Photo quiz. Blue-gray centrofacial hyperpigmentation

Partial substitution of Fe for Ni alone or both for Ni and Mn in the 5 V spinel cathode LiMn1.5Ni0.5O4 and been investigated by characterizing the samples by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR). charge-discharge measurements in lithium cells, electrochemical impedance spectroscopy (EIS), chronoamperometry (CA), and X-ray photoelectron spectroscopy (XPS). Tile Fe-substituted samples exhibit remarkably superior cycling performances and higher rate capabilities than the pristine LiMn1.5Ni0.5O4. The greatly enhanced electrochemical performances by the Fe substitution are attributed to the (i) stabilization of the structure with cation-disorder in the 16d octahedral sites of the spinel lattice, (ii) Suppression of the formation of a thick solid-electrolyte interfacial (SEI) layer due to the Fe-enrichment and Ni-deficiency on the surface, (iii) production of Mn3+ and the consequent enhancement in electronic conductivity, and (iv) much reduced polarization loss arising from both fast charge transfer kinetics and lithium ion diffusion kinetics in the bulk.