A disc-like Mo-metal cluster compound, Co2Mo3O8, as a high capacity anode for lithium ion batteries

Abstract

The disc-like Mo-metal cluster compound, Co₂Mo₃O₈ has been prepared by a one-step carbothermal reduction method from CoMoO₄ and characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution-transmission electron microscopy (HR-TEM), Fourier transform-infrared (FT-IR) and micro-Raman spectroscopy at room temperature. The Li-cycling behaviour was investigated by galvanostatic cycling and cyclic voltammetry (CV) in the voltage range of 0.005–3.0 V vs. Li at room temperature. It is inferred that the as prepared Co₂Mo₃O₈ delivered a high first cycle reversible capacity of 900 (±5) mA h g⁻¹ (17.9 moles of Li). The compound showed slow capacity degradation upon cycling and reached 425 (±5) mA h g⁻¹ (8.5 Li) at the end of the 40th cycle. The ball milled Co₂Mo₃O₈ (Co₂Mo₃O₈-B) showed similar cycling behaviour and a reversible capacity of 400 (±5) mA h g⁻¹ (7.8 Li) was observed at the end of the 30th cycle. Interestingly, the heat-treated Co₂Mo₃O₈ (Co₂Mo₃O₈-H) showed improved Li-storage behaviour when cycled under similar conditions. It showed almost a stable capacity of 790 (±5) mA h g⁻¹ (15.7 Li) at the end of 60th cycle. All the compounds showed excellent coulombic efficiency in the range of 97–99%. The electrochemical impedance spectroscopy (EIS) measurement is performed on a Li/Co₂Mo₃O₈ system at selected voltages during first discharge and charge cycling. The possible reaction mechanism is proposed based on CV, ex situ XRD, ex situ TEM and electrochemical impedance spectroscopy (EIS) data on the Li/Co₂Mo₃O₈ system.