Invited Presentation: Tailoring Cathode and Morphology of Discharge Products Toward High-Rate and Long-Life Lithium-Oxygenbatteries

The pressing concerns of reducing greenhouse gas emissions and the consumption of non-renewable fossil fuels have led to increased demand for electrochemical energy storage systems with energy densities that far exceed those of the lithium-ion battery. To this end, rechargeable lithium-oxygen (Li-O₂) batteries have been the focus of great interest due to their extremely high energy potentiality (up to 2-3 kWh kg^-1) because oxygen, the cathode material, is not necessarily stored in the battery, as with convention lithium-ion battery technology, but can be accessed from the environment. However, in practice, the system has presented many scientific and technological challenges since it was introduced in 1996. The decomposition of the organic electrolytes during the discharge and charge processes causes the battery instability and electrolyte depletion. The irregular precipitation of thick film-like or large toroidal-like insoluble discharge product, Li₂O₂, blocks the void of the O₂ cathode and eventually limits the battery performance, including the specific capacity, charge efficiency, and cycle life. The continuous accumulation of Li₂CO₃ and lithium carboxylates, resulting from side reactions of the Li₂O₂ and intermediate species, such as LiO₂* and O₂^-, with the cathode and the electrolyte on cycling, leads to cathode passivation and battery death. The insufficient structural stability of the cathode during Li₂O₂ formation/decomposition is also a limitation for battery cycle life.

In response, we have designed and fabricated a series of advanced cathodes, which hold many favorable properties including high electronic conductivity, tuned porous structure, and high electrocatlytic activity for both oxygen reduction and evolution reactions, to reduce the discharge/charge overpotentials to alleviate electrolyte decomposition, tailoring the deposition site and morphology of Li₂O₂ within cathode to enhance the specific capacity, charge efficiency, and cycle life. Finally, a free-standing honeycomb-like palladium-modified hollow spherical carbon deposited onto carbon paper cathode is successfully obtained, which endows a lithium-oxygen battery with high-rate capability (5900 mAh g^-1 at a current density of 1.5 A g^-1) and long-term (100 cycles at a current density of 300 mA g^-1 and a specific capacity limit of 1000 mAh g^-1) stability. These superior properties could be explained by the tailored deposition and morphology of the discharge products as well as the alleviated electrolyte decomposition compared with the conventional carbon cathode. The obtained performances provide a promising example and promote the effort to design more advanced cathode architecture for lithium-oxygen batteries.