Hierarchically structured MoO2/dopamine-derived carbon spheres as intercalation electrodes for lithium-ion batteries

A hydrogen peroxide initiated sol-gel process involving molybdenum transformation in the presence of dopamine (Dopa) hydrochloride excess produced the metastable precipitate composed of polydopamine (PDopa) spheres coated with Dopa preintercalated molybdenum oxide, (Dopa)xMoOy@PDopa. The hydrothermal treatment (HT) of the (Dopa)xMoOy@PDopa precursor resulted in the simultaneous carbonization of Dopa and molybdenum reduction generating MoO2 nanoplatelets distributed and confined on the surface of the Dopa-derived carbon matrix (HT-MoO2/C). The consecutive annealing (An) of the HT-MoO2/C sample at 600 °C under Ar atmosphere led to the formation of MoO2 with increased Mo oxidation state and improved structural stability (AnHT-MoO2/C). Annealing had also further facilitated interaction between the molybdenum-derived and Dopa-derived components resulting in the modification of the carbon matrix confirmed by Raman spectroscopy. Morphology of both materials is best described as Dopa-derived carbon spheres decorated with MoO2 nanoplatelets. These integrated metal oxide and carbon structures were tested as electrodes for lithium-ion batteries in the potential window that corresponds to the intercalation mechanism of charge storage. The AnHT-MoO2/C electrode showed enhanced electrochemical activity, with an initial specific discharge capacity of 260 mAh/g and capacity retention of 67% after 50 cycles, compared to the HT-MoO2/C electrode which exhibited an initial specific discharge capacity of 235 mAh g−1 and capacity retention of 47% after 50 cycles. The rate capability experiments revealed that the capacity of 93 mAh/g and 120 mAh/g was delivered by the HT-MoO2/C and AnHT-MoO2/C electrodes, respectively, when the current density was increased to 100 mA/g. The improved specific capacity, electrochemical stability, and rate capability achieved after annealing were attributed to higher crystallinity of MoO2, increased oxidation state of Mo, and formation of the tighter MoO2/carbon contact accompanied by the annealing assisted interaction between MoO2 and Dopa-derived carbon.