Redox-active glyme-Li tetrahalogenoferrate() solvate ionic liquids for semi-liquid lithium secondary batteries

• Published in: RSC advances Vol. 1; no. 7; pp. 4129 - 4136
• Main Authors: Kemmizaki, Yuta, Katayama, Yu, Tsutsumi, Hiromori, Ueno, Kazuhide
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 24.01.2020; The Royal Society of Chemistry
• Subjects: Anions; Catholytes; Cations; Chemistry; Electrolytes; Feasibility studies; Flammability; Ionic liquids; Ions; Liquid lithium; Lithium; Lithium batteries; Melting points; Raman spectra; Raman spectroscopy; Rechargeable batteries; Spectrum analysis; Storage batteries; Thermogravimetry
• ISSN: 2046-2069; 2046-2069
• DOI: 10.1039/c9ra10149g

Solvate ionic liquids (SILs), comprising long-lived, Li solvate cations and counter anions, serve as highly Li-ion-conductive and non-flammable electrolytes for use in lithium secondary batteries. In this work, we synthesized a series of novel redox-active glyme(oligoether)-Li salt-based SILs, consisting of a symmetric ([Li(G3)] + ) or asymmetric ([Li(G3Bu)] + ) triglyme-Li salt complex and redox-active tetrahalogenoferrate ([FeX] − (X = Br 4 , Cl 3 Br, Cl 4 )), for use as the catholyte in semi-liquid lithium secondary batteries. The successful formation of stable molten complexes of [Li(G3/G3Bu)][FeX] was confirmed by Raman spectroscopy and thermogravimetry. The melting point ( T m ) depended on both the molecular weights of the complex anions and the structures of the complex cations. [Li(G3)][FeCl 4 ] comprised complex cations with a symmetric structure, and the smallest complex anions showed the lowest T m of 28.2 °C. The redox properties of the [FeX] − /[FeX] 2− couple strongly suggested the suitability of [Li(G3/G3Bu)][FeX] as a catholyte. The discharge capacities of semi-liquid lithium secondary batteries utilizing the [Li(G3/G3Bu)][FeX] catholyte depended on the structure of the SILs, and the cell with [Li(G3)][FeCl 4 ] showed the highest capacity with relatively good capacity retention. This study confirmed the feasibility of the glyme-based redox-active SILs as catholytes for scalable redox-flow type batteries. Solvate ionic liquids (SILs), comprising long-lived, Li solvate cations and counter anions, serve as highly Li-ion-conductive and non-flammable electrolytes for use in lithium secondary batteries.