Co2Mo3O8/reduced graphene oxide composite: synthesis, characterization, and its role as a prospective anode material in lithium ion batteries

Abstract

A Co₂Mo₃O₈/reduced graphene oxide (Co₂Mo₃O₈/rGO) composite was synthesized by following a single step solid state reduction procedure. The prepared Co₂Mo₃O₈/rGO composite was characterized using a multitude of characterization techniques, which confirmed the formation of the composite. Electron micrographs clearly showed that the composite consisted of submicron sized (lateral) and 50 nm thick hierarchical hexagonal nanoplatelets of Co₂Mo₃O₈ attached to thin graphene layers of rGO. Raman scattering analysis not only confirmed the presence of Co₂Mo₃O₈ and rGO in the composite but also revealed that the defects present in rGO are more than that in GO. Through thermogravimetric analysis, the amount of rGO present in the composite was found to be ∼22% by weight. Co 2p, Mo 3d, C 1s and O 1s X-ray photoelectron energy peaks were clearly identified. The analysis of these peaks confirmed the oxidation states of the respective elements in stoichiometric Co₂Mo₃O₈. The as-synthesized Co₂Mo₃O₈/rGO composite was tested as an anode material in half-cell configured lithium ion batteries. When cycled at 60 mA g⁻¹ current density and in the 0.005–3.0 V range, the Co₂Mo₃O₈/rGO composite delivered an excellent reversible specific capacity of ∼954 mA h g⁻¹ that corresponds to 82% capacity retention at the end of the 60^th cycle, which is higher than the theoretical capacity of both Co₂Mo₃O₈ and graphene. Moreover, the Co₂Mo₃O₈/rGO composite exhibited excellent rate capability. A reversible specific capacity of 471 mA h g⁻¹ (at a current density of 1000 mA g⁻¹) was delivered at the end of the 31^st cycle. The value increased to 1006 mA h g⁻¹ when the current density was switched to 100 mA g⁻¹ at the end of the 36^th cycle. Redox peaks in the cyclic voltammetry (CV) curves revealed that electrochemical conversion and electrochemical adsorption and desorption type reaction mechanism are the primary reasons for lithium ion storage. A constant area under the CV curves throughout the tests was noticed, which is an indication of the stable capacity while the CV results are in line with the galvanostatic cycling (GC) results. From the CV and GC results, it is concluded that the higher specific capacity, longer cycle life, and better rate capability are due to the excellent synergy between Co₂Mo₃O₈ and rGO in the composite.