Facile Electrochemical Mg-Ion Transport in a Defect-Free Spinel Oxide

Inversion, that is, Mg/Mn antisite disorder, in a spinel oxide simultaneously causes blockage of favorable Mg2+ migration paths, raising activation barriers for diffusion, and it reduces the number of redox-active metals, limiting the maximum capacity in the spinel. An inversion-free spinel, MgCr1.5Mn0.5O4, was synthesized by exploiting the different intrinsic crystal field stabilization of redox-active Cr and Mn in the form of a solid solution. The capability of the tailored spinel to reversibly (de)intercalate Mg2+ at high redox potentials was investigated. The decrease in inversion dramatically lowered the electrochemical overpotential and hysteresis and enabled utilization of high potentials at ∼2.9 V (vs Mg/Mg2+) upon re-intercalation of Mg2+. A combination of characterization techniques reveals that the structural, compositional, and redox changes within the spinel oxide were consistent with the observed electrochemical Mg2+ activity. Quantification of selection solely to lattice Mg2+ upon the electrochemical reaction was investigated by monitoring nuclear magnetic resonance signals in isotope 25Mg-enriched spinel oxides. Our findings enhance the understanding of Mg2+ transport within spinel oxide frameworks and provide conclusive evidence for bulk Mg migration in oxide lattices at high redox potentials with minimized electrochemical hysteresis.