Optimized orbital occupancy of transition metal in spinel Ni-Co oxides with heteroatom doping for Aprotic Li-O2 battery

Abstract

Li-O2 battery is one of the most attractive energy storage technologies because of its extremely high energy density (3500 Wh kg−1). However, the main limitation of the battery is the high energy barrier during the formation and decomposition of the discharge product (Li2O2), which results in a series of problems such as large overpotential and poor cycle stability. Herein, the Fe-doped spinel Ni-Co oxides are employed as the cathode catalyst to decrease the energy barrier of the oxygen electrode reactions in Li-O2 battery. The batteries with Fe-doped Ni-Co oxides deliver a large discharge specific capacity of 16,727 mAh g−1 and remarkable durability of over 790 h at current density of 500 mA g−1. Based on the density functional theory (DFT) calculation, the optimized performance is attributed to the near-unity eg electron occupancy (1.32) in transition metal atom for iron-doped Ni-Co oxide as compared to that for undoped Ni-Co oxide (1.64). The near-unity eg electron occupancy can increase the covalency of transition metal-oxygen bonds and finally enhance the electrocatalytic activity. This study is helpful for deeply understanding the relationship between the surface electronic structure and catalytic activity of oxygen electrocatalysts in Li-O2 cells.