Poly(vinylidene fluoride)–polydiphenylamine composite electrospun membrane as high-performance polymer electrolyte for lithium batteries
Introduction
Polymer electrolytes consist of salts dissolved in solid polymers hold a key role in realizing the major goal of an all-solid-state rechargeable lithium battery [1], [2], [3]. Amorphous polymer electrolytes have been studied intensively for more than 30 years, however, the performance of the electrolyte has not increased substantially over that period. Conductivity in the order of ∼mS cm−1 was achieved in these systems. To enable high performance of the lithium battery, a polymer electrolyte should possess ionic conductivity greater than mS cm−1, good dimensional and thermal stability, an electrochemical stability window of ≥5.0 V, chemical compatibility with Li electrodes, ability to afford Li cycling (recharge) at an efficiency of greater than 99%. Therefore, it is essential to develop polymer electrolytes to suit for the modern needs in the power sources.
Poly(vinylidene fluoride), PVdF with its high mechanical stability and chemical inertness has been used as polymer electrolytes in lithium batteries [4]. PVdF is inherently polar due to the presence of electronegative fluorine atoms in the backbone structure [5]. The crystalline domains in PVdF hinder migration of lithium ions and lower the ionic conductivity. Polydiphenylamine (PDPA), a polymer of N-substituted aniline, is more soluble in common organic solvents [6], exhibits different redox characteristics than other poly(N-substituted anilines) [7], [8]. Recent studies show that backbone units of PDPA can be grafted with other polymeric chains to have novel functional properties [9].
Electrospinning is an efficient fabrication process that gives fibrous and porous membranes with an average diameter ranging from 100 nm to 5 μm [10], which are at least one or two order of magnitude smaller than the fibers produced from melt or solution spinning. Electrospinning technology has recently been extended in various fields like preparation of porous filters, biomedical materials, reinforcing components, cloths for electromagnetic wave shielding, sensors, electronic devices, etc. [11], [12], [13]. Reports on using electrospinning for making non-woven mats of polymer composites consisting of conducting polymer and a conventional polymer are scarce. Blends of polyaniline with poly(ethylene oxide) in chloroform were electrospun to produce filaments in the range of 4–20 nm [14]. PEO provides adequate viscosity to the composite solution to achieve electrospinning.
In the present study, PVdF–PDPA composite nanofibrous membranes (PVdF–PDPA-CFM) were prepared though electrospinning. β-Naphthalene sulfonic acid (NSA)-doped PDPA is fairly soluble in DMF in which PVdF (10% w/w) can form viscous solution and this suits for electrospinning the composite [15]. The migration of lithium ions in PVdF is expected to be enhanced by forming composites with PDPA that can have physicochemical interactions with C–F groups in PVdF. Improved ion mobility and ionic conductivity are therefore expected. Further, the presence of PDPA can also reduce the bulk resistance of the electrolyte.
Section snippets
Preparation of polymer electrolytes
Polydiphenylamine was prepared by the oxidative polymerization of diphenylamine (50 mM in 1 M NSA) with potassium peroxodisulfate (0.2 M in 1 M NSA) at 5 °C [16]. The green colored precipitate (NSA-doped PDPA) was filtered, washed with 1 M NSA and dried in vacuum oven. Adequate amounts of PVdF and NSA-doped PDPA were dissolved in DMF/acetone mixture (7:3 v/v). Electrospun composite membranes were prepared by using the methodology, as described elsewhere [15]. Electrospinning of the composite solution
Morphology and structure
Fig. 1 shows the FESEM images of electrospun fibrous membranes depicting the morphological variations between pristine PVdF membrane and PVdF–PDPA-CFM. PVdF membrane is white in color, whilst PVdF–PDPA-CFM is green in color (viewed through optical microscope). FESEM images clearly show the morphological variations between pristine PVdF membrane and PVdF–PDPA-CFM. Electrospun PVdF fibrous membranes have a nearly straightened and tubular structure with an average diameter of ∼500 nm (Fig. 1a).
Conclusions
PVdF–PDPA composite nanofibrous membranes were prepared by electrospinning and the polymer electrolytes were prepared by soaking the porous mats into an electrolyte solution. The polymer electrolyte based on PVdF–PDPA-CFM shows superior performances in terms of ionic conductivity, electrochemical stability window and good interfacial behaviour with electrode and proved to be a promising material component for high-performance lithium batteries.
Acknowledgements
This work was supported by Korean Research Foundation Grant (KRF-2006-J02402 and KRF-2006-C00001). The authors acknowledge the Korea Basic Science Institute (Daegu) and Kyungpook National University Center for providing Scientific Instrumentation. We also thank Samsung SDI, CRD Energy Lab for recording charge-discharge profiles.
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