Understanding the Improved Electrochemical Performances of Fe-Substituted 5 V Spinel Cathode LiMn_1.5Ni_0.5O₄

Abstract

Partial substitution of Fe for Ni alone or both for Ni and Mn in the 5 V spinel cathode LiMn 1.5 Ni 0.5 O 4 has 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). The Fe-substituted samples exhibit remarkably superior cycling performances and higher rate capabilities than the pristine LiMn 1.5 Ni 0.5 O 4 . 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 Mn 3+ 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.