Synthesis of Mo-cluster compound, LiHoMo3O8 by carbothermal reduction and its reactivity towards Li

Abstract

The mixed oxide containing the triangular molybdenum clusters, LiHoMo3O8, is prepared from the component oxides by one-step carbothermal reduction reaction at 750 °C. It was characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) techniques. Electrochemical studies were carried out at 24 °C and at 50 °C by galvanostatic cycling (range 0.005–3.0 V vs Li, at 30 mA/g) and cyclic voltammetry (CV). The first-cycle charge capacities at 24 °C and 50 °C after deep discharge to 0.005 V are 180 and 315 (±5) mAh/g, respectively, corresponding to ∼3.5 and ∼6.0 moles of Li per mole of LiHoMo3O8. Cycling up to 70 cycles at 24 °C shows a gradual increase in the reversible capacity and finally reaches 290 (±5) mAh/g (∼5.6 moles of Li). When cycled at 50 °C, the reversible capacity reaches 470 (±5) mAh/g (∼9.1 moles of Li) at the 40th cycle. The coulombic efficiency ranged from 96 to 98% (30–70 cycles; 24 °C). The differential capacity–voltage curves and the CV data show that the average potentials for the discharge- and charge process occur at 1.0 V and 2.2 V, respectively. The observed ex-situ XRD and HRTEM data on the electrodes in the discharged/charged states have been interpreted and a plausible reaction mechanism is proposed.