Coordination environment modulation to optimize d-orbit arrangement of Mn-based MOF electrocatalyst for lithium-oxygen battery

Nowadays lithium-oxygen (Li-O2) batteries with metal-organic frameworks (MOFs) based oxygen electrodes have suffered from the sluggish kinetics and irreversible behavior of Li2O2 formation/decomposition, which originates from weak orbit coupling with oxygen species caused by narrow orbit arrangement of metal sites in MOFs. Modulation of coordination environment of metal sites has been regarded as the vital strategy to tune 3d orbit arrangement and electron distribution of metal sites, which is favorable for altering electron transfer pathway and oxygen electrode reaction kinetics. Herein, electron-rich ferrocene acid (FcA), as grafted ligand, is introduced on Mn sites of Mn-MOF-74 (named as Mn-MOF-74-FcA) for facilitating the oxygen electrode reactions in Li-O2 batteries. In details, due to the strong electron delocalization on electron-rich FcA ligand, the resulted metal-ligand interaction between FcA and Mn sites distorts [MnOx] coordination sphere, which induces 3d orbits rearrangement of Mn sites from (dx2-y2/dxy, dz2, dxz/dyz) to (dx2-y2, dz2, dxz, dyz, dxy) and electron transfer from electron-rich FcA to Mn sites. And the strong orbit overlapping between LiO2 intermediate and widened Mn d-band ensures the electron exchange with each other and thus reduces the energy barrier of oxygen electrode reactions. Moreover, electron transfer from FcA ligand to Mn sites also leads to the decreased energy gap between valence band (VB) and conduction band (CB). As expected, Li-O2 battery with Mn-MOF-74-FcA exhibits low overpotential of 0.82 V and long lifespan of 276 cycles at the current density of 200 mA g−1.