Oxygen Deficiency as the Origin of the Disparate Behavior of LiM0.5Mn1.5 O 4 ( M = Ni , Cu ) Nanospinels in Lithium Cells

Abstract

A comparative study of the electrochemical properties of two spinels of nominal composition (M = Ni, Cu) was conducted. The doping element introduces significant structural changes in stoichiometry, cation distribution, and oxidation state of the elements. Whereas Ni spinels were virtually stoichiometric and exhibit mostly normal structure, the Cu samples were oxygen-deficient spinels with a greater contribution of the inverse structure. This was especially so in the sample obtained at 800°C, where X-ray photoelectron spectroscopy (XPS) revealed the presence of Its electron configuration favored the occupancy of tetrahedral positions and the resulting displacement of some ions to octahedral positions. All these features account for the increased content of the Cu spinels, consistent with electrochemical measurements. Ex situ XPS measurements confirmed the oxidation processes of Ni and Cu ions on charging the cell at 5.0 V, and the reversibility of the electrochemical reaction undergone by Ni. However, the reversibility of the reaction could not be demonstrated because of the low capacity delivered by the cell in the 5.0-4.5 V region. The structural defects in the Cu spinels have an adverse impact on cell performance. Compared to the Ni spinel, the Cu spinel possesses a low discharge capacity that fades faster on cycling. © 2005 The Electrochemical Society. All rights reserved.