Adjusting the Covalency of Metal-Oxygen Bonds in LaCoO by Sr and Fe Cation Codoping to Achieve Highly Efficient Electrocatalysts for Aprotic Lithium-Oxygen Batteries.

Developing high-efficiency dual-functional catalysts to promote oxygen electrode reactions is critical for achieving high-performance aprotic lithium-oxygen (Li-O) batteries. Herein, Sr and Fe cation-codoped LaCoO perovskite (LaSrCoFeO, LSCFO) porous nanoparticles are fabricated as promising electrocatalysts for Li-O cells. The results demonstrate that the LSCFO-based Li-O batteries exhibit an extremely low overpotential of 0.32 V, ultrahigh specific capacity of 26 833 mA h g, and superior long-term cycling stability (200 cycles at 300 mA g). These prominent performances can be partially attributed to the existence of abundant coordination unsaturated sites caused by oxygen vacancies in LSCFO. Most importantly, density functional theory (DFT) calculations reveal that codoping of Sr and Fe cations in LaCoO results in the increased covalency of Co 3d-O 2p bonds and the transition of Co from an ordinary low-spin state to an intermediate-spin state, eventually resulting in the transformation from nonconductor LCO to metallic LSCFO. In addition, based on the theoretical calculations, it is found that the inherent adsorption capability of LSCFO toward the LiO intermediate is reduced due to the increased covalency of Co 3d-O 2p bonds, leading to the formation of large granule-like LiO, which can be effectively decomposed on the LSCFO surface during the charging process. Notably, this work demonstrates a unique insight into the design of advanced perovskite oxide catalysts via adjusting the covalency of transition-metal-oxygen bonds for high-performance metal-air batteries.