A secondary battery, comprising a positive-electrode plate, a negative-electrode plate, a separator disposed between the positive-electrode plate and the negative-electrode plate, and an electrolyte solution, wherein the negative-electrode plate comprises an alkali metal layer and a polymer film layer provided on at least one surface of the alkali metal layer, the polymer film layer comprises a first surface and a second surface that are back away from each other in the thickness direction of the polymer film layer, the first surface being an outer surface of the polymer film layer facing away from the alkali metal layer, and the second surface being an inner surface of the polymer film layer facing towards the alkali metal layer, resistivity of the first surface is denoted as R1, resistivity of the second surface is denoted as R2, and the polymer film layer satisfies that resistivity increases continuously from R2 to R1, R1 is 102 Ω·cm-109 Ω·cm, R2 is 1 Ω·cm-104 Ω·cm.