In this study, to increase in the redox potential of three-electron redox reaction of Mo3+/Mo6+, a mixed anion system, oxyfluorides, is targeted. Crystal structures and electrode performance of a new series of xLiF–LiMoO2 binary system, Li1+xMoO2Fx, as oxyfluoride positive electrodes are systematically examined. The highest theoretical capacity based on Mo3+/Mo6+ is expected for x = 2 (Li3MoO2F2) in this binary system. Li3MoO2F2 was prepared by mechanical milling from LiMoO2 and LiF. A mixture of LiMoO2 and LiF was mechanically milled with a ZrO2 container and balls. Li3MoO2F2 is found to crystallize into an anion-/cation-disordered rocksalt structure with low crystallinity. Electrochemical properties of Li3MoO2F2 before and after mechanical milling are compared in Figure 1. The sample after mechanical milling delivers a large reversible capacity of ca. 280 mAh g-1, which nearly corresponds to that of theoretical capacity based on the two-electron redox reaction of Mo3+/Mo5+. However, higher average voltage for the molybdenum oxyfluoride is evidenced compared with those of oxides.
From these results together with structural and electrochemical data of Li1+xMoO2Fx (x= 0.5, 1, and 1.5), we will discuss the feasibility of molybdenum oxyfluoride materials with multi-electron redox reactions as positive electrode materials for rechargeable lithium batteries.
 R. Chen et al., Adv. Energy Mater., 5, 1401814 (2015).