Boosting the Electrocatalytic Activity of Co3O4 Nanosheets for a Li-O2 Battery through Modulating Inner Oxygen Vacancy and Exterior Co3+/Co2+ Ratio

Rechargeable Li-O-2 batteries have been considered as the most promising chemical power owing to their ultrahigh specific energy density. However, the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) result in high overpotential (similar to 1.5 V), poor rate capability, and even a short cycle life, which critically hinder their practical applications. Herein, we propose a synergistic strategy to boost the electrocatalytic activity of Co3O4 nanosheets for Li-O-2 batteries by tuning the inner oxygen vacancies and the exterior Co3+/Co2+ ratios, which have been identified by Raman spectroscopy, X-ray photoelectron spectroscopy, and X-ray absorption near edge structure spectroscopy. Operando Xray diffraction and ex situ scanning electron microscopy are used to probe the evolution of the discharge product. In comparison with bulk Co3O4, the cells catalyzed by Co3O4 nanosheets show a much higher initial capacity (similar to 24051.2 mAh g(-1)), better rate capability (8683.3 mAh g(-1)@400 mA g(-1)) and cycling stability (150 cycles@400 mA g(-1)), and lower overpotential. The large enhancement in the electrochemical performances can be mainly attributed to the synergistic effect of the architectured 2D nanosheets, the oxygen vacancies, and Co3+/Co2+ difference between the surface and the interior. Moreover, the addition of LiI to the electrolyte can further reduce the overpotential, making the battery more practical. This study offers some insights into designing high-performance electrocatalysts for Li-O-2 batteries through a combination of the 2D nanosheet architecture, oxygen vacancies, and surface electronic structure regulation.