An electric accumulator having an alkali metal as the reactant at the negative electrode, and a cationically conductive separator physically separating the reactant at the negative electrode from a reactant at the positive electrode, while permitting passage of cations of the alkali metal therethrough, characterized in that the reactant at the positive electrode comprising a mixture which, in the fully charged state, comprises 5 to 75 mole % of aluminium chloride and 95 to 25 mole % iron III chloride.