Enhanced cycling performance of rechargeable Li-O2 batteries via LiOH formation and decomposition using high-performance MOF-74@CNTs hybrid catalysts

Li-O-2 batteries have received much attention for next-generation energy storage devices due to their high specific energy. However, Li-O-2 batteries still face several challenges including low energy efficiency and poor cycle life, which are mainly caused by the low stability of electrolytes and cathodes towards aggressive reduced oxygen species, e.g., O-2(-) intermediate and Li2O2. It has been reported that water can be used as an effective additive in aprotic Li-O-2 batteries to increase the discharge capacity and to alleviate parasitic reactions by solvating and trapping the highly aggressive O-2(-) intermediate. In this study, Mn-MOF-74 nanoparticles directly grown on carbon nanotubes (Mn-MOF-74@CNTs) via a facile additive-mediated synthesis are proposed as catalytic cathode materials for Li-O-2 batteries to be operated in humid oxygen environment to generate less-reactive discharge product LiOH compared to Li2O2. Due to the formation of LiOH by the nano-architectured Mn-MOF-74@CNTs hybrid catalyst, Mn-MOF-74@CNTs-based oxygen cathode exhibits less side reactions during battery operation and much-enhanced cycling performance in humid oxygen containing 200 ppm moisture than those of conventional carbon cathodes (Ketjenblack and CNTs) in both dry and humid oxygen where Li2O2 was formed as discharge products. Furthermore, a series of controlled experiments and thermodynamic analysis are conducted to investigate the formation mechanism of LiOH. Based on the results, we report that the formation pathway of LiOH is a chemically-catalytic process via a chemical conversion of Li2O2 occurring at Mn2+/Mn3+ metal centers in Mn-MOF-74@CNTs hybrid, instead of an electrocatalytic process via a direct four-electron reduction of oxygen.