NOVELTY - A hybrid aqueous energy storage device is operated by providing the storage device with active cathode electrode material comprising lithium manganate cubic spinel material; deintercalating lithium ions from the material to form lithium free cubic spinel lambda -manganate active cathode electrode material; reversibly intercalating sodium ions from the electrolyte into the active cathode electrode material to form cubic spinel lambda -manganate active cathode electrode material; and reversibly deintercalating sodium ions from the active cathode electrode into electrolyte.    USE - Method for operating a hybrid aqueous energy storage device (claimed).    ADVANTAGE - The method uses water-based (aqueous) electrolyte which allows for use of thicker electrodes, less expensive separator and current collector materials, and benign and environmentally friendly materials for electrodes and electrolyte salts. The energy storage systems can be assembled in an open-air environment, resulting in a lower cost of production.    DETAILED DESCRIPTION - Operating a hybrid aqueous energy storage device comprising anode and cathode electrodes, separator, and sodium containing aqueous electrolyte, comprises providing the storage device in which the cathode electrode comprises an active cathode electrode material comprising lithium manganate cubic spinel material of formula LixMn2-zAlzO4; deintercalating lithium ions from the active cathode electrode material during initial charging of the device to form lithium free cubic spinel lambda -manganate active cathode electrode material; reversibly intercalating sodium ions from the electrolyte into the active cathode electrode material during subsequent discharge cycle to form cubic spinel lambda -manganate active cathode electrode material of formula AyMn2-zAl04; and reversibly deintercalating sodium ions from the active cathode electrode into electrolyte during subsequent charge cycle.    A= greater than or equal to 80 atom% Na;    x=less than 1.1;    y=less than 1;and    z=less than 0.1.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic view of a possible charge/discharge mechanism employed by hybrid energy storage device.