A high temperature rechargeable electrochemical cell comprises a housing in the form of a canister which is polygonal in cross-section so that it has a plurality of peripherally spaced corners. A solid electrolyte separator which is a conductor of sodium ions, separates the interior of the housing into an anode compartment containing an anode and a cathode compartment containing a cathode. The separator is tubular or cup-shaped, having a closed end and an open end, and having a plurality of peripherally spaced radially outwardly projecting ribs or lobes corresponding in number to the corners of the housing. The separator is concentrically located in the housing, with each lobe of the separator being peripherally aligned with, and projecting towards, one of said corners of the housing. Anchoring components anchoring the separator in position relative to the housing, are provided. The anchoring component comprises a first portion spanning a gap between the housing and the separator, between a pair of lobes of the separator, and a second portion protruding from the first portion and engaging a surface of a lobe of the separator. Sodium is provided as active anode material in the anode compartment, with the cell having an operating temperature at which the sodium is molten, while active cathode material is provided in the cathode compartment.