Abstract

Three series of spinels of nominal composition LiM_xMn_2 − xO₄ (M = Fe, Co, Ni; x ≈ 0.3) were prepared by using a carbonate-based precursor method followed by calcining in the air at 400, 600 and 900 °C. The spinels were characterized by chemical analysis, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), and tested as cathodes in lithium cells over the range 3.5 to 5.2 V vs Li/Li⁺. XPS data revealed that Co and Fe adopt a tervalent oxidation state whereas Ni is in a divalent state. No sign of Mn²⁺ was detected. Above 400 °C, all the spinels contain a small fraction of a transition element in their 8a tetrahedral sites, together with lithium ions. The three series have the ability to extract lithium above 4.5 V, which is accompanied by the oxidation of the dopant transition metal. The reversibility of this reaction increases with higher calcining temperatures as a result of better crystallinity and the smoother particle surfaces. However, the iron spinels depart from this trend; thus, the cell obtained from the sample heated at 600 °C possesses good cycling properties and maintains a capacity of ca. 100 A h kg⁻¹ upon cycling. By contrast, the Fe-containing spinel obtained at 900 °C loses its whole capacity after the first few cycles. Partial extrusion of iron as Fe₂O₃ and non-uniformity in particle size and shape may account for its poor electrochemical behavior. The Co- and Ni-containing spinels obtained at 900 °C lack these features; their cells exhibit excellent capacity retention upon extended cycling and are capable of delivering 100 and 120 A h kg⁻¹, respectively, at an average voltage of 4.5 V. They can supply a higher energy density than the common spinel LiMn₂O₄.