Chloroaluminate ionic liquids for low-temperature aluminum-ion batteries

•[BMIm]Cl-based ILs do not crystallize when cooled to –100 °C at rate of 10 °C∙min−1.•The presence of glass transition in [BMIm]Cl-based ILs was determined by DSC.•Conductivity hysteresis was observed in [EMIm]Cl-based ILs near the melting point.•[BMIm]Cl-based ILs remain liquid at temperatures above −35 °C.•[BMIm]Cl-based ILs can be used in AIB at temperatures below zero. The paper discusses the feasibility of 1-ethyl-3-methylimidazolium ([EMIm]Cl) and 1-butyl-3-methylimidazolium ([BMIm]Cl) chloroaluminate ionic liquids (ILs) as electrolytes for aluminum-ion batteries capable of delivering high performance at subzero temperatures. The melting temperatures of AlCl3–[EMIm]Cl and AlCl3–[BMIm]Cl ILs with the molar ratio of aluminum chloride to the chloride of imidazolium salt (N) equal to 1.3 and 1.5 were determined using DSC. The melting temperature of AlCl3–[EMIm]Cl is –21 and –32 °C at N = 1.3 and N = 1.5, respectively. The conductivity of AlCl3–[EMIm]Cl IL increases sharply at heating and decreases sharply near its melting temperature at cooling. The DSC curves for [BMIm]Cl-based ILs do not contain any peaks related to the melting or crystallization of the IL, and temperature dependences of conductivity contain no inflection points. Galvanostatic cycling and cyclic voltammetry demonstrated efficient performance of the aluminum-ion battery with an aluminum anode, graphene cathode and AlCl3–[BMIm]Cl IL, which delivered coulombic efficiency approaching 100 % at temperatures as low as –30 °C. The capacity of the graphene cathode was found to be 82.4, 63.0 and 48.5 mA∙h∙g−1 at 0, –20 and –30 °C, respectively.