We studied the self-discharge of Li/Li x Mn2O4 battery cells in relation to the corrosion of aluminum cathode substrates. We determined the self-discharge of the cells from the decline of their open-circuit voltage, observed aluminum corrosion with use of voltammetry on electrochemical corrosion cells with an aluminum working electrode, and detected the existence and change of passivation films on the aluminum substrates with ac impedance measurement. The results showed that for the two salts LiPF6 and LiClO4 that formed the electrolytes with a binary solvent of ethylene carbonate–dimethoxyethane, the presence of LiPF6 reduced the self-discharge of the battery cells, retarded the corrosion of aluminum, and assisted the formation of a passivation film on aluminum. On the other hand, pre-treating aluminum to 480°C seemed to have similar but weaker effects. We therefore concluded that corrosion of aluminum played a significant role in the self-discharge of Li/Li x Mn2O4 batteries with an aluminum cathode substrate, but the corrosion could be greatly reduced by the formation of protection films on the aluminum. Such films could be formed by either having LiPF6 salt in the electrolyte in contact with the aluminum or by pre-treating the aluminum to a high temperature. However, films formed in the former fashion seemed much more stable when the aluminum was carrying an Li x Mn2O4 material.