Electrochemical characteristics of phase-separated polymer electrolyte based on poly(vinylidene fluoride–co-hexafluoropropane) and ethylene carbonate
Introduction
Gel polymer electrolytes made from a polymer and a liquid electrolyte have been extensively studied for use as a separator in lithium polymer battery. The investigated polymers, which incorporate liquid electrolytes, include poly(vinylidene fluoride) (PVdF) [1], [2] poly(vinylidene fluoride–co-hexafluoropropane) (PVdF–HFP) [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], poly(methyl methacrylate) (PMMA) [5], [15], [16], [17], [18], polyacrylonitrile (PAN) [5], [19], [20], poly(vinyl chloride) (PVC) [5], [21], poly(ethylene oxide) (PEO) [5], [22], [23], and their blends [24], [25]. Among these polymers, PVdF–HFP has received special attention as one of the promising host polymer for polymer electrolytes because of its excellent mechanical strength and electrochemical stability.
In general, the ionic conductivity of the polymer electrolyte is increased with the increase of the amount of the liquid electrolyte incorporated into the polymer matrix. Many researches have been focused on increasing the liquid electrolyte content in PVdF–HFP matrix by the control of morphology. According to the Bellcore's technology, nano-porous PVdF–HFP film is prepared by extracting dibutyl phthalate (DBP) from the film composed of PVdF–HFP and DBP, then, the liquid electrolyte is impregnated into the nanoporous PVdF–HFP film [3], [4]. In order to avoid annoying extraction process, the use of a volatile plasticizer such as dimethyl phthalate (DMP) or the use of a soluble plasticizer such as PEO oligomer instead of DBP were also suggested [6], [26]. It has been reported that the development of micropore in the PVdF–HFP matrix significantly enhances ionic conductivity owing to the increased electrolyte uptake, resulting in the improved rate capability of the corresponding lithium batteries [1], [6], [26], [27]. The addition of SiO2 was known to be an effective way to generate micropore in the PVdF–HFP matrix [5], [6], [13], [14]. The phase inversion method using non-solvent could also realize microporous PVdF–HFP matrix [13], [14]. Alcohol was generally selected as a non-solvent in this case. Similar works have been made for PVdF based polymer electrolyte; porous PVdF matrix was formed by phase inversion process with a solvent/non-solvent system [1], [2], [27]. According to the previous works, the extraction method and phase inversion method are effective for generating porous structure. However, the solvents used for casting, extraction, and phase inversion do harm to battery performance and should be removed throughout an extensive drying process.
In this paper, PVdF–HFP based polymer electrolytes were prepared by introducing liquid electrolyte into a PVdF–HFP polymer matrix obtained from the casting of a homogeneous mixture of PVdF–HFP and solvents such as ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). After most of DMC and EMC are evaporated in the cast, a solid film of EC and PVdF–HFP was obtained. The remaining EC was then filled with the liquid electrolyte by soaking the PVdF–HFP/EC film into the liquid electrolyte. Since the casting solvents are selected from the solvents used in lithium ion battery, the residual solvents do not harm to battery performance, and thus, excessive drying to remove residual cast solvent is not required in this system. In addition, the production of the suggested system does not include extraction procedure to generate porous PVdF–HFP matrix. The morphological, spectroscopic and electrochemical investigation of the newly suggested PVdF–HFP based polymer electrolyte were made with varying the composition of the PVdF–HFP/EC matrix. We examined any relationship between the structure of the PVdF–HFP matrix and the ionic conductivity of the corresponding polymer electrolyte. The performances of the lithium ion polymer cell employing the PVdF–HFP based polymer electrolyte were also evaluated.
Section snippets
Preparation of the polymer electrolytes
PVdF–HFP (Kynar 2801, Mw=4.77×105) with copolymer ratio of VdF/HFP=88/12 was dried in oven under vacuum at 60 °C before usage. EC, DMC, and EMC were used as received without any treatment. The PVdF–HFP/EC films were prepared by the following procedures. The PVdF–HFP was added to the mixture of EC, DMC, and EMC. The ratio of DMC/EMC was set to 1/1 (wt./wt.) and EC content was varied as 0, 18.7, 27.8, 38.1 and 43.5 wt.% based on the total weight of the mixed solvent. The ratio of PVdF–HFP and the
Morphology of the PVdF–HFP/EC films
Fig. 1 shows the SEM microscopic images (magnification: 5000) of the PVdF–HFP/EC films with various EC contents. The EC was extracted with methanol from the PVdF–HFP/EC films before taking SEM measurements to clearly observe the site which the EC occupies. The films cast from the mixture of PVdF–HFP, EMC, and DMC were transparent in appearance and did not show any discernable pore structure in the SEM image as shown in Fig. 1(a). Because DMC and EMC are good solvents for PVdF–HFP and evaporate
Conclusion
A microporous PVdF–HFP matrix can be formed from the mixture of PVdF–HFP and EC/DMC/EMC without any extraction process. The porosity of the PVdF–HFP matrix increases with the increase of EC content in the PVdF–HFP/EC film. A significant increase of the porosity was observed at EC content of 62.6 wt.% due to the formation of the bulk EC phase. Below 62.6 wt.% EC content, most of EC interact with PVdF–HFP forming a new crystal structure. The development of micropore significantly increases the
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