Effects of Constituent Elements on the Electrochemical Characteristics of Composites of LiF and Several Spinel Oxides as Cathode Materials for Li-Ion Batteries

4LiF-MM’2O4 composites were synthesized via the mechanical milling of LiF and MM’2O4 (M = Mn, Mg, Zn; M’ = Mn Fe) for 72 h. In the obtained composites, the XRD peak broadened because of the milling, and the composites possessed a rock-salt-type structure. During charge–discharge measurements at 0.1 C, composites with spinel materials containing Mg showed particularly high discharge capacities; the discharge capacity of 4LiF-MMn2O4 and 4LiF-MFe2O4 was 310 mAh/g and 309 mAh/g, respectively. The discharge voltage was approximately 3.2 V for 4LiF-MgMn2O4 and approximately 2.8 V for 4LiF-MgFe2O4, and 4LiF-MgMn2O4 composites had the highest energy densities, exceeding 1000 Wh/kg. During cycle characteristic measurements with a cutoff voltage of 4.8 V and 4.4 V, the initial capacity retentions at the 100th cycle were 11% and 79%, respectively. The Coulombic efficiency was also better at a cutoff voltage of 4.4 V than that of 4.8 V, indicating that electrolyte decomposition has a significant influence on the cycle characteristics. Additionally, composites were synthesized via mechanical milling using various molar ratios of LiF and MgMn2O4. In xLiF-MgMn2O4 (x ≥ 3), the discharge potential was approximately 3.2 V, and the discharge capacity was higher than 250 mAh/g. The highest discharge capacity was observed for 4LiF-MgMn2O4 among xLiF-MgMn2O4 (x ≥ 3) composites.