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Article Cite This: Chem. Mater. 2018, 30, 252−261
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Polymer Electrolytes Containing Solvate Ionic Liquids: A New Approach To Achieve High Ionic Conductivity, Thermal Stability, and a Wide Potential Window Yuzo Kitazawa,†,# Kaori Iwata,†,# Ryosuke Kido,†,# Satoru Imaizumi,† Seiji Tsuzuki,‡ Wataru Shinoda,§ Kazuhide Ueno,† Toshihiko Mandai,∥ Hisashi Kokubo,† Kaoru Dokko,† and Masayoshi Watanabe*,† †
Department of Chemistry & Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan ‡ Research Center for Computational Design of Advanced Functional Materials (CD-FMat), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan § Department of Materials Chemistry, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ∥ Faculty of Science and Engineering, Graduate School of Engineering, Iwate University, 4-3-5 Ueda, Morioka, Iwate 020-8551, Japan S Supporting Information *
ABSTRACT: We describe here the electrochemical properties and battery performance of polymer electrolytes composed of ABA-triblock copolymers and Li-glyme solvate ionic liquids (SILs), which consist of the [Li(glyme)]+ complex cation and bis(trifluoromethanesulfoly)amide ([TFSA]−) anion, to simultaneously achieve high ionic conductivity, thermal stability, and a wide potential window. Three different block copolymers, consisting of a SILincompatible A segment (polystyrene, PSt) and SILcompatible B segments (poly(methyl methacrylate) (PMMA), poly(ethylene oxide) (PEO), and poly(butyl acrylate) (PBA)) were synthesized. The SILs were solidified with the copolymers through physical cross-linking by the selfassembly of the PSt segment. The thermal and electrochemical properties of the polymer electrolytes were significantly affected by the stability of the [Li(glyme)]+ complex in the block copolymer B segments, and the preservation of the SILs contributed to their thermal stabilities and oxidation stabilities greater than 4 V vs Li/Li+. Pulsed-field gradient spin−echo nuclear magnetic resonance measurements of the polymer electrolytes and molecular dynamics simulation indicate that the [Li(glyme)]+ complex cation is unstable in the PEO matrix because of the competitive coordination of the PEO chain and glyme with Li+. On the other hand, the complex structure of [Li(glyme)]+ is stable in the PMMA- and PBA-based polymer electrolytes because of the weak interaction between Li+ and the polymer chains. By use of the PMMA- and PBA-based polymer electrolytes, 4-V class Li batteries with a LiCoO2 cathode and a Li metal anode could be operated stably at 60 °C; in contrast, this was not possible using the PEO-based electrolyte.
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INTRODUCTION Lithium-ion batteries (LIBs) have been widely used as power sources for portable devices because of their high-energy densities. In general, LIBs are comprised of a transition-metal oxide cathode, a carbon anode, and a nonaqueous electrolyte. The liquid electrolytes are composed of aprotic organic solvents such as ethylene carbonate and diethyl carbonate containing a Li salt. Liquid electrolytes possess high ionic conductivities, but their volatility, flammability, and leakage are serious issues affecting their practical use. To enhance the safety of LIBs, room-temperature ionic liquids (RTILs) comprised solely of ions are attractive electrolytes and have been studied extensively.1 RTILs have high dissociativities without the addition of solvents and have characteristic © 2017 American Chemical Society
properties such as negligible volatility, nonflammability, high ionic conductivity, and wide liquid temperature range. RTILs can be gelled with polymers2−4 and nanoparticles of inorganic materials5−7 and can be used as quasi-solid-state electrolytes. We have previously reported polymer gels composed of a three-dimensionally cross-linked polymer network and a RTIL, so-called ion gels.4,8 Ion gels also exhibit high ionic conductivities comparable to those of neat RTILs because the prepared ion gels contain significant quantities of RTILs, Received: October 11, 2017 Revised: December 12, 2017 Published: December 13, 2017 252
DOI: 10.1021/acs.chemmater.7b04274 Chem. Mater. 2018, 30, 252−261
Article
Chemistry of Materials
dynamics (MD) simulations. In addition, thermogravimetric measurements were carried out to evaluate the thermal stability of the gels. The electrochemical stability of the polymer electrolytes was investigated by means of linear sweep voltammetry (LSV) and cyclic voltammetry (CV). Finally, the polymer electrolytes were tested in Li/LiCoO2 cells to demonstrate the stable operation depending on the structures of SIL-compatible segments. Here we show that the stability of [Li(glyme)]+ in the polymer network correlated well with thermal and electrochemical stabilities, and cycle performance of Li/LiCoO2 cells. Among the polymer electrolytes examined, PBA-based polymer electrolytes showing high stability of [Li(glyme)]+ and higher ionic conductivity demonstrated charge−discharge cycling stability with high Coulombic efficiency and good rate capability.
greater than 70 wt %, and the ion transport in the gel is decoupled from the polymer chain dynamics.9 Conventional RTILs are composed of onium cations and counteranions. The use of an onium cation-based RTIL in LIBs requires that the Li salts are dissolved in the RTIL to provide Li+-ion conductivity. However, the addition of Li salts generally increases the viscosity of the RTILs, resulting in a decrease in the ionic conductivity of the electrolytes.10−12 Moreover, the Li+ cation transference number (tLi) in a binary mixture of a RTIL and a Li salt can become as low as 0.1.13 This is because, in addition to the Li+ ions, the onium cation and the anion are mobile in the electrolyte. Furthermore, the solvation of Li+ ions by anionic species increases the hydrodynamic radius of the Li+ ions, resulting in the lower mobility of the Li+ ion.14,15 These aspects lead to the poor performance of RTIL-containing LIBs. In this study, to achieve a high ionic conductivity and high tLi in the ion gels, we used solvate ionic liquids (SILs).16 SILs are defined as ILs consisting of a salt and ligands that strongly coordinate to the component ions of the salt.16 Equimolar complexes of lithium bis(trifluoromethanesulfoly)amide (Li[TFSA]) and glymes, such as triglyme (G3) and tetraglyme (G4), are representative SILs.17−19 In Li-glyme SILs, the glyme (G3 or G4) coordinates to Li+ ion like a crown ether, forming a complex cation such as [Li(G3)]+ or [Li(G4)]+.20 In the Li-glyme SILs, almost all the glyme molecules form complexes with Li+, and a number of uncoordinated glyme molecules is very low (
