The invention provides a shell, a secondary battery, a
battery pack and an
energy storage system, the secondary battery can comprise the shell and a
battery cell, the shell comprises a
metal matrix and a
metal oxide layer, the
battery cell is arranged in the
metal matrix, the metal
oxide layer is arranged on the inner surface, facing the
battery cell, of the metal matrix, the
porosity of the metal
oxide layer is 2%-7%, and the
porosity of the metal oxide layer is 2%-7%. The metal oxide layer is a compact oxide layer, and the compact oxide layer has relatively
high density and relatively few defects, so that the compact oxide layer can be damaged by bombardment of more free electrons, and the
positive pressure resistance of the shell of the secondary battery is improved. Moreover, due to the existence of the metal oxide layer, the resistance value of an equivalent parallel
resistor in the
electrolytic capacitor can be increased, the leakage current of the secondary battery in a withstand
voltage test can be reduced, and the
joule heat generated in the withstand
voltage test can be further reduced, so that the energy released by the secondary battery can be reduced when
short circuit failure occurs, the probability of
valve opening explosion can be further reduced, and the service life of the secondary battery can be prolonged. Therefore, the use safety of the secondary battery is improved.