This application provides a secondary battery, including at least one battery unit assembly. The battery cell assembly includes a positive electrode plate, a negative electrode plate, and a separator disposed between the positive electrode plate and the negative electrode plate. An elongation rate of the separator is greater than 100%, the elongation rate of the separator includes an elongation rate in the length direction and/or an elongation rate in the width direction, a ratio of the elongation rate of the separator to a thickness of the active material layer of the positive electrode plate and/or negative electrode plate is 3.0%/µm to 8.0%/µm, and a ratio of the elongation rate of the separator to an elongation rate of the current collector of the positive electrode plate and/or negative electrode plate is greater than or equal to 60.