The ionic liquid 1-allyl-1-methylpyrrolidinium chloride was investigated as an additive to an ethylmagnesium bromide Grignard solution, for use as an advanced electrolyte in rechargeable Mg batteries. Electrochemical studies showed that the addition of the ionic liquid significantly enhanced the anodic limit of the Grignard solution by about 1V and improved the coulombic efficiency and the kinetics of Mg deposition-stripping reactions on the negative electrode. We have also demonstrated that the Grignard solution containing the ionic liquid works well as an electrolyte in a prototype cell consisting of a Mo6S8 Chevrel phase cathode and a Mg metal anode, where the original Grignard electrolyte does not function properly.