Silica-Polyethylene Glycol Hybrid Organic-Inorganic Solid Electrolytes By Sol-Gel for Lithium-Ion Battery Applications

Among the many applications proposed for solid electrolytes, their employment in lithium-based secondary batteries has the greatest impact in terms of technology and market potential [1]. Rechargeable thin film microbatteries have recently become the topic of widespread research for use in low power applications and energy storage systems for electric devices [2]. Common rechargeable batteries are based on a liquid electrolyte, which implies that there are several restrictions for their design and size due to the available separators and liquid electrolytes. Secondly, these liquid electrolytes carry the inherent risk of leakage [3]. Therefore the need for all-solid-state microbatteries arises, which can facilitate miniaturization, will create more flexibility for the design of stand-alone microelectronic devices [4]. In battery applications, the ionic conductivity is of paramount importance as the power density of the device depends on the electrolyte performance. Lithium-ion solid electrolytes offer many advantages such as ease of manufacturing, immunity from leakage, suppression of lithium dendrite formation, elimination of volatile organic liquids and mechanical flexibility. However, the main problem in achieving the expected applications is their low ionic conductivity [5]. The suitable combination of inorganic and organic materials into hybrid nanostructured thin film electrolytes could result in an interesting combination of properties able to substitute the liquid electrolyte [6]. This work reports on the preparation and structural properties of hybrid ionic conductor silica (SiO₂)-Polyethylene glycol (PEG_n) using a sol-gel approach. The materials were prepared by sol-gel method: Tetraethoxysilane (TEOS), Polyethylene glycol (PEGn) and Bis(trifluoromethane)sulfonimide lithium salt LiN(CF₃SO₂)₂ (LiTFSA) in a desired [O]/[Li] ratio (where oxygen considered are only those of the ether type), have been used as precursors. Several compositions were studied in order to evaluate the ionic conductivity and electrochemical stability. The thus obtained solutions were cast onto silicon moulds to obtain the desired solid membranes, having a translucent appearance, with thicknesses about 0.1 cm. The optimization of the processing parameters leads also to homogeneous and translucent coatings of few microns. The structural characterization was carried out by FTIR, Raman and RMN. The electrical properties of the materials have been determined by Electrochemical Impedance Spectroscopy (EIS). The lithium transference number and electrochemical stability window were analyzed to evaluate compatibility with the lithium metal and other possible electrodes. Ionic conductivities about 10^-5 S/cm at room-temperature has been found for [O]/ [Li] = 10.6 and PEG₄₀₀/TEOS = 1.0 ratios (Figure 1).

References

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[6] M. Aparicio, A. Jitianu, L. Klein, (eds.). “Sol-Gel Processing for Conventional and Alternative Energy, Advances in Sol-Gel Derived Materials and Technologies”. Ed. Springer Science + Business Media, New York, 2012.

Figure 1. Ionic conductivity values as a function of ratio [Li]/ [O].