Lithium Molybdenum Oxyfluorides As High-Capacity Positive Electrodes for Rechargeable Lithium Batteries

Abstract

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, Li9/7Nb2/7Mo3/7O2, 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, Li1+x MoO2F 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 Li1+x MoO2F 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.