Effects of operating temperature on Li-O2 battery with ionic liquid-based binary electrolyte

Abstract

Awaiting a breakthrough, the Li-oxygen battery (LOB) is considered a promising candidate to meet the high energy demands in the future. Among various critical challenges which hamper its development, the mystery of the electrolyte with optimal properties remains unsolved to this day. In this study, we comprehensively investigated the effects of operating temperature (20C, 40 °C and 60 °C) on the electrochemical performance of LOBs incorporated with room temperature ionic liquid (RTIL) and organic solvent binary electrolyte. We designed and investigated 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₂C₁im][Tf₂N]) RTIL and dimethyl sulfoxide (DMSO) organic solvent at various volume ratios ((4:1), (1:1), (1:4)). Among the binary electrolytes, ([C₂C₁im][Tf₂N]/DMSO (1:4)) delivered the highest discharge capacities of 3.70 Ah g ^{− 1} (20 °C), 4.0 Ah g ^{− 1} (40 °C) and 3.65 Ah g ^{− 1} (60 °C) as compared with pure [C₂C₁im][Tf₂N] and DMSO. Cycling stability tests showed superior stability of the binary electrolyte ([C₂C₁im][Tf₂N]/DMSO (1:4)) irrespective of the operating temperature. From viscosity and ionic conductivity measurements (at 20–60 °C), [C₂C₁im][Tf₂N]/DMSO (1:4) exhibited the highest ionic conductivity and the lowest viscosity compared with other binary electrolytes (even with pure electrolytes) at any given temperature. Cyclic voltammetry (CV) tests revealed the highest reaction rates for [C₂C₁im][Tf₂N]/DMSO (1:4) binary electrolytes than pure electrolytes. The superior performance of [C₂C₁im][Tf₂N]/DMSO (1:4) binary electrolyte was ascribed to enhanced stability against reactive intermediate species during oxygen reduction reaction (ORR), increased ionic conductivity, low viscosity (comparable with organic electrolytes), improved oxygen solubility, and relatively low evaporation rates.