ion batteries. These separators can be (i) dense polymers gelled by liquid electrolytes [6,7] or (ii) porous polymers where the pores are filled by liquid electrolytes [8–10]. The former are generally easy to process by extrusion, a non-costly and safe process. However, a suitable compromise between high electrolyte uptake, leading to high conductivities, and sufficient mechanical strength after soaking is not easy to reach in order to obtain thin films. The porous separators, which are the most common solution for the portable electronics devices at the moment, are based on a single polymer film or a multi-layered structure; their preparation processes, ensuring the porous structure, makes them much more costly but, at the same time, enables the decoupling of mechanical strength, realized by the polymeric inter-pore phase, and ionic conductivity, ensured by the liquid electrolyte filling the porous volume. Porous separators must therefore exhibit high open
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porosity and low tortuosity. In addition, a good wetting of the pores is essential to provide (i) high conductivities and (ii) good retention of the liquid electrolyte in the porous volume. The wetting ability depends on the interaction between the polymers and the liquid electrolyte. The most successful materials, such as the Celgard1 series, are either based on a single sheet of highly oriented polypropylene, or a tri-layered material, in which a porous polyethylene layer is sandwiched between 2 porous polypropylene layers, the latter solution increases the battery safety thanks to the temperature-induced shut-down effect. In case of these separators, the interaction is between the polar electrolyte and the nonpolar polymer is poor, and a significant drop in conductivity is observed [11,12]. Poly(vinylidene fluoride) (PVdF) homopolymer and its copolymers have been identified [14–16] as good candidates for macroporous separators as well, since they (i) provide a high thermal, mechanical and electrochemical stability, in particular towards oxidation, (ii) are often used as binder for both negative and positive electrodes, favoring a good electrolyte–electrode interface, and (iii) are polar polymers exhibiting fairly good affinity with the polar solvent of the electrolyte. Due to this affinity, the electrolyte does not only fill the porous volume of PVdF but also swells the polymeric phase of the separator. However, this swelling, which results in a decrease in the amount of crystallinity and of the melting point, is detrimental to the mechanical performance of the separator without enhancing the conductivity As this swelling increases with a decreasing crystallinity, the homopolymer, PVdF, is preferred compared to the copolymers, such as poly(vinylidene fluoride-co-hexafluoropropylene) (PVdFHFP). Multiple reinforcement routes have been suggested to improve the mechanical strength of the polymer; such as polymer crosslinking [17,18] or the combination with woven fabrics [19,20]. Another very promising strategy due to its low preparation cost and the large selection of materials relies on the formation of composites thanks to the addition of organic and inorganic fibers [21–23]. Among the most investigated fibrous materials, there are different types of cellulose nanofibers, which have seen an exponential growth in scientific communications and research efforts in the last 15 years [24,25], due to their renewable and nontoxic nature, the strong mechanical reinforcement which can be obtained, and their low cost. From mechanical disintegration of the wood pulp, microfibrillated cellulose (MFC) is obtained: it consists of both individual and aggregated nanofibrils made of alternating amorphous and crystalline domains, with an estimated width between 3–100 nm and a length exceeding 1 mm. A subsequent controlled acid hydrolysis allows the dissolution of amorphous domains, and yields high aspect ratio rod-like nanocrystals (nanocrystalline cellulose, NCC), or whiskers, having a diameter of 5 to 15 nm and length of 100 to 500 nm, depending on the cellulose source. The accepted mechanism of reinforcement of these materials is the formation of a 3-D percolating network due to the strong interactions (hydrogen bonds) between the nanofibers; if properly formed, the cellulosic network can also prevent the composite from flowing at temperatures exceeding the melting point of the polymeric matrix, thus increasing the operational range of the products [26]. Zhang et al. [27] reported the preparation of macroporous PVdF- regenerated cellulose composites by coagulating their N,Ndimethyl acetamide (DMAC)/LiCl solution with water, and that the composition affected the mechanical strength. Tang et al. [28] also prepared a similar composite, but by the use of PVdF grafted with maleic anhydride to address the compatibility issues which could potentially arise from mixing PVdF, which is highly hydrophobic, with polar cellulose. Finally, the group of R. Hashaikeh reported the
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use of macroporous PVdF-HFP membranes reinforced with NCC, produced by electrospinning, with improved separation properties for desalination purposes [29,30]. A mechanical reinforcement was reported, to some extent, by all the authors, but no percolating 3-D network was reportedly obtained in any of these works. With regard to electrochemical storage applications, so far not much attention has been devoted to the preparation of macroporous PVdF-cellulose composite membranes and their properties; however, the interest of this strategy has been already highlighted for dense PEO-based membranes and gelled acrylic matrix in both lithium polymer and lithium-ion applications, in which an important mechanical reinforcement has been observed with no significant impact on the electrochemical stability and performances [31–34]. We recently reported [35] the electrochemical performances of PVdF-NCC macroporous separators prepared by phase inversion in graphite/LiNi0.4Mn1.6O4 full cells. With respect to a commercial Celgard1 2400 separator, the nanocomposite membranes exhibited improved performances and cyclability, in particular at high C rates, thanks to their superior transport properties. Their superior performance has been further validated for a plug-in hybrid vehicle application under a DOE testing protocol [36]. In this contribution, we aim at broadening the knowledge of the physicochemical properties of these nanocomposites separators. Due to its industrial potential, phase inversion method was used to obtain the porous films. Their morphology, physicochemical and thermo-mechanical properties were investigated, and compared to a series of dense membranes, prepared by polymer casting, and then hotpressed. Finally, their ionic conductivity vs NCC loading was investigated with a standard lithium ion electrolyte. 2. EXPERIMENTAL 2.1. Materials PVdF homopolymer (Solef1 6020, 650,000 g mol1) was kindly provided by Solvay Solexis. Dry nanocrystalline cellulose was provided by FPInnovations. Dimethyl formamide (DMF, Carlo Erba, 99%) and ethanol (Sigma Aldrich, 95%) were used as received without further purification. The reference electrolyte for the electrochemical testing was prepared by dissolving lithium hexafluorophosphate (LiPF6) at a 1 M concentration in an equivolume mixture of dimethyl carbonate and ethylene carbonate. All the products were purchased from Sigma-Aldrich ( > 99%), and were dried before use. 2.2. Production of the nanocrystalline cellulose dispersion The dispersion of nanocrystalline cellulose in DMF was prepared by adding the NCC to the solvent (2 wt% was the standard concentration; it was raised up to 3 wt % to obtain the proper composition for the most loaded PVdF-NCC slurries), and then sonicating the whiskers/solvent mixture at room temperature for several cycles of 30 minutes with a VC 505 Vibra-CellTM using a 13 mm probe and storing the mixture in an ice bath for 10 min between each cycle to avoid excessive solvent evaporation and degradation. The quality of the dispersion was assessed by inspection of the dispersion between two crossed polarizers as widely accepted in literature: the presence of birefringence was used as the criterion for having obtained a good dispersion [37]. 2.3. Processing of the nanocomposite films PVdF was at first dissolved in DMF and the resulting solution was added to the NCC dispersion in order to obtain a concentration
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of cellulose whiskers in the final membranes of 6, 12, and 20 wt% and a PVdF concentration of 11.5 wt% with respect to the solvent. The resulting suspension was stirred for 8 h at 50 C and degassed under vacuum in order to remove the remaining air. In order to obtain dense films, the PVdF-NCC dispersion was cast into Petri dishes. The films were then left to evaporate at 60 C for 2 days and eventually dried under vacuum for 4 h until constant weight. The thickness varied between 15 and 40 mm, following the amount of dispersion cast onto the Petri dish. The samples were then pressed at 180 C in a laboratory hot press for 5 min in order to prepare dense membranes. To obtain macroporous membranes, the PVdF-NCC dispersion was spread over a glass support with a doctor blade, then
immerged into an ethanol bath for 10 min, as proposed by Djian [13]. The resulting films were then dried in the same way as described for the dense membranes. A thickness of about 20 mm was obtained. The processing strategies are resumed in Fig. 1. 2.4. Characterization methods Transmission electron micrographs (TEMs) of the cellulose whiskers were acquired using a JEOL 2100 F–200 kV instrument. Drops of the 10-fold diluted crystal suspension were deposited onto glow-discharged carbon-coated copper grids and the liquid in excess was blotted with filter paper. Uranyl acetate was used as
Fig. 1. Simplified scheme of the elaboration of the nanocomposite membranes.
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contrasting agent. Digital image analysis (ImageJ Software) was used in order to estimate the dimensions of the nanoparticles, and an average of 100 measures was retained. Observation of the final nanocomposite films was realized, after cryofracture in liquid nitrogen and coating with a thin graphite layer, by means of a scanning electron microscope (SEM) LEO Stereoscan 440, with an accelerating voltage of 15 or 20 kV. The porosity of the macroporous membranes was calculated using the following equation: porosity (%) = (1- (rm/rth) x 100
(1)
where rm is the membrane density and was determined in distilled water or ethanol by means of a density measurement kit (Mettler Toledo), and rth is the theoretical density of the membranes, calculated from the density of PVdF (1.78 g/cm3) and cellulose (1.50 g/cm3). The weight loss as a function of temperature was assessed by means of thermogravimetric analysis (TGA), using a Q500 device (TA Instruments). The experiments were performed in double, under air flux, between r.t. and 600 C, with a heating ramp of 10 C. min1. Determination of the crystallinity and melting point Tm was performed by differential scanning calorimetry (DSC), using a TA Instrument DSC 2920 CE (Gemini BV) and applying a temperature ramp of 2 C min1. Each sample (around 3 mg) was sealed in an aluminum pan inside an argon-filled Inert Lab glove box having an oxygen and water content below 20 ppm. The degree of crystallinity of the polymeric phase, xp, was calculated using the following equation:
xp (%) = (DHm/DH0m) x w x 100
(2) H0m being the enthalpy of
DHm being the enthalpy of melting and D fusion of the repeating unit of a PVdF perfect crystal of infinite size, estimated as 104.6 J g1 from literature data [38] and w being the weight fraction of polymeric material in the composite. Infrared (IR) analysis was carried out using a FT-IR Spectrum One instrument (Perkin Elmer) in the frequency range 650– 4000 cm1. Each measurement was repeated at least twice to ensure reproducibility. The fraction of a crystalline phase was determined through the equation proposed by Gregorio et al. [39]: Fa = (1-[Aa/(Aa + 1.3 x Ab)])
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electrochemical impedance spectroscopy (EIS) using an HP 4192A Impedance Analyser. Measurements were carried out in the frequency range 5 Hz 1 MHz with a SwagelokTM-type cell in two-electrode configuration with stainless steel blocking electrodes (0.785 cm2). An AC amplitude of 5 mV was used and data were collected taking 10 points per decade. The temperature was equilibrated for one hour prior to each measurement. The uncertainties of conductivity values are estimated to be 0.3 mS.cm1. 3. RESULTS AND DISCUSSION 3.1. Materials and shaping 3.1.1. Nanocrystalline cellulose and its dispersion in DMF MFC and NCC are readily dispersed in water up to a 6–8 wt % concentration. However, it’s much harder to disperse them in an organic solvent compared to water due to the less favorable surface interactions between the solvent and the cellulose surface groups. Van den Berg et al. [40] demonstrated that a proper dispersion of tunicate whiskers at low concentration ( 0.1 wt%) could be achieved in different organic solvents; nevertheless, for industrial applications a more concentrated dispersion should be readily available. From the works of Azizi Samir [5], dimethylformamide is known to be a suitable solvent to obtain a 2 wt% NCC dispersion: this prompted us to select this solvent for our study. From the TEM image (Fig. 2) the whiskers appear homogeneously dispersed in the solvent, without forming agglomerates. A key information is their aspect ratio (the ratio between length and diameter), which in turn is crucial to calculate the percolation threshold vRc of the whiskers. This parameter is defined as the critical amount beyond which the fillers loading vR form a 3-D network within the polymer matrix, and is linked to the aspect ratio by the following equation: vRc = 0.7/(L/d) where L/d is the aspect ratio [5].
(3)
where Aa and Ab are the absorption fractions of the a and b phase, respectively. The absorbance around 767 cm1 (CF2 bending and skeletal bending) was used as Aa and the maximum absorbance around 840 cm1 (CH2 rocking) as Ab. The contact angles of deionized water on some selected macroporous separators were evaluated using an OCA 15plus contact angle system (Neurtek Instruments, Spain). The values were obtained by averaging values from more than ten drops. The tensile testing of film samples was carried out at room temperature and relative humidity at a cross head speed of 0.05 mm min1 with a RSA-G2 analyzer (TA instruments) and a Minimat 2000 material tester. Each value is an average of two measurements. Dynamic mechanical analysis (DMA) was carried out with a QA 800 analyzer (TA Instruments) equipment working in tensile mode. The measurements were performed in isochronal conditions at 1 Hz and at a strain amplitude of 0.05%. Film thickness was determined before each measurement. Subsequently, the system was cooled down to 80 C and the viscoelastic moduli during the heating run were recorded up to 300 C at 2 C min1. Two samples were used to characterize each film. Conductivities of electrolyte-soaked macroporous PVDF-NCC and Celgard separators between 0 C and 60 C were determined by Fig. 2. TEM image (380000) of the NCC whiskers dispersed in DMF.
(4)
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In our case, visual analysis of the TEM image allowed us to obtain a value of 16.55 4.30 for the aspect ratio, and then a vRc value of 4.2% in volume, or 3.6% in weight; all the calculated values are coherent with values proposed in literature for hardwood whiskers such as those used in our study. The Storage Modulus of a 100% NCC film prepared by casting was measured by means of a DMA analysis, since the obtained samples appeared too brittle for an elongation test such as performed in literature: the value obtained at r.t. is 4,400 MPa, which is higher than the values proposed by Dufresne et al. ( 500 MPa), who performed an elongation measurement [41]. 3.1.2. Separators elaboration and morphology In order to obtain macroporous separators, the phase inversion method was applied, using the DMF (solvent)/ethanol (nonsolvent) couple, and a protocol very similar to the one proposed by Djian et al. [14]. Two series of PVdF based membranes were prepared with 0, 6, 12, or 20 wt% NCC, as resumed in Table 1; dense and macroporous membranes are indicated, respectively, with the acronym PVDF-xD or PVdF-xP, where x corresponds to the amount of cellulose incorporated As revealed by SEM analysis (Fig. 3), the macroporous separators are sponge-like. A visual analysis with the software ImageJ of the surface pores size indicates that this parameter is little affected by the NCC loading until 20 wt%, where the membranes clearly look less porous. In parallel, the porosity measurements indicate that the overall porosity of the nanocomposites decreases from a maximum of 76% for a non-reinforced separator to 62% for a 20 wt% NCC-containing one. From the visual examination, the pore size is only slightly affected by the whiskers addition up to 12 wt%, with most of the pore diameters spanning between 2 and 5 mm, whereas, with the addition of 20 wt% NCC, values between 0.8 and 2 mm are reached. The pore size and porosity values for the NCC-free PVdF separators are coherent with those found by Djian et al. [13]. We attribute the sharp decrease in porosity mainly to the higher viscosity induced by the whiskers in the cast slurry, which was found to have increased from 1.20 to a maximum of 2.60 mPa.s. As shown in Fig. 4, the hot-pressed nanocomposites membranes are dense with no visible open porosity, similarly to what was obtained by Tang et al. [28]. The presence of the whiskers is revealed by the presence of white spots on the membranes whose size, due to optical effects, is not directly related to the whiskers’ size. TGA was also performed to assess the thermal stability of our nanocomposites and, above all, to confirm the amount of whiskers effectively present in the membranes. The results (Fig. 5) show that the membrane’s degradation starts before 300 C in both macroand dense membranes and is assigned to the oxidative degradation of the SO4 functional groups at the whiskers surface [42]. The PVdF degradation takes place at about 450 C and no influence of the presence of the whiskers on the degradation temperature is Table 1 Morphology parameters for the nanocomposite membranes. Sample
NCC content (%)
Porosity (%)
Pore Size (mm)
PVdF-0D PVdF-6D PVdF-12D PVdF-20D PVdF-0P PVdF-6P PVdF-12P PVdF-20P
0 6.2 0.5 13.0 1.0 20.9 1.0 0 6.4 0.5 12.6 1.0 19.1 1.0
<1% <1% <1% <1% 77 1 72 1 67 1 62 1
<0.05 <0.05 <0.05 <0.05 25 25 25 0.8 2
observed. Examination of the TGA curve for pure NCC reveals a water loss at 100 C due to the moisture uptake of cellulose even after prolonged drying. This is not the case for the macroporous and dense membranes, indicating a more efficient water removal from NCC in composite membranes. This may be explained by (i) the hydrophobicity of the PVdF matrix, embedding the NCC, and (ii) the removal of water traces by the formation of an azeotrope with the ethanol bath during phase inversion. Calculations of the weight loss were performed to confirm the amount of whiskers effectively present in the membranes. The results (see Table 1) are in good agreement with the expected values and, more importantly, point out that during the fabrication of the macroporous separators no fillers are lost in the non-solvent bath, which is very promising for a potential upscaling of this method. Due to the good agreement between theoretical and experimental NCC content, the former was used to perform the theoretical storage modulus calculations (see section 3.2.3.). 3.2. Physicochemical characterizations of the nanocomposite separators 3.2.1. Crystallinity and crystal phases content The results of DSC analyses are presented in Table 2. At a first glance, one can observe that macroporous membranes present a higher crystallinity and melting point (Tm) than the dense ones. This has already been observed by Karabelli et al. on another PVdF grade (Kynar by Arkema), and was attributed to the process duration and the thermal treatment: when the membranes are quenched at 180 C, the polymeric chains do not have enough time to properly rearrange and crystallize, contrarily to the phase inversion process [15,19] which is a longer and softer process. The NCC addition to the dense membrane decreases the crystallinity and Tm of the polymer matrix, but its influence is weak in the range of loading which we investigated: the crystallinity of the dense membranes is decreased from 44% to 41% and its Tm decreases from 169 C to 167 C. For the macroporous membranes, the effect is negligible. These results are in good accordance with the findings by Zhang et al. on PVdF-cellulose composites who found that a filler content up to 20 wt% led to a decrease in crystallinity inferior to 5% [27]. Bodkhe et al. also reported a small decrease in crystallinity for PVdF dense membranes at low content of NCC or MFC, and suggested that this phenomenon is due to the heterogeneous crystallization of the PVdF chains on the cellulosic surface and that its appearance is correlated to the preferential formation of the energetically unfavored b phase instead of the most common a phase [43]. IR spectroscopy results of our samples are reported in Table 2 and in Fig. 6, and show that the addition of cellulose crystals strongly decreases the a phase amount from more than 50% to less than 30%, thus confirming Bodkhe’s findings. In this regard, dense and macroporous nanocomposites exhibit a similar decrease of the a phase content with respect to the NCC-free separators, and the major effect appears to be the presence or absence of the whiskers in the films. In literature, the obtainment of the b phase was typically attributed to the stretching of the a phase, which is otherwise favored in most conditions. However, the arrangement of the chains in this phase increases its dipolar moment, as also reported by Sundaray et al., who found that the use of electrospinning, a technique in which polymeric solutions are exposed to a strong electrical field, for the production of porous PVdF membranes led to a strong b phase enhancement [44]. In the case of cellulose, the attractive electrostatic and ion-dipole forces between OH groups at its surface and fluorine atoms of PVdF are the possible reasons for its ability to prevent the reversion of the b phase into a. The calculated relative amount of a phase i.e. 50% in non-reinforced
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Fig. 3. SEM images (1000) of the solution side of porous PVdF separators with a) 0% (b) 6% c) 12% and d) 20% wt of NCC fillers.
Fig. 4. SEM images (10000) of the cross-section of dense PVdF separators with a) 0% and b) 20% wt of CNC fillers.
samples, between 20% and 30% in nanocomposites is in agreement with the calculations of Bodkhe et al., who estimated the initial a phase content in their PVdF sample to exceed 60%, and to be decreased between 20% and 30% for the range of 0.5 to 2 wt% of NCC loading. Finally, the temperature of the a-relaxation, associated to the glass transition temperature (Tg), was also obtained for some samples as the maximum of the peak of the damping factor (tand) in DMA analyses (results not shown). The influence of the NCC loading is negligible for both macroporous and dense membranes, which is in accordance with the results of our DSC analyses and results found in literature for other cellulose-based nanocomposites [5,15,31].
PVDF macroporous separator strongly decreases with the NCC increase as the contact angle notably decreases (Table 3): since the porosity tends to slightly decrease as the NCC content increases, this increased wettability can only originate from the hydrophilic cellulose crystals, despite the fact that they are embedded in the PVdF polymer matrix. In particular, we must emphasize the total wetting of the sample PVDF-20P, after some time. The results are in good agreement with the findings of Lalia et al., who reported an increased hydrophilicity for macroporous PVdF-HFP membranes with the addition of NCC up to 4 wt% [30] This is a twofold technological asset, since it allows the battery to be quickly filled by the polar electrolyte and to better retain the liquid in the pore volume.
3.2.2. Contact angle measurements Table 3 presents the values for the contact angle of water on some macroporous separators. The high hydrophobicity of the
3.2.3. Thermo-mechanical properties First, tensile tests at room temperature were performed on nanocomposites, and a summary of the results is presented in
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Fig. 5. Thermogravimetric analysis of dry dense (top, left) and macroporous (top, right) nanocomposite PVDF-CNC separators, and pure CNC whiskers (bottom, left) under Air flux (10 C min1).
Table 2 Melting point, enthalpy of melting, degree of crystallinity, and a-phase content of the polymeric phase in the nanocomposite membranes. Sample
Tm ( C)
DHf (J g1)
xp (%)
a-phase content (%)
PVdF-0D PVdF-6D PVdF-12D PVdF-20D PVdF-0P PVdF-6P PVdF-12P PVdF-20P
169 169 168 167 169 170 169 170
46 41 38 34 61 59 54 50
44 42 41 41 59 61 59 60
54 26 27 28 51 24 27 20
Table 4. The presence of the nanocrystals leads to an increase of Young’s Modulus of dense membranes from 730 to 1150 MPa; an even more promising 4-fold increase was observed in macroporous membranes, from 35 MPa for PVdF-0P to 145 MPa for PVdF12P. We also observed that the elongation at break, for both dense
and macroporous membranes, decreases when NCC is added, presumably due to the formation of a rigid network. This behavior is coherent with reports by Zhang et al. [27], Tang et al. [28], Hashaikeh et al. [29,30] for PVdF-based and by Azizi Samir et al. [31] and POE-based nanocomposites, respectively. As the cellulose content is increased, the stiffness increases gradually; however, for 20 wt% cellulose, the macroporous composite begins to lose its mechanical strength, which could be due to local agglomeration of fibers or cracks in the membranes. The presence of a maximum in the mechanical properties is also observed by Hashaikeh et al. [30] The increase of mechanical strength at r.t. is a common effect of fibers addition in the matrix; the mechanical reinforcement on the stogage modulus of the composite (Ec) becomes significant at temperatures higher than the glass transition temperature (Tg, 30 C for PVdF). However, this does not prove that a 3-D network is fully obtained: in certain composites the percolation is incomplete due to the nature of the interactions between polymer chains and cellulose crystals, and a drastic loss in mechanical
Fig. 6. Infrared spectra of dry dense (left) and macroporous (right) nanocomposite PVDF-NCC separators between 600 and 1000 cm1. The asterisks signal the peaks used to calculate the fraction of a phase.
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Table 3 Contact angle of deionized water on some nanocomposite membranes. Sample
PVdF-0P
PVdF-6P
PVdF-20P
Contact angle ( )
143.3 0.8
67.8 4.0
20.2 3.2
Table 4 Thermomechanical parameters at 25 and 200 C for the nanocomposite membranes. Sample
Young’s Modulus (MPa)
Elongation at break (%)
Experimental Storage Modulus at 25 C (MPa)
Experimental Storage Modulus at 200 C (MPa)
Calculated Storage Modulus at 200 C (MPa)
PVdF-0D PVdF-6D PVdF12D PVdF20D PVdF-0P PVdF-6P PVdF12P PVdF20P
730 30 850 40 950 30
55 5 49 4 34 4
910 30 1284 40 1510 40
/ 83 5 185 10
/ 75 251
1150 30
22 3
2010 50
448 10
546
35 4 48 6 145 10
29 3 15 1 61
42 4 68 5 153 7
/ / /
/ / 80
99 9
2 0.3
n.a.
/
208
strength was reported above the melting point of the matrix Tm [45,46]. Considering the hydrophobic nature of PVdF, its compatibility with a polar hydrophilic filler such as NCC might be questioned. A proof for the successful formation of a rigid percolating network within the matrix can be obtained at temperatures exceeding the Tm of PVdF: if a percolating network is effectively present, it becomes the only source of mechanical strength, and a stable plateau in mechanical strength should be observed, whose value can be effectively predicted from the equation: Ec = c ER
(5)
where ER is the mechanical modulus of the percolating phase (in our case, 4,400 MPa), and c is a parameter linked to the loading:
c¼ð
vR vRc b Þ 1 vRc
ð6Þ
where b is equal to 0.4 for a 3-D network [47]. To further study the effect of the fibers reinforcement and assess the presence of a 3D network, we performed DMA analyses
between 100 C and 300 C in the elastic domain. The results are presented in Fig. 7. For dense membranes (Fig. 7, left), at r.t., the incorporation of NCC led to a spectacular improvement of the tensile modulus, which increased from 730 MPa (a value in agreement with that reported by Karabelli [14]) to 1150 MPa. After the PVdF melting, the modulus of composites remained constant, up to temperatures close to 300 C, where the cellulose degradation likely took place. This proves that a rigid percolating NCC network was formed in the composites through whisker/ whisker hydrogen bonds interactions. To the best of our knowledge, this is the first time that the successful formation of percolating network of cellulose fibers was reported in a hydrophobic polymer such as PVdF. Experimental modulus at 200 C for 6 wt% loading is in good agreement with the theoretical value predicted by equation (5) (Table 4). For higher loadings, the difference between the two values is more significant, which might be related to a partial agglomeration of the fillers, reducing the effective network density.
Fig. 7. Storage modulus vs temperature for dense (left) and macroporous (right) PVdF-NCC membranes.
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The DMA results for macroporous membranes (Fig. 7, right) confirmed the promising reinforcement at room temperature. Unlike dense membranes, the mechanical strength is remarkably increased even before the Tg: such a difference is partially due to the effective porosity of the samples, i.e., the porosity decreases with the incorporation of NCC, being in agreement with the results reported by Karabelli et al. for a series of PVdF membranes with different porosity [14]. However, after the melting temperature, the membrane PVdF-12P, whose loading was above the percolation threshold, didn’t exhibit a plateau in storage modulus. This indicates an incomplete formation of a percolating network, which may be due to the presence of a large amount of pores and the confinement of part of the crystals at their surface, thus diluting the effective loading. A promising strategy to overcome this problem relies on the chemical modification of the whiskers surface, in order to enhance their compatibility with different hydrophobic matrix [46,48,49] and then favor the presence of NCC in the PVdF matrix part. With regard to the sample PVdF-20P, its extreme brittleness made it impossible to perform the DMA test properly. Overall, a very promising mechanical reinforcement could be highlighted, which is an important technological asset for the processability of dry membranes at the scale of a pilot line. 3.3. Ionic conductivity In case of the PVdF homopolymer, the fabrication of macroporous separators presents a much larger technological interest over dense ones, due to their high electrolyte uptake. Indeed, weak conductivities are obtained with dense membranes swelled by LP30, with 1.6 102 mS cm1 at 30 C obtained without NCC. Although the electrolyte uptake appears to be slightly increased with the incorporation of cellulose, the conductivity reaches only a maximum of 2.5 102 mS cm1 with 20 wt% of NCC, still insufficient for commercial applications. The ionic conductivities seff of macroporous membranes soaked in LP30 are reported in Fig. 8, which shows also the conductivity for the pure electrolyte and for Celgard1 2400. In the temperature range we investigated (0 to 60 C), PVdF-0P samples exhibited the highest conductivities (1.89 to 6.55 mS cm-1 at 0 C and 60 C respectively, 3.73 mS.cm-1 at 25 C) among the separators tested herein, in agreement with the fact that it possesses the highest porosity (76%), while the least porous Celgard1 2400 (less than 40% of porosity [12]) presents the lowest performance. The addition of NCC leads to a decrease in ionic
conductivity, down to 1.45 mS.cm1 for PVdF-20P at 25 C, which also ehxibits the lowest porosity (61%). Nonetheless, it is still performing better than the Celgard1 separator. The measured conductivities of non-reinforced PVdF and Celgard1 are in agreement with previous measurement by Djian et al. [13] and Karabelli et al. [19] on macroporous PVdF separators presenting a porosity of 80%. It is also worth noticing that, for high NCC loadings, the conductivities are slightly less enhanced with the temperature increase, i.e. PVdF-0P is 3.5 times more conductive than PVdF-20P at r.t., but this ratio increases to 3.9 at 60 C. This is likely to be due to the decreasing amount of PVdF polymeric phase which can be swollen by the electrolyte (NCC is assumed not to swell) and thus contribute to the ionic transport. At the same size, an incomplete NCC network might be less effective against the reduction of the mean pore size due to swelling and compression of the macroporous membrane. Although the porosity of a separator is a major factor in obtaining high conductivity, it is not the only one, since the pores have to be completely filled by the liquid electrolyte. Assuming that the swelling by the electrolyte of the PVdF polymeric interpores phase did not significantly contribute to the overall conductivity, the conductivity will be affected by the interconnection between pores, as previously observed by Djian et al. [13]. The MacMullin number, NM, and tortuosity, t , parameters are used to characterize this behaviour. Both are defined by Eqs.7 and 8: NM = s0/seff
(7)
t = (NM x e)1/2
(8)
where s0 is the conductivity of the pure liquid electrolyte, seff is the conductivity of the porous membrane plus liquid electrolyte pair, and e is the porosity ratio. The calculated values presented in Table 5. The MacMullin numbers, NM, range between 3 and 16. As expected, the highest pore volume led to the lowest NM value (i.e., the highest conductivity). The tortuosity increases with the NCC loading, which may be associated to the decrease of the average pore size. It is worth noticing that for comparable tortuosity values (2.19 vs 2.26), PVdF-20P still presents a 2-fold increase over Celgard1 2400, thanks to an increased porous volume. For the practical use of separators within batteries, the wetting ability of the separator is crucial; in this regard, we deem that the large pore size and the presence of polar fillers are certainly a technological asset of our products. However, the large pore size might also be a drawback for the long-term cyclability of the devices, since it doesn’t permit the separator to effectively act as a barrier against particles migration between the electrodes. Finally, the addition of NCC induces a decrease of both the pore volume and the pore size, which appear largely responsible for the conductivity decrease, whereas it is clearly advantageous from a safety point of view. In order to optimize these porous structures, further studies are currently being conducted.
Table 5 Conductivity, MacMullin number, and tortuosity at 25 C for the nanocomposite porous membranes and Celgard1 2400.
Fig. 8. Ionic conductivity between 0 and 60 C for PVdF-CNC macroporous separators soaked in LP30.
Sample
seff (mS cm1)
NM
t
PVdF-0P PVdF-6P PVdF-12P PVdF-20P Celgard1 2400
3.73 0.4 2.53 0.30 2.08 0.15 1.45 0.10 0.73 0.10
3.16 4.66 5.66 8.13 16.10
1.57 1.79 1.92 2.19 2.26
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4. CONCLUSIONS Nanocomposites macroporous separators of PVdF/NCC were successfully prepared by phase inversion. Their physicochemical and thermo-mechanical properties were fully investigated, and the results were compared to those obtained for reference dense membranes produced by hot-pressing. SEM and TGA analyses showed that this process does not affect the final loading of NCC in the porous nanocomposites, and decreases the porosity and the pore size of the macroporous separators. DSC measurements shown that the cellulose whiskers present a negligible effect on the amount of crystallinity of the polymer matrix. We confirmed that the use of polar whiskers induces the preferential formation of the polar b phase of PVdF, and improve the wettability of the macroporous separators by polar electrolytes. The addition of the cellulose whiskers induces a large increase in mechanical strength, in both dense and macroporous membranes, up to 4-fold for the latter. The successful formation of a rigid percolating network in the dense PVdF membranes was observed for the first time. For macroporous PVdF, the ionic conductivity decreases by adding NCC, due to the porosity decrease and the lower pore interconnection; nonetheless, macroporous membranes exhibit a largely improved conductivity with respect to a commercial standard. The use of NCC to reinforce hydrophobic macroporous separators is an attractive concept, as it allows for it to improve their mechanical strength while maintaining appropriate conductivity levels for lithium-ion batteries. ACKNOWLEDGEMENTS Financial support from the European Commission within the AMELIE project (265910) under the Seventh Framework Programme (7th FWP) is gratefully acknowledged. Dr Grégory Chauve (FPInnovations) is gratefully acknowledged for his precious advice concerning the cellulose dispersions. We also gratefully thank Mrs. Cécile Bruzzese and Prof. Alain Dufresne (LGP2) for their help with mechanical characterizations. Mr. Riccardo Scarrazini and Dr. Houssemeddine Benattia are acknowledged for their help in the membranes preparation. Solvay is acknowledged for the supply of PVdF. References [1] C. Chauvin, F. Alloin, C. Iojoiu, J.-Y. Sanchez, New polymer electrolytes based on ether sulfate anions for lithium polymer batteries: part II: conductivity and transport properties of blended and cross-linked ionomers, Electrochimica Acta 51 (2006) 5954–5960. [2] A. Nishimoto, K. Agehara, N. Furuya, T. Watanabe, M. Watanabe, High ionic conductivity of polyether-based network polymer electrolytes with hyperbranched side chains, Macromolecules 32 (1999) 1541–1548. [3] R. Bouchet, S. Maria, R. Meziane, A. Aboulaich, L. Lienafa, J.P. Bonnet, T.N.T. Tran, D. Bertin, D. Gigmes, D. Devaux, R. Denoyel, M. Armand, Single-ion BAB triblock copolymers as highly efficient electrolytes for lithium-metal batteries, Nature Materials 12 (2013) 452–457. [4] F. Croce, L. Persi, B. Scrosati, F. Serraino-Fiory, E. Plichta, M.A. Hendrickson, Role of the ceramic fillers in enhancing the transport properties of composite polymer electrolytes, Electrochimica Acta 46 (2001) 2457–2461. [5] M.A.S. Azizi Samir, F. Alloin, J.-Y. Sanchez, N. El Kissi, A. Dufresne, Preparation of Cellulose Whiskers Reinforced Nanocomposites from an Organic Medium Suspension, Macromolecules 37 (2004) 1386–1393. [6] B. Scrosati, C.A. Vincent, Polymer electrolytes: the key to lithium polymer batteries, MRS Bulletin 25 (2000) 28–30. [7] J. Saunier, F. Alloin, J.Y. Sanchez, Electrochemical and spectroscopic studies of polymethacrylonitrile based electrolytes, Electrochimica Acta 45 (2000) 1255–1263. [8] A.M. Stephan, Review on gel polymer electrolytes for lithium batteries, European Polymer Journal 42 (2006) 21–42. [9] S.S. Zhang, A review on the separators of liquid electrolyte Li-ion batteries, Journal of Power Sources 164 (2007) 351–364. [10] K. Murata, S. Izuchi, Y. Yoshihisa, An overview of the research and development of solid polymer electrolyte batteries, Electrochimica Acta 45 (2000) 1501–1508.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Nanocomposite poly(vynilidene fluoride)/nanocrystalline cellulose porous membranes as separators for lithium-ion batteries Marco Bollolia,b,* , Claire Antonellic, Yannick Molméreta,b , Fannie Alloina,b,d, Cristina Iojoiua,b,d, Jean-Yves Sancheza,b,c,* a
Univ. Grenoble Alpes, LEPMI, F-38000 Grenoble, France CNRS, LEPMI, F-38000 Grenoble, France Universidad Carlos III de Madrid, Materials Sciences Leganés, Spain d Réseau sur le Stockage Electrochimique de l'Energie (RS2E), CNRS, FR3459, 33 Rue Saint Leu, 80039 Amiens Cedex, France b c
A R T I C L E I N F O
Article history: Received 18 April 2016 Received in revised form 5 August 2016 Accepted 5 August 2016 Available online 6 August 2016 Keywords: PVdF nanocomposite phase inversion nanocrystalline cellulose NCC separator
A B S T R A C T
Nanocomposite materials were obtained from poly(vinylidene fluoride) (PVdF) as matrix polymer and a stable DMF suspension of nanocrystalline cellulose (NCC) as the reinforcing phase. Porous and dense nanocomposite membranes were prepared by non-solvent induced phase separation (NIPS) and film casting methods, respectively. The resulting films were characterized regarding their structuration, i.e., the content of crystalline phases, as well as their transport and thermo-mechanical properties. The presence of the fillers led to a mechanical reinforcement, associated with a lower strain at break. For dense nanocomposites, a thermal stabilization at temperatures higher than the melting temperature was highlighted and ascribed to the formation of a rigid cellulosic network within the matrix. The superior electrochemical performances together with the observed reinforcement effect render these porous nanocomposites membranes as interesting candidates for the replacement of commercial polyolefinbased microporous separators in lithium-ion batteries. ã 2016 Elsevier Ltd. All rights reserved.
1. INTRODUCTION Lithium-ion and lithium metal-polymer batteries are at present the most competitive power sources for high capacity energy storage because of their high energy density and power capability. In both technologies, the electrolyte does not only serve as ion conductore but moreover as separator to prevent the direct contact between the electrodes. One approach, largely predominant in the latter technology, consists in using a macromolecular solvent, generally poly(oxyethylene) POE based, in order to guarantee a sufficient dissociation and solvation of the ionic species and, at the same time, to ensure the mechanical separation between electrodes to prevent short-circuits. In this case, an improvement of the mechanical properties of the polymer film has been shown to be crucial. Different strategies were explored such as the use of (i) cross-linked polyether networks [1,2], (ii) block copolymers [3], or different fillers blended in the membranes [4,5]. Although these
* Corresponding authors at: Univ. Grenoble Alpes, LEPMI, F-38000 Grenoble, France. E-mail addresses: [email protected] (M. Bolloli), [email protected] (J.-Y. Sanchez). http://dx.doi.org/10.1016/j.electacta.2016.08.020 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
polyethers can be effectively swelled by liquid electrolytes and substantially reinforced, they are nevertheless unstable in the presence of current positive electrodes operating above 4 V vs Li+/Li. Separators based on polyolefins or fluorinated polymers, compatible with high-voltage cathodes, are therefore more widely used in lithium-ion batteries. These separators can be (i) dense polymers gelled by liquid electrolytes [6,7] or (ii) porous polymers where the pores are filled by liquid electrolytes [8–10]. The former are generally easy to process by extrusion, a non-costly and safe process. However, a suitable compromise between high electrolyte uptake, leading to high conductivities, and sufficient mechanical strength after soaking is not easy to reach in order to obtain thin films. The porous separators, which are the most common solution for the portable electronics devices at the moment, are based on a single polymer film or a multi-layered structure; their preparation processes, ensuring the porous structure, makes them much more costly but, at the same time, enables the decoupling of mechanical strength, realized by the polymeric inter-pore phase, and ionic conductivity, ensured by the liquid electrolyte filling the porous volume. Porous separators must therefore exhibit high open
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porosity and low tortuosity. In addition, a good wetting of the pores is essential to provide (i) high conductivities and (ii) good retention of the liquid electrolyte in the porous volume. The wetting ability depends on the interaction between the polymers and the liquid electrolyte. The most successful materials, such as the Celgard1 series, are either based on a single sheet of highly oriented polypropylene, or a tri-layered material, in which a porous polyethylene layer is sandwiched between 2 porous polypropylene layers, the latter solution increases the battery safety thanks to the temperature-induced shut-down effect. In case of these separators, the interaction is between the polar electrolyte and the nonpolar polymer is poor, and a significant drop in conductivity is observed [11,12]. Poly(vinylidene fluoride) (PVdF) homopolymer and its copolymers have been identified [14–16] as good candidates for macroporous separators as well, since they (i) provide a high thermal, mechanical and electrochemical stability, in particular towards oxidation, (ii) are often used as binder for both negative and positive electrodes, favoring a good electrolyte–electrode interface, and (iii) are polar polymers exhibiting fairly good affinity with the polar solvent of the electrolyte. Due to this affinity, the electrolyte does not only fill the porous volume of PVdF but also swells the polymeric phase of the separator. However, this swelling, which results in a decrease in the amount of crystallinity and of the melting point, is detrimental to the mechanical performance of the separator without enhancing the conductivity As this swelling increases with a decreasing crystallinity, the homopolymer, PVdF, is preferred compared to the copolymers, such as poly(vinylidene fluoride-co-hexafluoropropylene) (PVdFHFP). Multiple reinforcement routes have been suggested to improve the mechanical strength of the polymer; such as polymer crosslinking [17,18] or the combination with woven fabrics [19,20]. Another very promising strategy due to its low preparation cost and the large selection of materials relies on the formation of composites thanks to the addition of organic and inorganic fibers [21–23]. Among the most investigated fibrous materials, there are different types of cellulose nanofibers, which have seen an exponential growth in scientific communications and research efforts in the last 15 years [24,25], due to their renewable and nontoxic nature, the strong mechanical reinforcement which can be obtained, and their low cost. From mechanical disintegration of the wood pulp, microfibrillated cellulose (MFC) is obtained: it consists of both individual and aggregated nanofibrils made of alternating amorphous and crystalline domains, with an estimated width between 3–100 nm and a length exceeding 1 mm. A subsequent controlled acid hydrolysis allows the dissolution of amorphous domains, and yields high aspect ratio rod-like nanocrystals (nanocrystalline cellulose, NCC), or whiskers, having a diameter of 5 to 15 nm and length of 100 to 500 nm, depending on the cellulose source. The accepted mechanism of reinforcement of these materials is the formation of a 3-D percolating network due to the strong interactions (hydrogen bonds) between the nanofibers; if properly formed, the cellulosic network can also prevent the composite from flowing at temperatures exceeding the melting point of the polymeric matrix, thus increasing the operational range of the products [26]. Zhang et al. [27] reported the preparation of macroporous PVdF- regenerated cellulose composites by coagulating their N,Ndimethyl acetamide (DMAC)/LiCl solution with water, and that the composition affected the mechanical strength. Tang et al. [28] also prepared a similar composite, but by the use of PVdF grafted with maleic anhydride to address the compatibility issues which could potentially arise from mixing PVdF, which is highly hydrophobic, with polar cellulose. Finally, the group of R. Hashaikeh reported the
39
use of macroporous PVdF-HFP membranes reinforced with NCC, produced by electrospinning, with improved separation properties for desalination purposes [29,30]. A mechanical reinforcement was reported, to some extent, by all the authors, but no percolating 3-D network was reportedly obtained in any of these works. With regard to electrochemical storage applications, so far not much attention has been devoted to the preparation of macroporous PVdF-cellulose composite membranes and their properties; however, the interest of this strategy has been already highlighted for dense PEO-based membranes and gelled acrylic matrix in both lithium polymer and lithium-ion applications, in which an important mechanical reinforcement has been observed with no significant impact on the electrochemical stability and performances [31–34]. We recently reported [35] the electrochemical performances of PVdF-NCC macroporous separators prepared by phase inversion in graphite/LiNi0.4Mn1.6O4 full cells. With respect to a commercial Celgard1 2400 separator, the nanocomposite membranes exhibited improved performances and cyclability, in particular at high C rates, thanks to their superior transport properties. Their superior performance has been further validated for a plug-in hybrid vehicle application under a DOE testing protocol [36]. In this contribution, we aim at broadening the knowledge of the physicochemical properties of these nanocomposites separators. Due to its industrial potential, phase inversion method was used to obtain the porous films. Their morphology, physicochemical and thermo-mechanical properties were investigated, and compared to a series of dense membranes, prepared by polymer casting, and then hotpressed. Finally, their ionic conductivity vs NCC loading was investigated with a standard lithium ion electrolyte. 2. EXPERIMENTAL 2.1. Materials PVdF homopolymer (Solef1 6020, 650,000 g mol1) was kindly provided by Solvay Solexis. Dry nanocrystalline cellulose was provided by FPInnovations. Dimethyl formamide (DMF, Carlo Erba, 99%) and ethanol (Sigma Aldrich, 95%) were used as received without further purification. The reference electrolyte for the electrochemical testing was prepared by dissolving lithium hexafluorophosphate (LiPF6) at a 1 M concentration in an equivolume mixture of dimethyl carbonate and ethylene carbonate. All the products were purchased from Sigma-Aldrich ( > 99%), and were dried before use. 2.2. Production of the nanocrystalline cellulose dispersion The dispersion of nanocrystalline cellulose in DMF was prepared by adding the NCC to the solvent (2 wt% was the standard concentration; it was raised up to 3 wt % to obtain the proper composition for the most loaded PVdF-NCC slurries), and then sonicating the whiskers/solvent mixture at room temperature for several cycles of 30 minutes with a VC 505 Vibra-CellTM using a 13 mm probe and storing the mixture in an ice bath for 10 min between each cycle to avoid excessive solvent evaporation and degradation. The quality of the dispersion was assessed by inspection of the dispersion between two crossed polarizers as widely accepted in literature: the presence of birefringence was used as the criterion for having obtained a good dispersion [37]. 2.3. Processing of the nanocomposite films PVdF was at first dissolved in DMF and the resulting solution was added to the NCC dispersion in order to obtain a concentration
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of cellulose whiskers in the final membranes of 6, 12, and 20 wt% and a PVdF concentration of 11.5 wt% with respect to the solvent. The resulting suspension was stirred for 8 h at 50 C and degassed under vacuum in order to remove the remaining air. In order to obtain dense films, the PVdF-NCC dispersion was cast into Petri dishes. The films were then left to evaporate at 60 C for 2 days and eventually dried under vacuum for 4 h until constant weight. The thickness varied between 15 and 40 mm, following the amount of dispersion cast onto the Petri dish. The samples were then pressed at 180 C in a laboratory hot press for 5 min in order to prepare dense membranes. To obtain macroporous membranes, the PVdF-NCC dispersion was spread over a glass support with a doctor blade, then
immerged into an ethanol bath for 10 min, as proposed by Djian [13]. The resulting films were then dried in the same way as described for the dense membranes. A thickness of about 20 mm was obtained. The processing strategies are resumed in Fig. 1. 2.4. Characterization methods Transmission electron micrographs (TEMs) of the cellulose whiskers were acquired using a JEOL 2100 F–200 kV instrument. Drops of the 10-fold diluted crystal suspension were deposited onto glow-discharged carbon-coated copper grids and the liquid in excess was blotted with filter paper. Uranyl acetate was used as
Fig. 1. Simplified scheme of the elaboration of the nanocomposite membranes.
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contrasting agent. Digital image analysis (ImageJ Software) was used in order to estimate the dimensions of the nanoparticles, and an average of 100 measures was retained. Observation of the final nanocomposite films was realized, after cryofracture in liquid nitrogen and coating with a thin graphite layer, by means of a scanning electron microscope (SEM) LEO Stereoscan 440, with an accelerating voltage of 15 or 20 kV. The porosity of the macroporous membranes was calculated using the following equation: porosity (%) = (1- (rm/rth) x 100
(1)
where rm is the membrane density and was determined in distilled water or ethanol by means of a density measurement kit (Mettler Toledo), and rth is the theoretical density of the membranes, calculated from the density of PVdF (1.78 g/cm3) and cellulose (1.50 g/cm3). The weight loss as a function of temperature was assessed by means of thermogravimetric analysis (TGA), using a Q500 device (TA Instruments). The experiments were performed in double, under air flux, between r.t. and 600 C, with a heating ramp of 10 C. min1. Determination of the crystallinity and melting point Tm was performed by differential scanning calorimetry (DSC), using a TA Instrument DSC 2920 CE (Gemini BV) and applying a temperature ramp of 2 C min1. Each sample (around 3 mg) was sealed in an aluminum pan inside an argon-filled Inert Lab glove box having an oxygen and water content below 20 ppm. The degree of crystallinity of the polymeric phase, xp, was calculated using the following equation:
xp (%) = (DHm/DH0m) x w x 100
(2) H0m being the enthalpy of
DHm being the enthalpy of melting and D fusion of the repeating unit of a PVdF perfect crystal of infinite size, estimated as 104.6 J g1 from literature data [38] and w being the weight fraction of polymeric material in the composite. Infrared (IR) analysis was carried out using a FT-IR Spectrum One instrument (Perkin Elmer) in the frequency range 650– 4000 cm1. Each measurement was repeated at least twice to ensure reproducibility. The fraction of a crystalline phase was determined through the equation proposed by Gregorio et al. [39]: Fa = (1-[Aa/(Aa + 1.3 x Ab)])
41
electrochemical impedance spectroscopy (EIS) using an HP 4192A Impedance Analyser. Measurements were carried out in the frequency range 5 Hz 1 MHz with a SwagelokTM-type cell in two-electrode configuration with stainless steel blocking electrodes (0.785 cm2). An AC amplitude of 5 mV was used and data were collected taking 10 points per decade. The temperature was equilibrated for one hour prior to each measurement. The uncertainties of conductivity values are estimated to be 0.3 mS.cm1. 3. RESULTS AND DISCUSSION 3.1. Materials and shaping 3.1.1. Nanocrystalline cellulose and its dispersion in DMF MFC and NCC are readily dispersed in water up to a 6–8 wt % concentration. However, it’s much harder to disperse them in an organic solvent compared to water due to the less favorable surface interactions between the solvent and the cellulose surface groups. Van den Berg et al. [40] demonstrated that a proper dispersion of tunicate whiskers at low concentration ( 0.1 wt%) could be achieved in different organic solvents; nevertheless, for industrial applications a more concentrated dispersion should be readily available. From the works of Azizi Samir [5], dimethylformamide is known to be a suitable solvent to obtain a 2 wt% NCC dispersion: this prompted us to select this solvent for our study. From the TEM image (Fig. 2) the whiskers appear homogeneously dispersed in the solvent, without forming agglomerates. A key information is their aspect ratio (the ratio between length and diameter), which in turn is crucial to calculate the percolation threshold vRc of the whiskers. This parameter is defined as the critical amount beyond which the fillers loading vR form a 3-D network within the polymer matrix, and is linked to the aspect ratio by the following equation: vRc = 0.7/(L/d) where L/d is the aspect ratio [5].
(3)
where Aa and Ab are the absorption fractions of the a and b phase, respectively. The absorbance around 767 cm1 (CF2 bending and skeletal bending) was used as Aa and the maximum absorbance around 840 cm1 (CH2 rocking) as Ab. The contact angles of deionized water on some selected macroporous separators were evaluated using an OCA 15plus contact angle system (Neurtek Instruments, Spain). The values were obtained by averaging values from more than ten drops. The tensile testing of film samples was carried out at room temperature and relative humidity at a cross head speed of 0.05 mm min1 with a RSA-G2 analyzer (TA instruments) and a Minimat 2000 material tester. Each value is an average of two measurements. Dynamic mechanical analysis (DMA) was carried out with a QA 800 analyzer (TA Instruments) equipment working in tensile mode. The measurements were performed in isochronal conditions at 1 Hz and at a strain amplitude of 0.05%. Film thickness was determined before each measurement. Subsequently, the system was cooled down to 80 C and the viscoelastic moduli during the heating run were recorded up to 300 C at 2 C min1. Two samples were used to characterize each film. Conductivities of electrolyte-soaked macroporous PVDF-NCC and Celgard separators between 0 C and 60 C were determined by Fig. 2. TEM image (380000) of the NCC whiskers dispersed in DMF.
(4)
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In our case, visual analysis of the TEM image allowed us to obtain a value of 16.55 4.30 for the aspect ratio, and then a vRc value of 4.2% in volume, or 3.6% in weight; all the calculated values are coherent with values proposed in literature for hardwood whiskers such as those used in our study. The Storage Modulus of a 100% NCC film prepared by casting was measured by means of a DMA analysis, since the obtained samples appeared too brittle for an elongation test such as performed in literature: the value obtained at r.t. is 4,400 MPa, which is higher than the values proposed by Dufresne et al. ( 500 MPa), who performed an elongation measurement [41]. 3.1.2. Separators elaboration and morphology In order to obtain macroporous separators, the phase inversion method was applied, using the DMF (solvent)/ethanol (nonsolvent) couple, and a protocol very similar to the one proposed by Djian et al. [14]. Two series of PVdF based membranes were prepared with 0, 6, 12, or 20 wt% NCC, as resumed in Table 1; dense and macroporous membranes are indicated, respectively, with the acronym PVDF-xD or PVdF-xP, where x corresponds to the amount of cellulose incorporated As revealed by SEM analysis (Fig. 3), the macroporous separators are sponge-like. A visual analysis with the software ImageJ of the surface pores size indicates that this parameter is little affected by the NCC loading until 20 wt%, where the membranes clearly look less porous. In parallel, the porosity measurements indicate that the overall porosity of the nanocomposites decreases from a maximum of 76% for a non-reinforced separator to 62% for a 20 wt% NCC-containing one. From the visual examination, the pore size is only slightly affected by the whiskers addition up to 12 wt%, with most of the pore diameters spanning between 2 and 5 mm, whereas, with the addition of 20 wt% NCC, values between 0.8 and 2 mm are reached. The pore size and porosity values for the NCC-free PVdF separators are coherent with those found by Djian et al. [13]. We attribute the sharp decrease in porosity mainly to the higher viscosity induced by the whiskers in the cast slurry, which was found to have increased from 1.20 to a maximum of 2.60 mPa.s. As shown in Fig. 4, the hot-pressed nanocomposites membranes are dense with no visible open porosity, similarly to what was obtained by Tang et al. [28]. The presence of the whiskers is revealed by the presence of white spots on the membranes whose size, due to optical effects, is not directly related to the whiskers’ size. TGA was also performed to assess the thermal stability of our nanocomposites and, above all, to confirm the amount of whiskers effectively present in the membranes. The results (Fig. 5) show that the membrane’s degradation starts before 300 C in both macroand dense membranes and is assigned to the oxidative degradation of the SO4 functional groups at the whiskers surface [42]. The PVdF degradation takes place at about 450 C and no influence of the presence of the whiskers on the degradation temperature is Table 1 Morphology parameters for the nanocomposite membranes. Sample
NCC content (%)
Porosity (%)
Pore Size (mm)
PVdF-0D PVdF-6D PVdF-12D PVdF-20D PVdF-0P PVdF-6P PVdF-12P PVdF-20P
0 6.2 0.5 13.0 1.0 20.9 1.0 0 6.4 0.5 12.6 1.0 19.1 1.0
<1% <1% <1% <1% 77 1 72 1 67 1 62 1
<0.05 <0.05 <0.05 <0.05 25 25 25 0.8 2
observed. Examination of the TGA curve for pure NCC reveals a water loss at 100 C due to the moisture uptake of cellulose even after prolonged drying. This is not the case for the macroporous and dense membranes, indicating a more efficient water removal from NCC in composite membranes. This may be explained by (i) the hydrophobicity of the PVdF matrix, embedding the NCC, and (ii) the removal of water traces by the formation of an azeotrope with the ethanol bath during phase inversion. Calculations of the weight loss were performed to confirm the amount of whiskers effectively present in the membranes. The results (see Table 1) are in good agreement with the expected values and, more importantly, point out that during the fabrication of the macroporous separators no fillers are lost in the non-solvent bath, which is very promising for a potential upscaling of this method. Due to the good agreement between theoretical and experimental NCC content, the former was used to perform the theoretical storage modulus calculations (see section 3.2.3.). 3.2. Physicochemical characterizations of the nanocomposite separators 3.2.1. Crystallinity and crystal phases content The results of DSC analyses are presented in Table 2. At a first glance, one can observe that macroporous membranes present a higher crystallinity and melting point (Tm) than the dense ones. This has already been observed by Karabelli et al. on another PVdF grade (Kynar by Arkema), and was attributed to the process duration and the thermal treatment: when the membranes are quenched at 180 C, the polymeric chains do not have enough time to properly rearrange and crystallize, contrarily to the phase inversion process [15,19] which is a longer and softer process. The NCC addition to the dense membrane decreases the crystallinity and Tm of the polymer matrix, but its influence is weak in the range of loading which we investigated: the crystallinity of the dense membranes is decreased from 44% to 41% and its Tm decreases from 169 C to 167 C. For the macroporous membranes, the effect is negligible. These results are in good accordance with the findings by Zhang et al. on PVdF-cellulose composites who found that a filler content up to 20 wt% led to a decrease in crystallinity inferior to 5% [27]. Bodkhe et al. also reported a small decrease in crystallinity for PVdF dense membranes at low content of NCC or MFC, and suggested that this phenomenon is due to the heterogeneous crystallization of the PVdF chains on the cellulosic surface and that its appearance is correlated to the preferential formation of the energetically unfavored b phase instead of the most common a phase [43]. IR spectroscopy results of our samples are reported in Table 2 and in Fig. 6, and show that the addition of cellulose crystals strongly decreases the a phase amount from more than 50% to less than 30%, thus confirming Bodkhe’s findings. In this regard, dense and macroporous nanocomposites exhibit a similar decrease of the a phase content with respect to the NCC-free separators, and the major effect appears to be the presence or absence of the whiskers in the films. In literature, the obtainment of the b phase was typically attributed to the stretching of the a phase, which is otherwise favored in most conditions. However, the arrangement of the chains in this phase increases its dipolar moment, as also reported by Sundaray et al., who found that the use of electrospinning, a technique in which polymeric solutions are exposed to a strong electrical field, for the production of porous PVdF membranes led to a strong b phase enhancement [44]. In the case of cellulose, the attractive electrostatic and ion-dipole forces between OH groups at its surface and fluorine atoms of PVdF are the possible reasons for its ability to prevent the reversion of the b phase into a. The calculated relative amount of a phase i.e. 50% in non-reinforced
M. Bolloli et al. / Electrochimica Acta 214 (2016) 38–48
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Fig. 3. SEM images (1000) of the solution side of porous PVdF separators with a) 0% (b) 6% c) 12% and d) 20% wt of NCC fillers.
Fig. 4. SEM images (10000) of the cross-section of dense PVdF separators with a) 0% and b) 20% wt of CNC fillers.
samples, between 20% and 30% in nanocomposites is in agreement with the calculations of Bodkhe et al., who estimated the initial a phase content in their PVdF sample to exceed 60%, and to be decreased between 20% and 30% for the range of 0.5 to 2 wt% of NCC loading. Finally, the temperature of the a-relaxation, associated to the glass transition temperature (Tg), was also obtained for some samples as the maximum of the peak of the damping factor (tand) in DMA analyses (results not shown). The influence of the NCC loading is negligible for both macroporous and dense membranes, which is in accordance with the results of our DSC analyses and results found in literature for other cellulose-based nanocomposites [5,15,31].
PVDF macroporous separator strongly decreases with the NCC increase as the contact angle notably decreases (Table 3): since the porosity tends to slightly decrease as the NCC content increases, this increased wettability can only originate from the hydrophilic cellulose crystals, despite the fact that they are embedded in the PVdF polymer matrix. In particular, we must emphasize the total wetting of the sample PVDF-20P, after some time. The results are in good agreement with the findings of Lalia et al., who reported an increased hydrophilicity for macroporous PVdF-HFP membranes with the addition of NCC up to 4 wt% [30] This is a twofold technological asset, since it allows the battery to be quickly filled by the polar electrolyte and to better retain the liquid in the pore volume.
3.2.2. Contact angle measurements Table 3 presents the values for the contact angle of water on some macroporous separators. The high hydrophobicity of the
3.2.3. Thermo-mechanical properties First, tensile tests at room temperature were performed on nanocomposites, and a summary of the results is presented in
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Fig. 5. Thermogravimetric analysis of dry dense (top, left) and macroporous (top, right) nanocomposite PVDF-CNC separators, and pure CNC whiskers (bottom, left) under Air flux (10 C min1).
Table 2 Melting point, enthalpy of melting, degree of crystallinity, and a-phase content of the polymeric phase in the nanocomposite membranes. Sample
Tm ( C)
DHf (J g1)
xp (%)
a-phase content (%)
PVdF-0D PVdF-6D PVdF-12D PVdF-20D PVdF-0P PVdF-6P PVdF-12P PVdF-20P
169 169 168 167 169 170 169 170
46 41 38 34 61 59 54 50
44 42 41 41 59 61 59 60
54 26 27 28 51 24 27 20
Table 4. The presence of the nanocrystals leads to an increase of Young’s Modulus of dense membranes from 730 to 1150 MPa; an even more promising 4-fold increase was observed in macroporous membranes, from 35 MPa for PVdF-0P to 145 MPa for PVdF12P. We also observed that the elongation at break, for both dense
and macroporous membranes, decreases when NCC is added, presumably due to the formation of a rigid network. This behavior is coherent with reports by Zhang et al. [27], Tang et al. [28], Hashaikeh et al. [29,30] for PVdF-based and by Azizi Samir et al. [31] and POE-based nanocomposites, respectively. As the cellulose content is increased, the stiffness increases gradually; however, for 20 wt% cellulose, the macroporous composite begins to lose its mechanical strength, which could be due to local agglomeration of fibers or cracks in the membranes. The presence of a maximum in the mechanical properties is also observed by Hashaikeh et al. [30] The increase of mechanical strength at r.t. is a common effect of fibers addition in the matrix; the mechanical reinforcement on the stogage modulus of the composite (Ec) becomes significant at temperatures higher than the glass transition temperature (Tg, 30 C for PVdF). However, this does not prove that a 3-D network is fully obtained: in certain composites the percolation is incomplete due to the nature of the interactions between polymer chains and cellulose crystals, and a drastic loss in mechanical
Fig. 6. Infrared spectra of dry dense (left) and macroporous (right) nanocomposite PVDF-NCC separators between 600 and 1000 cm1. The asterisks signal the peaks used to calculate the fraction of a phase.
M. Bolloli et al. / Electrochimica Acta 214 (2016) 38–48
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Table 3 Contact angle of deionized water on some nanocomposite membranes. Sample
PVdF-0P
PVdF-6P
PVdF-20P
Contact angle ( )
143.3 0.8
67.8 4.0
20.2 3.2
Table 4 Thermomechanical parameters at 25 and 200 C for the nanocomposite membranes. Sample
Young’s Modulus (MPa)
Elongation at break (%)
Experimental Storage Modulus at 25 C (MPa)
Experimental Storage Modulus at 200 C (MPa)
Calculated Storage Modulus at 200 C (MPa)
PVdF-0D PVdF-6D PVdF12D PVdF20D PVdF-0P PVdF-6P PVdF12P PVdF20P
730 30 850 40 950 30
55 5 49 4 34 4
910 30 1284 40 1510 40
/ 83 5 185 10
/ 75 251
1150 30
22 3
2010 50
448 10
546
35 4 48 6 145 10
29 3 15 1 61
42 4 68 5 153 7
/ / /
/ / 80
99 9
2 0.3
n.a.
/
208
strength was reported above the melting point of the matrix Tm [45,46]. Considering the hydrophobic nature of PVdF, its compatibility with a polar hydrophilic filler such as NCC might be questioned. A proof for the successful formation of a rigid percolating network within the matrix can be obtained at temperatures exceeding the Tm of PVdF: if a percolating network is effectively present, it becomes the only source of mechanical strength, and a stable plateau in mechanical strength should be observed, whose value can be effectively predicted from the equation: Ec = c ER
(5)
where ER is the mechanical modulus of the percolating phase (in our case, 4,400 MPa), and c is a parameter linked to the loading:
c¼ð
vR vRc b Þ 1 vRc
ð6Þ
where b is equal to 0.4 for a 3-D network [47]. To further study the effect of the fibers reinforcement and assess the presence of a 3D network, we performed DMA analyses
between 100 C and 300 C in the elastic domain. The results are presented in Fig. 7. For dense membranes (Fig. 7, left), at r.t., the incorporation of NCC led to a spectacular improvement of the tensile modulus, which increased from 730 MPa (a value in agreement with that reported by Karabelli [14]) to 1150 MPa. After the PVdF melting, the modulus of composites remained constant, up to temperatures close to 300 C, where the cellulose degradation likely took place. This proves that a rigid percolating NCC network was formed in the composites through whisker/ whisker hydrogen bonds interactions. To the best of our knowledge, this is the first time that the successful formation of percolating network of cellulose fibers was reported in a hydrophobic polymer such as PVdF. Experimental modulus at 200 C for 6 wt% loading is in good agreement with the theoretical value predicted by equation (5) (Table 4). For higher loadings, the difference between the two values is more significant, which might be related to a partial agglomeration of the fillers, reducing the effective network density.
Fig. 7. Storage modulus vs temperature for dense (left) and macroporous (right) PVdF-NCC membranes.
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The DMA results for macroporous membranes (Fig. 7, right) confirmed the promising reinforcement at room temperature. Unlike dense membranes, the mechanical strength is remarkably increased even before the Tg: such a difference is partially due to the effective porosity of the samples, i.e., the porosity decreases with the incorporation of NCC, being in agreement with the results reported by Karabelli et al. for a series of PVdF membranes with different porosity [14]. However, after the melting temperature, the membrane PVdF-12P, whose loading was above the percolation threshold, didn’t exhibit a plateau in storage modulus. This indicates an incomplete formation of a percolating network, which may be due to the presence of a large amount of pores and the confinement of part of the crystals at their surface, thus diluting the effective loading. A promising strategy to overcome this problem relies on the chemical modification of the whiskers surface, in order to enhance their compatibility with different hydrophobic matrix [46,48,49] and then favor the presence of NCC in the PVdF matrix part. With regard to the sample PVdF-20P, its extreme brittleness made it impossible to perform the DMA test properly. Overall, a very promising mechanical reinforcement could be highlighted, which is an important technological asset for the processability of dry membranes at the scale of a pilot line. 3.3. Ionic conductivity In case of the PVdF homopolymer, the fabrication of macroporous separators presents a much larger technological interest over dense ones, due to their high electrolyte uptake. Indeed, weak conductivities are obtained with dense membranes swelled by LP30, with 1.6 102 mS cm1 at 30 C obtained without NCC. Although the electrolyte uptake appears to be slightly increased with the incorporation of cellulose, the conductivity reaches only a maximum of 2.5 102 mS cm1 with 20 wt% of NCC, still insufficient for commercial applications. The ionic conductivities seff of macroporous membranes soaked in LP30 are reported in Fig. 8, which shows also the conductivity for the pure electrolyte and for Celgard1 2400. In the temperature range we investigated (0 to 60 C), PVdF-0P samples exhibited the highest conductivities (1.89 to 6.55 mS cm-1 at 0 C and 60 C respectively, 3.73 mS.cm-1 at 25 C) among the separators tested herein, in agreement with the fact that it possesses the highest porosity (76%), while the least porous Celgard1 2400 (less than 40% of porosity [12]) presents the lowest performance. The addition of NCC leads to a decrease in ionic
conductivity, down to 1.45 mS.cm1 for PVdF-20P at 25 C, which also ehxibits the lowest porosity (61%). Nonetheless, it is still performing better than the Celgard1 separator. The measured conductivities of non-reinforced PVdF and Celgard1 are in agreement with previous measurement by Djian et al. [13] and Karabelli et al. [19] on macroporous PVdF separators presenting a porosity of 80%. It is also worth noticing that, for high NCC loadings, the conductivities are slightly less enhanced with the temperature increase, i.e. PVdF-0P is 3.5 times more conductive than PVdF-20P at r.t., but this ratio increases to 3.9 at 60 C. This is likely to be due to the decreasing amount of PVdF polymeric phase which can be swollen by the electrolyte (NCC is assumed not to swell) and thus contribute to the ionic transport. At the same size, an incomplete NCC network might be less effective against the reduction of the mean pore size due to swelling and compression of the macroporous membrane. Although the porosity of a separator is a major factor in obtaining high conductivity, it is not the only one, since the pores have to be completely filled by the liquid electrolyte. Assuming that the swelling by the electrolyte of the PVdF polymeric interpores phase did not significantly contribute to the overall conductivity, the conductivity will be affected by the interconnection between pores, as previously observed by Djian et al. [13]. The MacMullin number, NM, and tortuosity, t , parameters are used to characterize this behaviour. Both are defined by Eqs.7 and 8: NM = s0/seff
(7)
t = (NM x e)1/2
(8)
where s0 is the conductivity of the pure liquid electrolyte, seff is the conductivity of the porous membrane plus liquid electrolyte pair, and e is the porosity ratio. The calculated values presented in Table 5. The MacMullin numbers, NM, range between 3 and 16. As expected, the highest pore volume led to the lowest NM value (i.e., the highest conductivity). The tortuosity increases with the NCC loading, which may be associated to the decrease of the average pore size. It is worth noticing that for comparable tortuosity values (2.19 vs 2.26), PVdF-20P still presents a 2-fold increase over Celgard1 2400, thanks to an increased porous volume. For the practical use of separators within batteries, the wetting ability of the separator is crucial; in this regard, we deem that the large pore size and the presence of polar fillers are certainly a technological asset of our products. However, the large pore size might also be a drawback for the long-term cyclability of the devices, since it doesn’t permit the separator to effectively act as a barrier against particles migration between the electrodes. Finally, the addition of NCC induces a decrease of both the pore volume and the pore size, which appear largely responsible for the conductivity decrease, whereas it is clearly advantageous from a safety point of view. In order to optimize these porous structures, further studies are currently being conducted.
Table 5 Conductivity, MacMullin number, and tortuosity at 25 C for the nanocomposite porous membranes and Celgard1 2400.
Fig. 8. Ionic conductivity between 0 and 60 C for PVdF-CNC macroporous separators soaked in LP30.
Sample
seff (mS cm1)
NM
t
PVdF-0P PVdF-6P PVdF-12P PVdF-20P Celgard1 2400
3.73 0.4 2.53 0.30 2.08 0.15 1.45 0.10 0.73 0.10
3.16 4.66 5.66 8.13 16.10
1.57 1.79 1.92 2.19 2.26
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4. CONCLUSIONS Nanocomposites macroporous separators of PVdF/NCC were successfully prepared by phase inversion. Their physicochemical and thermo-mechanical properties were fully investigated, and the results were compared to those obtained for reference dense membranes produced by hot-pressing. SEM and TGA analyses showed that this process does not affect the final loading of NCC in the porous nanocomposites, and decreases the porosity and the pore size of the macroporous separators. DSC measurements shown that the cellulose whiskers present a negligible effect on the amount of crystallinity of the polymer matrix. We confirmed that the use of polar whiskers induces the preferential formation of the polar b phase of PVdF, and improve the wettability of the macroporous separators by polar electrolytes. The addition of the cellulose whiskers induces a large increase in mechanical strength, in both dense and macroporous membranes, up to 4-fold for the latter. The successful formation of a rigid percolating network in the dense PVdF membranes was observed for the first time. For macroporous PVdF, the ionic conductivity decreases by adding NCC, due to the porosity decrease and the lower pore interconnection; nonetheless, macroporous membranes exhibit a largely improved conductivity with respect to a commercial standard. The use of NCC to reinforce hydrophobic macroporous separators is an attractive concept, as it allows for it to improve their mechanical strength while maintaining appropriate conductivity levels for lithium-ion batteries. ACKNOWLEDGEMENTS Financial support from the European Commission within the AMELIE project (265910) under the Seventh Framework Programme (7th FWP) is gratefully acknowledged. Dr Grégory Chauve (FPInnovations) is gratefully acknowledged for his precious advice concerning the cellulose dispersions. We also gratefully thank Mrs. Cécile Bruzzese and Prof. Alain Dufresne (LGP2) for their help with mechanical characterizations. Mr. Riccardo Scarrazini and Dr. Houssemeddine Benattia are acknowledged for their help in the membranes preparation. Solvay is acknowledged for the supply of PVdF. References [1] C. Chauvin, F. Alloin, C. Iojoiu, J.-Y. Sanchez, New polymer electrolytes based on ether sulfate anions for lithium polymer batteries: part II: conductivity and transport properties of blended and cross-linked ionomers, Electrochimica Acta 51 (2006) 5954–5960. [2] A. Nishimoto, K. Agehara, N. Furuya, T. Watanabe, M. Watanabe, High ionic conductivity of polyether-based network polymer electrolytes with hyperbranched side chains, Macromolecules 32 (1999) 1541–1548. [3] R. Bouchet, S. Maria, R. Meziane, A. Aboulaich, L. Lienafa, J.P. Bonnet, T.N.T. Tran, D. Bertin, D. Gigmes, D. Devaux, R. Denoyel, M. Armand, Single-ion BAB triblock copolymers as highly efficient electrolytes for lithium-metal batteries, Nature Materials 12 (2013) 452–457. [4] F. Croce, L. Persi, B. Scrosati, F. Serraino-Fiory, E. Plichta, M.A. Hendrickson, Role of the ceramic fillers in enhancing the transport properties of composite polymer electrolytes, Electrochimica Acta 46 (2001) 2457–2461. [5] M.A.S. Azizi Samir, F. Alloin, J.-Y. Sanchez, N. El Kissi, A. Dufresne, Preparation of Cellulose Whiskers Reinforced Nanocomposites from an Organic Medium Suspension, Macromolecules 37 (2004) 1386–1393. [6] B. Scrosati, C.A. Vincent, Polymer electrolytes: the key to lithium polymer batteries, MRS Bulletin 25 (2000) 28–30. [7] J. Saunier, F. Alloin, J.Y. Sanchez, Electrochemical and spectroscopic studies of polymethacrylonitrile based electrolytes, Electrochimica Acta 45 (2000) 1255–1263. [8] A.M. Stephan, Review on gel polymer electrolytes for lithium batteries, European Polymer Journal 42 (2006) 21–42. [9] S.S. Zhang, A review on the separators of liquid electrolyte Li-ion batteries, Journal of Power Sources 164 (2007) 351–364. [10] K. Murata, S. Izuchi, Y. Yoshihisa, An overview of the research and development of solid polymer electrolyte batteries, Electrochimica Acta 45 (2000) 1501–1508.
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