Iron Doping Effects in NiMn2O4 Structures with Hierarchical Porosity: Stabilization of the Cubic Spinel Phase and Kinetics Activation for Li Storage

Spinel-type mixed oxides possess richer redox reactions for Li⁺ storage and at least two orders of magnitude higher electrical conductivity than binary metal oxides. This talk demonstrates the synthesis of  NiMn₂O₄ composite with three-dimensional macro/mesoporous network and various iron doping amount via a facile and scalable nanocasting method. Multi-scale pores were integrated in the structure through the structural engineering: interconnected macropores were reverse-replicated by employing the polymeric sacrificial template; whereas mesopores were formed upon the pyrolysis of the polymer during heat treatment. Through tailoring the metal precursor ratio, the tetragonal sites of divalent nickel could be preferentially substituted by iron, resulted in the stabilized cubic phase of quaternary oxide Fe_xNi_1-xMn₂O₄. Combining the elemental analysis from XPS and EELS techniques as well as crystal field theory, we concluded the stabilization mechanism of iron doping in our spinel structure system. Electrochemical characterizations exhibited the best of the composites prepared as such was capable to deliver high specific capacity, remarkable rate capability (2 A.g^-1) and very good cycle life for Li⁺ storage (600 cycles).