Metallic MoS2 nanosheets: multifunctional electrocatalyst for the ORR, OER and Li-O2 batteries.

Lithium-oxygen batteries (LOBs) possess the highest theoretical specific density among all types of lithium batteries, making them ideal candidates to replace the current Li ion batteries for next-generation electric vehicle applications. However, designing highly active catalysts with high electronic conductivities to kinetically accelerate the sluggish oxygen reduction/evolution reactions (ORR/OER) is still a big challenge. This work was dedicated to developing two-dimensional (2D) trigonal phase MoS2 (1T-MoS2) nanosheets as a highly active electrocatalyst for LOBs for the first time. Metallic 1T-MoS2 prepared via in situ liquid-redox intercalation and exfoliation was hybridized with functionalized carbon nanotubes (CNTs) to form freestanding, binder-free oxygen electrodes. The 1T-MoS2/CNT electrode exhibited excellent electrochemical performances with a high reversible capacity of 500 mA h g-1 at a current density of 200 mA g-1 for more than 100 cycles owing to the catalytically active surfaces of 1T-MoS2 accessible by Li+ ions and O2. Density functional theory (DFT) calculations identified the catalytically active basal planes in 1T-MoS2 during the ORR as well as the initial ORR path during LOB cycles. The results based on the rotational ring disk electrode (RRDE) experiments also supported the findings from the DFT calculations, where the 1T-MoS2 basal planes are active for both the ORR and OER, not the semiconducting hexagonal MoS2 (2H-MoS2) whose edges are only electrocatalytically active. This study sheds light on the use of metallic 1T-MoS2 as a multifunctional oxygen electrocatalyst for LOB applications with enhanced ORR and OER activities.