NOVELTY - The electrochemical cell (10) comprises a positive electrode having an active intercalation cathode material of treated bentonite, a negative electrode having an active anode material, and an electrolyte (16) positioned in contact with the positive and negative electrodes. When the negative electrode comprises magnesium, the electrolyte is a solid gel polymeric electrolyte, and when the negative electrode comprises sodium, the electrolyte is a molten salt electrolyte at the operating temperature of the battery.    USE - The electrochemical cell is useful for a secondary battery such as magnesium-based battery and a sodium-based battery, and useful in plug-in hybrid-type vehicles.    ADVANTAGE - The slender electrochemical cell exhibits improved energy density, lower electrical/mechanical resistance per unit mass without comparable changes in thermal resistance, optimizes the power density, and can be simply and economically manufactured with improved lifespan and durability.    DETAILED DESCRIPTION - The electrochemical cell (10) comprises a positive electrode having an active intercalation cathode material of treated bentonite, a negative electrode having an active anode material, and an electrolyte (16) positioned in contact with the positive and negative electrodes. When the negative electrode comprises magnesium, the electrolyte is a solid gel polymeric electrolyte, and when the negative electrode comprises sodium, the electrolyte is a molten salt electrolyte at the operating temperature of the battery. When a melting point of the electrolyte is higher than a melting point of sodium, the electrochemical cell further comprises a separator placed between the negative electrode and the electrolyte, where the separator is beta-alumina solid electrolyte. The treated bentonite is fired in a discharged state before it is assembled in the electrochemical cell. The solid gel polymeric electrolyte possesses an amorphous structure.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a first battery.    Electrochemical cell (10)    Electrical terminal (11)    Electrode material (12)    Electrolyte (16)    Housing (17)    Active cathode material. (18)