Characteristics of Gel Electrolyte Composed of PEO-Based Polymer / Solvated Ionic Liquid for High Performance Li-S Batteries

Abstract

IntroductionLithium-sulfur (Li-S) batteries as new secondly battery system type are attracted attention. The sulfur positive electrode of this system have theoretical capacity of 1672 mAhg-1, which shows approximately 10 times higher than conventional Li-ion battery positive electrode material (LiCoO2). The problem of this Li-S batteries is dissolution of sulfur component of positive electrode which reached a state of reaction intermediate (Li2S m : m=2~8) into electrolyte during charge / discharge processes. The dissolution and diffusion of Li2S m into electrolyte solution lead significant degradation of cycle characteristics and irreversible side reaction of Li-S batteries. Currently, as functional electrolyte solution that suppresses dissolution of Li2S m , “Solvate Ionic Liquid (SIL)” has been proposed [1]. SIL is equimolar mixed electrolyte of glyme solvent and Li salt, and has strongly interaction between glyme / Li+ to form complex cation. SIL was suppressed dissolution of Li2S m because of weak Lewis acidity / basesity. By application of SIL electrolyte, Li-S battery exhibited stable charge / discharge reaction more than 800 cycles [2]. However, equimolar composition of SIL was destroyed at electrolyte / electrode interface during charge and discharge, and free glyme which have not coordinated Li+ is temporary formed. Thus, it is dynamically possible that Li2S m dissolution into generated free glyme [3]. To solve this problem, we attempted increasing carrier cation (suppression of free glyme generation), solidification of SIL electrolyte by introduction of polymer matrix (diffusion suppression of dissolved chemical species).ExperimentalIn this study, all procedures of sample preparation were conducted in argon-filled glovebox. At first, as highly concentrated SILs, [Li a (G4)1]TFSA a (a = 1 and 1.25) (G4 : CH3-O-(C2H4O)4-CH3, LiTFSA: LiN(SO2CF3)2 = 1 : a) were prepared [4]. Gel electrolyte were prepared by mixing of SILs (a = 1, 1.25), polyether-based P(EO/PO) macromonomer and photo initiator. Composition ratio of gel electrolyte were SILs : polymer = 7 : 3, 8 : 2, 9 : 1 (wt%), respectively. After stirring, samples casted to glass substrate and polymerized by UV irradiation to obtain thin-films, which was transparent and self-standing gel electrolyte films. Prepared electrolyte films ware evaluated by differential scanning calorimetry (DSC) and AC impedance measurements.Result and discussionFig.1(a) show DSC thermograms of prepared liquid and solid electrolytes. All electrolytes exhibited glass transition temperatures (T gs). Gel electrolyte showed lower T g than that neat polymer electrolyte (without SIL). From the results, it was considered that mobility of PEO chain was increased by SIL electrolyte solutions. With increasing SIL amount into the gel electrolyte, T g appeared low temperature side, and therefore, the gel electrolyte with high SIL composition should indicate high ionic conductivity.Fig.1(b) shows temperature dependence of the ionic conductivity (σ) of gel electrolytes, respectively. Owing to increase of viscosity with high Li salt concentration, σ value of high Li salt composition SIL electrolyte (a=1.25) was lower than equimolar SIL (a=1). At 303 K, σ of [Li1(G4)1]TFSA1 : polymer = 8 : 2 electrolyte was 1.5×10−3 Scm−1, which was almost same σ value with SIL electrolyte solution.Fig.1(c) shows charge/discharge curves of Li-S cell using gel electrolyte of [Li1(G4)1]TFSA1 : polymer = 8 : 2. The Li-S battery using the equimolar SIL of gel electrolyte was confirmed first discharge capacity of 1100 mAhg-1. However, this system showed unstable voltage profiles during charge process, and this charge capacity exhibited larger than that discharging capacity. From the unstable and large voltage in this charging process, Li2S m was dissolved into gel electrolyte and side reactions occurred, was considered. This result correlates with the report that the co-migration of glyme molecule and Li cation is destroyed in when PEO matrix introduction by NMR spectroscopy method measurement [5].