Polymer Electrolytes Containing Solvate Ionic Liquids: A New Approach To Achieve High Ionic Conductivity, Thermal Stability, and a Wide Potential Window

• Published in: Chemistry of materials 2018-01, Vol.30 (1), p.252-261
• Main Authors: Kitazawa, Yuzo, Iwata, Kaori, Kido, Ryosuke, Imaizumi, Satoru, Tsuzuki, Seiji, Shinoda, Wataru, Ueno, Kazuhide, Mandai, Toshihiko, Kokubo, Hisashi, Dokko, Kaoru, Watanabe, Masayoshi
• Format: Article
• Language: English
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/acs.chemmater.7b04274

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 SIL-incompatible A segment (polystyrene, PSt) and SIL-compatible 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 self-assembly 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.