An electrolyte element (10) comprises a perforated sheet (11) of non-reactive metal such as an aluminium-bearing ferritic steel, and a non-permeable ceramic layer (16b) of sodium-ion-conducting ceramic bonded to one face of the perforated sheet (11) by a porous ceramic sub-layer (16a). The perforated sheet (11) may be of thickness in the range 50 μm up to 500 μm, and the thickness of the non-permeable ceramic layer (16b) may be no more than 50 μm, for example 20 μm or 10 μm. Thus the electrolyte properties are provided by the non-permeable thin layer (16b) of ceramic, while mechanical strength is provided by the perforated sheet (11). The electrolyte element (10) may be used in a rechargeable molten sodium-metal halide cell, in particular a sodium/nickel chloride cell (20). It makes cells with increased power density possible.