Controlling Operating Voltages in Molybdenum Oxide Anodes through Inductive Effects

Abstract

As Li-ion batteries are more widely adopted, it becomes important to identify new battery electrode materials made from a greater diversity of elements while improving stability, extracted power, and the ability to charge and discharge rapidly. Early transition metals, such as Nb and Mo, are relatively earth-abundant elements, with oxides that are candidates for next-generation anode materials. However, Mo oxides are limited as battery electrodes due to their intermediate redox voltages. Lowering this voltage could open the door for high-performance Mo-based oxide anodes. Oxidation of the voltage can be employed by adding redox-inactive cations of electropositive elements. This strategy is named the induction effect and has been proposed as a design principle to increase cathode voltages. However, its use on the anode side appears less common. Here, we compare the ionization energy, electrochemistry, and Li diffusivity of the compound Li2Mo4O13 with MoO3, which both start out as Mo6+ compounds before lithiation. The ionization energy values extracted from ultraviolet photoemission spectroscopy support the hypothesis that the alkali metal cation pushes up the valence band. Electrochemical studies in half-cells against Li/Li+ indicate that Li2Mo4O13 can reversibly take up two additional Li ions in the unit cell. Moreover, the addition of Li to the molybdenum oxide structure lowers the voltage by 300 mV for the Mo6+/5+ couple compared to the same redox couple in MoO3 and retains high Li+ diffusivity. This work, in conjunction with experimental redox voltages extracted from prior literature on Ti4+/3+ and Nb5+/4+ redox couples, demonstrates the utility of using inductive effects to tailor the operating voltage of candidate anode materials.