Electrolyte

An electrolyte comprising a first solvent, a second solvent and a compound of formula (I): wherein: X is Al or B; M+ is a metal cation; R1 in each occurrence is independently a linear, branched or cyclic C1-40 alkyl in which one or more non adjacent C atoms of the C1-40 alkyl, other than the C atom bound to O of OR1 or a terminal C atom of the C1-40 alkyl, may be replaced with O; and one or more H atoms may be replaced by F, wherein two R1 groups may be linked to form a chain; the first and second solvents are miscible; the first solvent has a Hansen solubility polar parameter δp of at least 5 at 20°C e.g. propylene carbonate; and the second solvent is an aromatic solvent e.g. toluene. Also shown is a metal ion battery comprising and anode, a cathode and the electrolyte.