The demand for the increase in energy density of rechargeable lithium batteries are still growing. Recently, the use of two-electron redox of vanadium ions coupled with a high electronegativity anion in the crystal lattice is proposed as an effective strategy to enhance the energy density of positive electrode materials.[1] Theoretical capacities of lithium insertion materials depend on both total extractable lithium ions and electron numbers in the structure of positive electrodes. Therefore, in addition to the lithium-enrichment in the structure, the use of multi-electron redox processes for transition metals is expected to be an important strategy to further increase energy density of positive electrode materials. We have recently proposed a new lithium-excess oxide, Li_9/7Nb_2/7Mo_3/7O₂, which delivers a large reversible capacity of ca. 290 mAh g^-1 based on highly reversible three-electron redox of Mo³⁺/Mo⁶⁺.[2] Three-electron redox of molybdenum ions is expected to be a promising system to further increase energy density of positive electrode materials with less transition metal ions. Nevertheless, redox potential of Mo³⁺/Mo⁶⁺in the oxide framework structure is relatively low for positive electrodes.

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, Li_1+xMoO₂F_x, 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.[2]

From these results together with structural and electrochemical data of Li_1+xMoO₂F_x (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.

References

[1] R. Chen et al., Adv. Energy Mater., 5, 1401814 (2015).

[2] S. Hoshino, et al., and N. Yabuuchi, ACS Energy Letters, 2, 733 (2017).