Conversion-type cathode materials such as perovskite-type MF3 (M = Fe and Ti) show promise for use in large-scale lithium-ion batteries by virtue of their low cost and large specific capacities. However, the FeF3 cathode shows a large overpotential during discharge-charge cycles, such that the rechargeable capacity is almost gone after a few cycles. To overcome this drawback, we elucidated the detailed mechanism of the deterioration of the rechargeable capacity for the FeF3 cathode. In the initial cycle, the cathode returned to the initial FeF3 structure from LiF and Fe. However, the diffraction peak of LiF and Fe after the 20th cycle was sharper than that after the initial discharge state; that is, the growth of the LiF and Fe crystal contributed to the lower cyclability of FeF3. On the other hand, the TiF3 cathode showed an initial discharge/charge capacity of 730/620mAhg-1 between 0.5 and 4.0V, and the discharge-charge overpotential of TiF3 was still smaller than that of FeF3. In addition, the cyclability of the TiF3 cathode were better than that of the FeF3 cathode, not only in insertion reaction region, but also in conversion reaction region.
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