Synthesis of Cobalt Manganese Oxide-Carbon Nanotubes Composite Microspheres As a Bifunctional Cathode Catalyst for Li-O2 Batteries
Authors: Kim Jung Hyun 
Published: February 2020

Although lithium-ion batteries (LIBs) have been widely used in electronic devices, intercalation limits their energy densities, rendering them incapable of meeting the increasing demand for high energy density. Lithium-oxide (Li-O2) batteries are appealing energy storage and conversion devices owing to their remarkably high energy densities (~3500 Wh kg-1), ~8 times higher than those of current LIBs. However, despite great promise, Li-O2 batteries still suffer from low round-trip efficiency, poor cycle life, and voltage decay. It has been revealed that this limited electrochemical performance is mainly attributed to the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the oxygen electrode.

The electrochemical performance of Li-O2 batteries is highly dependent on both the materials and architecture of the oxygen electrode. Transition metal oxide nanomaterials have attracted enormous interest as low-cost alternative catalysts to noble-metal catalysts capable of catalyzing both the ORR and OER. In particular, materials with mixed metal compounds play an important role in electrocatalytic reactions, since mixed valence species enhance electrocatalytic activity. However, the poor electronic conductivity limits the catalytic activity of the mixed-valant transition metal oxide. Therefore, nanostructured materials comprising efficient catalyst nanocrystals and carbon materials with high electrical conductivity, high electrocatalytic activities for the ORR and OER, and a well-designed porous structure to accommodate the reaction products have to be developed. A commercial air electrode should be formed by a simple printing process from slurry containing the cathode powders, organic binder, and solvent. Macroporous cathode powders with uniform composition, several microns in size, spherical morphology, and narrow size distribution should be used to form the uniform air electrode in the commercial process. However, studies on cathode microspheres with optimum morphological characteristics and high electrocatalytic activities for ORRs and OERs for Li–O2 batteries are rare. In this study, Mn-Co-O catalyst was studied as electrocatalyst showing high efficiencies for the ORR and OER for Li-O2 batteries. In addition, the optimum structure of carbon nanotube (CNT) microspheres was designed as efficient support material for catalysts with high electrical conductivity and high accommodation ability for Li2O2 products. Consequently, novel and uniquely structured CNT microspheres decorated with Mn-Co-O nanocrystals with a high electrocatalytic activity were designed and synthesized as cathode material for high performance Li–O2 batteries.

Macroporous Mn-Co-O-CNT composite microspheres designed as efficient cathode material for Li-O2 batteries were prepared by one-pot spray pyrolysis. The macroporous structure of CNTs facilitated the wetting of the electrolyte and diffusion of oxygen within the electrode and provided plenty of space to accommodate Li2O2 species formed during the discharge process. The Li-O2 batteries with macroporous Mn-CO-O-CNT composite microspheres showed superior properties as cathode material. The new concept studied in this project can be efficiently applied to the development of highly efficient cathode materials for advanced Li-O2 batteries.

Figure 1