Evaluating the functions of the key dopant elements in multi-metal oxide electrocatalysts for high-performance Li-O2 batteries

Multi-metal oxides (MMOs) have emerged as promising electrocatalysts for high-performance Li-O2 batteries (LOBs) because of their tailored intrinsic properties. However, the complex nature of MMOs, with multiple elements involved, has made it challenging to fully comprehend the electrochemical behaviour of MMOs in relation to the metal composition. Herein, we systematically synthesized a series of spinel MMOs, including Ni-Co-Fe, Mn-Co-Fe, and Ni-Mn-Fe oxides, by doping Ni, Co, or Mn into spinel Fe3O4 via a hydrothermal method. The properties of the obtained MMOs, including metal element cooperation, oxygen vacancy, and substitution positions of the metal cations, are carefully examined. We further evaluated the obtained MMOs as electrocatalysts in LOBs to investigate the dependency between their metal composition and electrocatalytic properties. Interestingly, we found the ratio of Fe3+/Fe2+ in the MMOs can be changed by matching different doping elements, affecting the cycling stability of LOBs with Fe-based MMOs. It is also revealed that Ni-Co-Fe oxides have stronger adsorption of O2 and LiO2 and better electrical conductivity than Mn-Co-Fe and Ni-Mn-Fe oxides. In addition, more oxygen vacancy in the crystal structure of MMOs can be obtained by doping more Ni and Co atoms, which facilitates electron/Li+ transportation and oxygen adsorption. With the increased amount of the doped Ni and Co in the Ni-Co-Fe oxides, the LOBs demonstrated an improved specific capacity from 8120 mAh g−1 to 10698 mAh g−1 at the same current density of 200 mA g−1. This systematic investigation of the MMO composition has provided a profound understanding of MMOs electrocatalysts, offering valuable insights for designing and optimizing MMO-based electrocatalysts for advanced LOBs.