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

Abstract

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 = Br₄, Cl₃Br, Cl₄)), 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)][FeCl₄] 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)][FeCl₄] 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.