New Lithium-Excess High-Capacity Positive Electrode Materials with Mo3+/Mo6+ three-Electron Redox

Abstract

The demand for the increase in energy density of lithium batteries are steadily growing. The use of three-electron redox of transition metals is expected to be a plausible strategy to further increase the energy density of positive electrode materials with less transition metal ions. Historically, three-electron redox reaction of transition metals is reported only for Cr3+/Cr6+, e.g., Li1.2Cr0.4Mn0.4O2.[1] In this study, the reaction of Mo3+/Mo6+ is targeted as electrode materials. Since conventional layered system, LiMo3+O2, has one mole of Li in the formula unit, only one-electron redox of Mo3+/Mo4+ is used. Therefore, Mo3+ is diluted in lithium-excess oxide, Li3NbO4 [2] as the model material, according to the chemical formula of x LiMoO2 – (1 – x) Li3NbO4 binary system. The highest reversible capacity based on Mo3+/Mo6+ is expected with x = 0.6 (Li9/7Nb2/7Mo3/7O2) in this binary system. Our trials to synthesize Li9/7Nb2/7Mo3/7O2 were, however, failed by conventional solid-state reaction. Mixtures of LiMoO2 and Li3NbO4 were obtained regardless of different synthesis conditions. Therefore, Li9/7Nb2/7Mo3/7O2 was prepared by mechanochemical route from LiMoO2 and Li3NbO4 as precursors. A mixture of LiMoO2 and Li3NbO4 was mechanically ball-milled at room temperature with a ZrO2 container and balls. XRD patterns of LiMoO2, Li3NbO4, and Li9/7Nb2/7Mo3/7O2 prepared by ball-milling are compared in Figure 1a. An X-ray diffraction pattern of Li9/7Nb2/7Mo3/7O2 is assigned to a cation disordered rocksalt structure with low crystallinity. Electrochemical properties of Li9/7Nb2/7Mo3/7O2 before and after mechanical-milling were examined in Li cells as shown in Figure 1b. The sample before mechanical-milling shows initial discharge capacity of ca. 80 mAh g-1, and a voltage profile is the same with LiMoO2. On the other hand, the sample after mechanical-milling delivers a reversible capacity of ca. 290 mAh g-1, which nearly corresponds to that of theoretical capacity based on the redox reaction of Mo3+/Mo6+. Moreover, three-electron redox of Mo is supported by X-ray absorption spectroscopy without the contribution of Nb for charge compensation. From these results, we will discuss the possibility of a new series of high-capacity positive electrode materials with three-electron redox of Mo3+/Mo6+ for rechargeable lithium batteries.