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

Inversion, that is, Mg/Mn antisite disorder, in aspinel oxide simultaneously causes blockage of favorable Mg2+migration paths, raising activation barriers for diffusion, and itreduces the number of redox-active metals, limiting the maximumcapacity in the spinel. An inversion-free spinel, MgCr1.5Mn0.5O4, wassynthesized by exploiting the different intrinsic crystalfieldstabilization of redox-active Cr and Mn in the form of a solidsolution. The capability of the tailored spinel to reversibly(de)intercalate Mg2+at high redox potentials was investigated.The decrease in inversion dramatically lowered the electrochemicaloverpotential and hysteresis and enabled utilization of highpotentials at similar to 2.9 V (vs Mg/Mg2+) upon re-intercalation of Mg2+.A combination of characterization techniques reveals that thestructural, compositional, and redox changes within the spinel oxidewere consistent with the observed electrochemical Mg2+activity. Quantification of selection solely to lattice Mg2+upon theelectrochemical reaction was investigated by monitoring nuclear magnetic resonance signals in isotope25Mg-enriched spinel oxides.Ourfindings enhance the understanding of Mg2+transport within spinel oxide frameworks and provide conclusive evidence for bulkMg migration in oxide lattices at high redox potentials with minimized electrochemical hysteresis.