NOVELTY - Sodium-halogen secondary cell (S1), comprises: (a) negative electrode compartment (15) comprising negative electrode that comprises sodium, where negative electrode electrochemically oxidizes to release sodium ions during discharge and reduces sodium ions to form sodium metal during recharge; (b) positive electrode compartment (25) comprising current collector (30) disposed in liquid positive electrode solution (35) that comprises at least one of halogen and halide; and (c) sodium ion conductive electrolyte membrane (40) that separates negative electrode from positive electrode solution.    USE - The sodium-halogen secondary cell is useful in grid-scale electrical energy storage systems and electrical vehicles.    ADVANTAGE - The sodium-halogen secondary cell: reduces overall battery complexity and total cost by eliminating cells, interconnect hardware, and charge control circuitry; and utilizes a pumping mechanism which reduces the total amount of sodium in the negative electrode compartment at any time, thus reducing the damage and/or danger that could occur if the positive electrode solution were to contact the negative electrode.    DETAILED DESCRIPTION - Sodium-halogen secondary cell (S1), comprises: (a) a negative electrode compartment (15) comprising a negative electrode that comprises sodium, where the negative electrode electrochemically oxidizes to release sodium ions during discharge and electrochemically reduces sodium ions to form sodium metal during recharge; (b) a positive electrode compartment (25) comprising a current collector (30) disposed in a liquid positive electrode solution (35) that comprises at least one of a halogen and a halide; and (c) a sodium ion conductive electrolyte membrane (40) that separates the negative electrode from the liquid positive electrode solution. INDEPENDENT CLAIMS are also included for:    (1) a molten sodium secondary cell (S2), comprising: (a) as above per se, where the sodium metal negative electrode is molten as the cell operates, a positive electrode compartment comprising a current collector disposed in a liquid positive electrode solution that comprises sodium halide, and (c) as above per se;    (2) a secondary cell (S3) comprising a negative electrode compartment comprising a negative electrode, where the negative electrode comprises a material comprising sodium metal and sodium intercalating carbon anode, and the negative electrode remains in a solid state as the secondary cell operates, (b) and (c) as above per se, and a non-aqueous anolyte solution disposed between the negative electrode and the electrolyte membrane; and    (3) a halogen-containing secondary cell, comprising a negative electrode compartment comprising a negative electrode that comprises a metal, where the negative electrode compartment further comprises sodium ions and halide anions, (b) and (c) as above per se.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the sodium-halogen secondary cell comprising a molten sodium negative electrode.    Negative electrode compartment (15)    Positive electrode compartment (25)    Current collector (30)    Liquid positive electrode solution (35)    Sodium ion conductive electrolyte membrane (40)