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