Mechanism of iron integration into LiMn1.5Ni0.5O₄ for the electrocatalytic oxygen evolution reaction

Spinel-type LiMn1.5Ni0.5O₄ has been paid temendrous consideration as an electrode material because of its low cost, high voltage, and stabilized electrochemical performance. Here, we demonstrate the mechanism of iron (Fe) integration into LiMn1.5Ni0.5O₄ via solution methods followed by calcination at a high temparature, as an efficient electrocatalyst for water splitting. Various microscopic and structural characterizations of the crystal structure affirmed the integration of Fe into the LiMn1.5Ni0.5O₄ lattice and the constitution of the cubic LiMn1.38Fe0.12Ni0.5O₄ crystal. Local structure analysis around Fe by extended X-ray absorption fine structure (EXAFS) showed Fe3+ ions in a six-coordinated octahedral environment, demonstrating incorporation of Fe as a substitute at the Mn site in the LiMn1.5Ni0.5O₄ host. EXAFS also confirmed that the perfectly ordered LiMn1.5Ni0.5O₄ spinel structure becomes disturbed by the fractional cationic substitution and also stabilizes the LiMn1.5Ni0.5O₄ structure with structural disorder of the Ni²⁺ and Mn⁴⁺ ions in the 16d octahedral sites by Fe²⁺ and Fe³⁺ ions. However, we have found that Mn³⁺ ion production from the redox reaction between Mn⁴⁺ and Fe²⁺ influences the electronic conductivity significantly, resulting in improved electrochemical oxygen evolution reaction (OER) activity for the LiMn1.38Fe0.12Ni0.5O4 structure. Surface-enhanced Fe in LiMn1.38Fe0.12Ni0.5O₄ serves as the electrocatalytic active site for OER, which was verified by the density functional theory study.