Cathode Properties of Rocksalt-type LiFeOF for Li-Ion Batteries

The iron-based conversion-type cathode materials attract attention due to the low cost and the high theoretical capacity. Among them, Fe₂O₃ has the highest theoretical capacity; FeF₃ has the highest discharge voltage, and FeOF shows the highest energy density. However, these conversion-type cathodes can’t use in Li-ion type cell with carbonaceous anode, because they don’t include lithium-ion in the initial composition. Recently, K. Kang group reported LiF-FeF₂ nano-composite can be used as cathode for Li-ion batteries [1]. Actually, LiF-FeF₂ nanocomposite showed that the reversible specific capacity was ca. 190 mAh/g with the average voltage of 3.6 V. In this research, we tried to prepare LiF-FeO composite as the novel cathode materials for Li-ion batteries by dry mechanical ball-milling method. Moreover, the cathode properties of LiF-FeO nanocomposite were evaluated against lithium metal or Li₄Ti₅O₁₂ (LTO).

LiF-FeO was prepared by mixing of LiF and FeO with a molar ratio of LiF : FeO = 1.2 : 1. The mixture was dry ball-milled in Ar for 24, 48 and 72 h, respectively. After mechanical ball milling, 5 wt % acetylene black (AB) was added to 70 wt % mixture in Ar for 24 h. Then, the 75 wt % obtained LiF-FeO/carbon composite power was also mixed with 20 wt % AB and 5 wt % PTFE teflon binder and punched in the form of disks (10 mm in diameter). The electrochemical performance was evaluated with a 2032 coin-type cell using a non-aqueous electrolyte (1M LiPF₆ in EC:DMC=1:1 in volume) against Li metal anode or LTO anode.

Rocksalt-type LiFeOF was obtained from LiF and FeO by the dry ball-milling method under ambient pressure. The reversible capacity of LiFeOF was 292 mAh/g with an average voltage of 2.5 V and an energy density over 700 mWh/g, which is higher than the theoretical energy density of LiFePO₄ (Fig. 1). In addition, we confirmed that a LiFeOF can actually work as cathode in Li-ion type cell with LTO anode. The initial charge and discharge capacities per cathode weight were 180 mAh/g and 149 mAh/g with an 83% discharge/charge efficiency. The discharge capacity was 130 mAh/g even after 30 cycles, and the cycling efficiency of LiFeOF was 87%. The reversibly expansion and contraction of structure of LiFeOF during cycling was measured with XRD.