Preparation of porous, chemically cross-linked, PVdF-based gel polymer electrolytes for rechargeable lithium batteries

Abstract

This study reports the development of a new system of porous, chemically cross-linked, gel polymer electrolytes based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF–HFP) copolymer as a polymer matrix, polyethylene glycol (PEG) as a plasticizer, and polyethylene glycol dimethacrylate (PEGDMA) as a chemical cross-linking oligomer. The electrolytes are prepared by a combination of controlled evaporation and thermal polymerization of PEGDMA. PVdF–HFP/PEG/PEGDMA gel polymer electrolytes with a composition of 5/3/2 exhibit both high ambient ionic conductivity, viz., >1 mS cm⁻¹, and a high tensile modulus of 52 MPa, because of their porous and network structures. All the blends of electrolytes are electrochemically stable up to 5 V versus Li/Li⁺ in the presence of 1 M LiPF₆/ethylene carbonate–diethyl carbonate (EC–DEC). With these polymer electrolytes, rechargeable lithium batteries composed of carbon anode and LiCoO₂ cathode have acceptable cycleability and a good rate capability.

Introduction

Many gel polymer electrolytes comprising polymer matrices, plasticizing organic solvents and alkali metal salts have been intensively studied for applications in rechargeable lithium batteries and other electrochemical devices [1], [2], [3]. Electrolytes with reduced thickness and improved ionic conductivities are technical goals for increasing the power density. Gel polymer electrolytes in the form of very thin films act simultaneously as transport for lithium ions, separator, and binder between the negative (anode) and positive (cathode) electrodes. Although gel polymer electrolytes with high ionic conductivity (>1 mS cm⁻¹) can usually be achieved by adding large amounts of organic solvents, they do not have sufficient mechanical ruggedness to withstand winding and stacking during manufacturing, or the stress that arises from morphological deformation of the electrode during repeated charge–discharge [3], [4], [5], [6].

Various approaches to increasing the mechanical strength have been proposed recently. Inorganic fillers, such as fume silica, zeolite, clay, Al₂O₃ or glass fiber have been added to strengthen the dimensional stability of gel polymer electrolytes [7], [8], [9], [10], [11]. Microphase-separated method has also been adopted and found to be effective. Systems such as PEO/PMMA [12], PVdF/HDPE [13], PEG/PEO [14] and PMMA/PVC [15] fall into this category. Microporous membranes of polyolefins such as polyethylene and polypropylene, impregnated with gel polymer electrolytes have been developed [5], [16]. In these membrane-supported electrolytes, polyolefins possess excellent mechanical properties and the gel can trap electrolyte solution without leakage. Another type has been prepared via the chemical cross-linking method [17], [18], [19], [20]. Most gel systems today are prepared by forming temporary physical cross-links via physical interactions such as crystallites or hydrogen bonds between the chains. These gel polymer electrolytes may undergo solvent exudation from the physical cross-linked structure on long storage, and may also dissolve and lose their mechanical integrity at high temperature [21]. By contrast, chemically cross-linked gel polymer electrolytes possess chemical cross-links that are permanent and remain thermally stable up to the decomposition temperature of the components. Hence, the chemical cross-linking method clearly shows not only better dimensional stability, but also thermal stability. It exhibits no solvent exudation and is easy to control [21], [22]. Therefore, this method has been used in the present study to develop new gel polymer electrolytes.

Beside the concern over mechanical strength, the process to fabricate gel polymer electrolytes requires a moisture-controlled environment because of the high sensitivity of lithium salts to water, which amounts to high equipment cost and inconvenient operation. Researchers of Telcordia Technologies (formerly Bellcore) overcame these difficulties and developed microporous poly(vinylidene fluoride-co-hexafluoropropylene (PVdF–HFP)-based polymer electrolytes [23], [24], [25], [26], in which the microporous structures made by the plasticization/extraction procedure can enhance the electrolyte uptake, thus improving the ionic conductivity and rate capability. Consequently, a critical moisture-controlled environment is only needed during the last activation step. Nevertheless, the extraction step is still inconvenient—it increases the production cost and presents safety concerns related to the handling of large volumes of extracting solvents. Therefore extraction-less processes to prepare microporous polymer electrolytes have been developed by using volatile plasticizers [27], a phase inversion process with controlled evaporation (solvent/non-solvent) [28], [29], [30], and liquid immersion [31], [32], [33].

In this study, an attempt is made to combine both the chemical cross-linking method and the controlled evaporation process to prepare porous, chemically cross-linked, gel polymer electrolytes. The advantage of this combined approach is that the ionic conductivity and mechanical properties can be controlled independently by designing properly the structure of the polymer electrolyte, and by selecting a suitable liquid electrolyte solution. Furthermore, the polymer membranes can be manufactured in an ambient environment.