Tuning Defects in the MoS2/Reduced Graphene Oxide 2D Hybrid Materials for Optimizing Battery Performance

Abstract

Rechargeable aqueous zinc ion batteries (AZIB) are an emerging topic in the battery research due to the high volumetric capacity of the zinc anode (5855 mAh/cm3), low cost, environmental friendliness and safety. But the development of the zinc ion batteries has been hindered by the slow insertion/extraction kinetics of the multivalent Zn2+ ions in the host structure mainly due to the larger ionic radii of the hydrated zinc ion (~4.60 Å) and stronger electrostatic interaction of the divalent metal cation with the host structure that monovalent Li+ ions. These factors make the existing Li-ion battery (LIB) cathode materials unfit for the intercalation/deintercalation of Zn2+ ion. Transition metal dichalcogenides like Molybdenum disulfide (MoS2) have caught the attention as a host for both monovalent and divalent ion storage due to their two-dimensional layered structure and high theoretical specific capacity for Li-ion storage (670 mAh g-1). The MoS2 structure consists of a two-dimensional (2D) layered structure with a layer of molybdenum atoms covalently bonded between two layers of sulfur atoms. The triatomic layers of MoS2 are linked by weak van der Waals forces similar to graphene, which can effectively accommodate the volume expansion to facilitate reversible intercalation/deintercalation of metal ions. Despite the advantages, MoS2 suffers from low electrical conductivity and pulverization of the structure after few intercalation/deintercalation cycles which causes rapid capacity fading. These problems can be mitigated by modifying the structure through (1) increasing the interlayer spacing (2) introducing active defects in the MoS2 structure to enhance Zn2+ ion adsorption, and (3) forming a hybrid structure with carbonaceous materials to improve the electrical conductivity, mechanical strength, and structural stability of MoS2 layers. Among which, defect engineering has been recently explored as an effective approach to enhancing the specific capacity of MoS2 towards the storage of monovalent and divalent ions including Li+, Na+, Zn2+ ions.In this work, the preparation of a set of hybrid materials consisting of Molybdenum disulfide (MoS2) nanopatches on reduced graphene oxide (rGO) nanosheets with controllable defect density and its impact on the storage of Li+ and Zn2+ ions will be presented and discussed. The MoS2/rGO hybrid materials are synthesized by applying the novel microwave specific heating of graphene oxide and molecular molybdenum precursors followed by a 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 2D layered structures whose defect density is tunable by controlling the annealing temperature at 250°C (MoS2/rGO-250), 325°C (MoS2/rGO-325) and 600°C (MoS2/rGO-600), respectively. The defect-free MoS2/rGO-600 material performs well as an anode for Li+ ion intercalation with a high specific capacity of 519 mAh gMoSx -1 (~3.1 Li+ ions per MoS2). Other samples show comparable results, indicating the insignificant effect by the defects. In contrast, the Zn-ion storage properties strongly depend on the defects in the MoS2 adlayer. The highly defective MoS2/rGO-250 hybrid prepared by annealing at 250°C shows the highest initial Zn-ion storage capacity of ~300 mAh gMoSx -1 (~1.8 Zn2+ per MoS2) and close to 100% coulombic efficiency after 3 cycles, which is dominated by pseudocapacitive surface reactions at the edges or defects in the MoS2 nanopatches. The fast fading in initial cycles can be mitigated by applying higher charge/discharge currents or extended cycles. These results provide critical insights to improve metal ion storage properties by defect engineering of the MoS2 in the MoS2/rGO hybrid materials.