Macromolecular NanotechnologyEffect of nano-clay on ionic conductivity and electrochemical properties of poly(vinylidene fluoride) based nanocomposite porous polymer membranes and their application as polymer electrolyte in lithium ion batteries
Graphical abstract
Highlights
► PVdF–clay nanocomposite membranes are prepared by phase inversion method. ► Composite polymer gel electrolytes had unique transport and electrochemical properties. ► The cells assembled with composite polymer gel electrolyte exhibited better battery performance. ► Nano-clay plays a key role in the electrochemical properties of composite polymer gel electrolyte.
Introduction
Polymer gel electrolytes (PGEs) prepared by soaking porous polymer membranes in liquid electrolytes provide safer and lighter high performance batteries with leak proof construction as compared to traditional liquid electrolytes [1]. Polymers such as poly(vinylidenefluoride) (PVdF) [2] and its co-polymer poly(vinylidenefluoride-co-hexafluoropropylene) {P(VdF-co-HFP)} [1], poly(methyl methacrylate) (PMMA) [3], poly(acrylonitrile) (PAN) [4] and its blends [5] have been widely studied as host polymers for preparing PGEs. Among these, fluoro-polymers have received great attention due to their high dielectric constant and affinity to electrolyte solution. PGES based on them shows high ionic conductivity (in the range of 10−4–10−3 S cm−1 at room temperature) and good electrochemical stability [1], [2]. Also, PVdF possesses many remarkable properties, such as good thermal stability under operating and processing temperature, non-flammability, excellent chemical resistance in combination with very low creep and high mechanical strength. Unfortunately, pristine PVdF is partially soluble in organic liquid electrolyte that is generally used for preparation of PGEs [5]. This could result in loss of mechanical strength and may cause internal short circuit leading to cell failure. The crystalline part of PVdF hinders the migration of Li+ ions and hence the batteries with PGEs based on PVdF shows lower charge/discharge properties and poor rate capability [6]. Recently many groups have reported that these problems can be resolved by the addition of nano-sized ceramic fillers [1], [7], [8], [9], [10]. Prasanth et al. [7] studied the influence of Al2O3, BaTiO3 and SiO2 on the electrochemical properties of polymer electrolytes (PEs) based on P(VdF-co-HFP). These studies showed that the incorporation of ceramic fillers caused an enhancement in ionic conductivity, ion transference number and lithium electrode–electrolyte interfacial stability. Wall et al. [8] also reported that addition of nano-fillers (SiO2) in poly(ethylene glycol) dimethyl ether causes an increase in elastic modulus and mechanical strength, and simultaneously an increase in ionic conductivity due to the increase in flexibility of the polymer chains and number of charge carriers. Addition of filler introduces the Lewis acid–base interaction between the polar surface group of the inorganic filler and electrolyte ionic species, which generates additional sites for ion migration thus improving ion mobility and ionic conductivity. In addition, ceramic nano-fillers enhance the mechanical properties of PEs. Composite PEs incorporating SiO2, BaTiO3, Al2O3, ZrO2 and TiO2 are widely reported [1], [7], [9], [10]. The main cause of lower ionic conductivity of PGEs is attributed to the ion pair formation, due to the columbic interaction between cations and anions, which reduces the number of mobile charge carriers. It is reported that the incorporation of nano-clay into the host polymer matrix is an effective method to minimize the ion pair formation [11]. The presence of clay in the PEs could directly affect the mobility of cations while avoiding the mobility of counter anions [12]. The cationic charges on the surface of clay platelets act as Lewis acid centers and compete with Li+ cations to form complexes with the polymer. This causes structural modification and promotion of Li+ conducting pathways and lowering of ionic coupling, which promotes the lithium salt dissociation. Polymer–clay nanocomposite also shows superior thermal stability, dimensional stability, chemical resistance and barrier properties by the addition of 0.5–5 wt.% nano-clay [12], [13], [14], [15], [16], [17], [18]. In-situ intercalation polymerization, solution intercalation and polymer melt intercalation are the principal processing routes used for preparing polymer–clay nanocomposites [19]. The solution intercalation approach adopted in the present study is considered as a simple and effective method for preparing polymer–clay nanocomposite. There are only a few reports on polymer–clay nanocomposite PGE based on PVdF and its co-polymer P(VdF-co-HFP) [13], [14], [20].
In the present study, an attempt is made to develop an organic–inorganic composite PGE for lithium ion batteries (LIBs). There are few related reports on PGEs based on polymer–clay nanocomposites [12], [13], [14], [15], [16], [17], [18]. The intercalated or exfoliated clay plays a distinct role in ion conduction in these electrolytes. Fully exfoliated clay morphology is expected to yield the highest ionic conductivity. It is reported that ultra sonication is a good method to achieve delamination of layered nano-clay to prepare polymer–clay nanocomposite [21]. In the present work clay suspension is prepared by ultrasonication. To our best knowledge this is the first report on polymer–clay nanocomposite based PGEs with high porosity and controlled properties prepared by wet non-solvent phase inversion processing method. Porous PVdF–clay nanocomposite membranes with varying clay content are prepared and activated with 1 M LiPF6 in EC/DEC. PVdF is particularly selected in this study due to its partial solubility in EC/DEC helps in forming a gel with leak proof construction while the presence of clay platelets maintain the mechanical stability of the nanocomposite PGE. The electrochemical properties of resulting PGEs show their suitability for use in lithium ion batteries.
Section snippets
Preparation of PVdF–clay nanocomposite
Poly(vinylidenefluoride) (PVdF), Mw = 53,4000 (Sigma–Aldrich) was vacuum dried at 60 °C for 6 h before use. Di-tallow dimethylamine (quaternary amine) treated Nanomer 1.28 E natural Wyoming montmorillonite/smectite (Hydrated Sodium Calcium Aluminum Magnisium Silicate Hydroxide) (Nanocor) clay was selected, because it is relatively inexpensive, widely available and has good intercalation capability. In order to allow dispersion of clay in an organic medium like PVdF, a surface modification of clay is
Morphological characterizations
Scheme 2 shows the schematic representation of the different types of polymer–clay nanocomposites formed due to the intercalation of polymer into the clay galleries. The first one is the conventional microcomposite (Scheme 2a), where the polymer molecules are unable to intercalate into the clay galleries and it leads to the formation of phase separated composite. The properties of these composites are similar to the traditional micro composite. The second is the intercalated polymer–clay
Conclusions
Self supported PVdF–clay nanocomposite PGEs containing 0–4 wt.% clay loading are prepared by wet non-solvent phase inversion method and their electrochemical properties have been evaluated. The intercalation/exfoliation of the clay into the polymer matrix is confirmed by XRD. Addition of organically modified clays to PVdF promotes the phase transformation of the polymer crystals. The PGEs are prepared by encapsulating the micro porous membranes with 1 M LiPF6 in EC/DEC. The liquid electrolyte is
Acknowledgments
This work was supported by funding from the National Research Foundation, Clean Energy Research Project, Grant number NRF2009EWT-CERP001-036. The authors also thankfully acknowledge Nutan Gupta, W.F. Mak and V. Aravindan for helpful discussion and assistance to finish this work.
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