Tuning the defects in MoS2/reduced graphene oxide 2D hybrid materials for optimizing battery performance

Abstract

This study reports the preparation of a set of hybrid materials consisting of molybdenum disulfide (MoS₂) nanopatches on reduced graphene oxide (rGO) nanosheets by microwave specific heating of graphene oxide and molecular molybdenum precursors followed by thermal annealing in 3% H₂ and 97% Ar. The microwave process converts graphene oxide to ordered rGO nanosheets that are sandwiched between uniform thin layers of amorphous molybdenum trisulfide (MoS₃). The subsequent thermal annealing converts the intermediate layers into MoS₂ nanopatches with two-dimensional layered structures whose defect density is tunable by controlling the annealing temperature at 250, 325 and 600 °C, respectively. All three MoS₂/rGO samples and the MoS₃/rGO intermediate after the microwave step show a high Li-ion intercalation capacity in the initial 10 cycles (over 519 mA h g_MoSx⁻¹, ∼3.1 Li⁺ ions per MoS₂) which is attributed to the small MoS₂ nanopatches in the MoS₂/rGO hybrids while the effect of further S-rich defects is insignificant. In contrast, the Zn-ion storage properties strongly depend on the defects in the MoS₂ nanopatches. The highly defective MoS₂/rGO hybrid prepared by annealing at 250 °C shows the highest initial Zn-ion storage capacity (∼300 mA h g_MoSx⁻¹) and close to 100% coulombic efficiency, which is dominated by pseudocapacitive surface reactions at the edges or defects in the MoS₂ nanopatches. The fast fading in the initial cycles can be mitigated by applying higher charge/discharge currents or extended cycles. This study validates that defect engineering is critical for improving Zn-ion storage.