Characterization of poly(vinylidenefluoride-co-hexafluoropropylene)-based polymer electrolyte filled with rutile TiO2 nanoparticles

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Abstract

Various amounts of nanoscale rutile TiO2 particle are used as fillers in the preparation of poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP)-based porous polymer electrolytes. Physical, electrochemical and transport properties of the electrolyte films are investigated in terms of surface morphology, thermal and crystalline properties, swelling behavior after absorbing electrolyte solution, chemical and electrochemical stabilities, ionic conductivity, and compatibility with lithium electrode. Contrary to reported inorganic fillers showing the maximum content lower than 50 wt.%, the self-supporting polymer electrolyte films can be obtained even when using higher content of 70 wt.% rutile TiO2 nanoparticles. The physical and electrochemical properties of polymer membrane are highly improved by the addition of TiO2 nanoparticles as good dispersion of fillers, low liquid uptake but adequate ionic conductivity, excellent electrochemical stability, and stabilized interfacial resistance with lithium electrode. An emphasis should be put on the fact that the sufficient ionic conductivity obtained is led by the liquid medium within nano-pores as well as effective ion transport supported by rutile TiO2. As a result, the sample with 30–40 wt.% rutile TiO2 is confirmed as the best polymer electrolyte for rechargeable lithium batteries.

Introduction

Over the last decade, rechargeable lithium batteries have been an essential power source in portable electronics such as cellular phone, laptop computer, etc. Recent studies for the battery are almost focused on the material examinations to achieve higher performances of high capacity, long cycle life, and improved safety. In material study for the battery, the materials for good polymer electrolyte can be significant in allowing higher ionic conductivity, effective lithium-ion transport, and the interfacial stability with lithium metal electrodes.

Polymer electrolytes based on poly(vinylidenefluoride) (PVdF) have been studied widely in various types such as gel (immobilized liquid electrolyte in polymer matrix) [1], [2] and hybrid membranes (captured liquid within porous polymer film) [3], [4]. These variations have been made for satisfying at once two conflicting properties of high ionic conductivity (dependable on the liquid) and good mechanical strength (on the polymer matrix). Recently, poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP) has been shown a high potential as the polymer electrolyte material [3], [4], [5], [6], [7], [8], [9], [10] of rechargeable lithium batteries because of its high solubility and the lower crystallinity and glass transition temperature. The success of PVdF-HFP polymer electrolytes is now broadly confirmed by the developments in lithium polymer batteries [11], [12].

It has been known that the addition of nanoscale inorganic fillers, such as alumina (Al2O3), silica (SiO2), titania (TiO2), to the polymer electrolyte resulted in the improvements of transport properties as well as mechanical and electrochemical properties [9], [10], [13], [14], [15]. Particularly, the TiO2 nanoparticles in the polymer electrolyte were confirmed to play useful roles in increasing the ionic conductivity and the cation transference number [16], which were not attributable to the polymer segmental motion but probably to the dipole interaction of TiO2 with the polymer component [17]. Moreover, the previous work [10] using anatase TiO2 nanoparticles showed much lower interfacial resistance between the polymer electrolyte and lithium metal electrode by the solid-solvent role of the filler.

As a next step of PVdF-HFP/TiO2 (anatase) system studied previously [10], we continue to investigate the PVdF-HFP polymer electrolyte system with the nanocrystalline rutile TiO2. Anatase and rutile TiO2's have the same chemistry but different structures: the octahedrons of the former share four edges and thus the four fold axis, whereas those of the latter share two edges with other octahedrons and form chains. Though it becomes difficult to extract definitely the effect of crystalline phase difference due to somewhat crystallographical complexity, the present work can tell at least what role or interaction the nanoscale rutile TiO2 plays or associates in the PVdF-HFP/TiO2 (rutile) polymer electrolyte system. From this study, the addition effect of rutile TiO2 will be also investigated as well as the determination of its optimal content showing the best electrochemical and transport properties as the polymer electrolyte for rechargeable lithium batteries.

Section snippets

Experimental

The PVdF-HFP and TiO2 used were, respectively, a commercially available fluoro-copolymer KynarFlex® 2801 (Atofina Chemicals, 12 mol% of hexafluoropropylene) and a rutile-type titanium oxide powder (ST-480, Titan Industry, Japan) having the primary particle size of 20–30 nm. Supplier of ST-480 said that the particle surfaces were passed through some Al2O3/SiO2 pre-treatment to allow surface hydrophilicity, and we worked on with the TiO2 under ultralow humidity condition (in a dry room). Prior to

Results and discussion

As mentioned in the experimental section, it is noticeable that the self-supporting polymer films filled can be obtained even when the rutile TiO2 nanoparticle is added up to more than 70 wt.%. This is very dissimilar to the fact that the polymer films filled with SiO2 [9] and anatase TiO2 [10] could be prepared with the maximum filler contents of 35 and 50 wt.%, respectively. It may be considered that the higher filler content could be given by the high dispersion ability and good

Concluding remarks

Physical and electrochemical characterization of the PVdF-HFP/TiO2 (rutile) polymer electrolyte using the electrolyte solution of 1 M LiPF6/(EC/DMC) (1:1) are carried out in the present study. The addition of rutile TiO2 nanoparticles to the PVdF-HFP polymer matrix leads eventually to improve the mechanical strength and to preserve the polymeric feature even for high filler content. Also, it may provide low liquid uptake but adequate ionic conductivity by the some dipole interaction of rutile

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