Discharge and Charge Reaction of Perovskite-type <i>M</i>F₃ (<i>M</i> = Fe and Ti) Cathodes for Lithium-ion Batteries

Conversion-type cathode materials such as perovskite-type <i>M</i>F₃ (<i>M</i> = 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 730/620 mAh g⁻¹ between 0.5 and 4.0 V, 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.