Polymeric Gel Electrolyte Consisting of Network Polymer Using Tri-Functional Glycerol Ester for Li-Ion Batteries

Abstract

Development of a safer electrolyte has become a key issue for realizing larger sizes of lithium-ion batteries (LIBs) for energy storage and transportation uses. Flame-retardant additives and/or ionic liquids are considered to be effective components that form safer electrolyte solutions. However, those systems still have risks coming from electrolyte leakage due to their liquid states. Solidification of the electrolyte is one of the effective ways to improve the safety of the battery system. In order to ensure sufficient ionic conductivity, a number of polymeric gel electrolyte systems have so far been proposed. We have investigated in this paper a novel polymeric gel electrolyte based on a network polymer having tri-functional glycerol ester for lithium-ion batteries. A triply branched monomer possessing a terminal olefin unit was newly synthesized from glycerol ethoxylate (M n~1,000) and 10-undecenoly chloride (3 eq.) (Fig. 1 (a)). The electrolyte consisting of 1 M LiPF6/ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 by vol.) was added to a mixture of trimethylolpropane tris(3-mercaptopropionate) (Fig.1 (b)) and the triply branched monomer with a smaller amount of benzophenone sensitizer (1:1:0.05 by molar ratio). The resulting mixed solution was applied to a glass plate, and was broaden. Finally, the monomer-coated glass plate was exposed to UV irradiation to get a gel film. A transparent polymeric gel electrolyte was obtained as a self-standing film. The ionic conductivity of the gel electrolyte was measure by an AC method and found to be 5.2, 3.4, and 2.8 mS cm-1 (at 297 K) for the films containing 10, 20, and 30 wt% of polymer matrix, respectively. The electrochemical stability toward anodic oxidation was investigated for the gel electrolyte by linear sweep voltammetry (LSV). The anodic oxidation current was observed as a current peak at 4.2 V. This was attributed to the oxidative decomposition of the thiol group in the polymer matrix1). Thus, the present polymeric gel electrolyte can be used in a potential range up to 4.2 V. The application to the graphite negative and lithium iron phosphate (LiFePO4) electrodes will be reported in the presentation. Reference 1) M.-H. Ryou, et al., Electrochim. Acta, 60, 23 (2012). Figure 1