Abstract: Self-discharge reaction of lithium-ion battery is inevitable, it leads to the reduction of the capacity, but also seriously affects the cycle life. The factors that affected self-discharge reaction are micro powder, burr, magnetic materials and so on. Among them, the magnetic metal impurity has the greatest influence. In this paper, the effect of adding different amounts of nano-iron powder on cathode materials was studied. After stored at 55 ℃ for 7 days, the self-discharge rate of the battery with 100 ppm of magnetic impurity is twice that of the battery with 50 ppm of iron powder. Magnetic impurities can increase the self-discharge rate of the battery. Cycling experiments showed that the residual capacity of pure cathode material（magnetic material ＜ 100 ppb） remained at 97% after 450 cycling at 1 C, which was better than others.