Magnetic impurity effects on self-discharge capacity, cycle performance, and rate capability of LiFePO₄/C composites

Battery manufacturers pay a great deal of attention to the reproducibility of LiFePO₄/C composites. Poor reproducibility of self-discharge capacity, cycle performance, and rate capability seriously affects the balance of battery packs. We have found that the above properties strongly depend on the level of magnetic impurities present. The existence of trace magnetic impurities produced from a sintering process can significantly poison the self-discharge capacity of LiFePO₄/C materials. In this work, the magnetic impurity-removed LiFePO₄/C composites exhibited the best discharge capacity of 158 mAh g⁻¹ at 0.2 C and excellent cycle performance with a capacity retention of 92% after 500 cycles at 5 C. Self-discharge experiments showed that the purified LiFePO₄/C maintains 97% residual capacity after a week at 0.5 C, which is better than the others containing magnetic impurities. We applied a new alternative current magnetic susceptibility device to quantify the impact of magnetic impurities on LiFePO₄/C composites. This method is rapid, effective, inexpensive, and nondestructive, which makes it a useful quality control tool for practical lithium battery applications.