Abstract: An alkali polysulphide flow battery, components, systems and compositions for use with an alkali polysulphide flow battery and a method of manufacturing and operating a flow battery system are provided. An ion-selective separator composition for a battery having an anode and an alkali metal sulfide or polysulfide cathode is provided. The separator composition includes an alkali metal ion conducting separator film for separating the anode and the cathode, a carbon layer disposed to a cathode side of the film and an alkali metal ion conductor layer disposed to an anode side of the carbon layer.