Cation Ordering in Substituted LiMn 2 O 4 Spinels
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Cation Ordering in Substituted LiMn2O4 Spinels P. Strobel 1, A. Ibarra-Palos 1 and C. Poinsignon 2 1 Laboratoire de Cristallographie CNRS, BP 166, 38042 Grenoble Cedex 9, France 2 LEPMI, ENSEEG, BP 75, 38402 Saint-Martin d'Hères Cedex, France
ABSTRACT In order to overcome the capacity fading of LiMn2O4 in lithium batteries, various substitutions for Mn have been proposed. The structural implications of substitution in LiMn2-xMxO4 with x = 0.5, i.e. with exactly 1/4 octahedral (16d-site) cations replaced, are investigated here. For this stoichiometry, cationic ordering was known previously for M = Mg and Zn, resulting in a superstructure with primitive cubic symmetry. Given the poor chemical contrast of X-ray diffraction between Mn and Co, Ni or Cu, LiMn1.5M0.5O4 samples were studied by neutron diffraction and IR spectroscopy. Both techniques show the occurence of cationic ordering for M = Ni and Cu, but not for Co or Ga. In the case of M = Zn, further complication due Li/Zn ordering on the tetrahedral (8a) site is well resolved by FTIR. This investigation shows that the main driving force for octahedral cation ordering is the charge difference between Mn and M atoms.
INTRODUCTION The spinel compound LiMn2O4 is one of the major candidates for positive electrode materials in lithium batteries [1, 2]. However, its practical use is beset by capacity fading, that numerous groups have sought to reduce by partial substitution of manganese by transition metals M [3]. The presence of M elements such as Cr, Fe, Co, Ni or Cu gives rise to an extra charge plateau at high potential and gives interest to compositions with relatively high substitution levels [4]. An especially interesting stoichiometry is LiMn1.5M0.5O4 (or Li2Mn3MO8), where the replacement of exactly 1/4 of the manganese yields strictly tetravalent Mn if M is divalent. On the basis of X-ray diffraction data, most electrochemical studies on such materials assumed a random substitution of M on the manganese sites (octahedral 16d sites in the spinel structure). However, the 3/4-1/4 ratio can favour ordering of the 16d cations, as in the wellknown compound Li2Mn3MgO8 [5]. This has also been predicted from ab initio studies for other dopants [6]. In this work, a number of LiMn1.5M0.5O4 compounds containing various divalent or trivalent M species are investigated. Using neutron diffraction and FTIR spectroscopy, we will show that compositions are indeed ordered for M = Ni and Cu, whereas the chemical contrast in X-ray diffraction is too weak to establish this by X- ray diffraction. The general trends of random vs. ordered 16d site occupation in lithium manganese oxide spinels as a function of dopant charge and size is discussed. EXPERIMENTAL DETAILS Samples were prepared by solid state reaction using Li2CO3, highly divided MnO2 and appropriate oxides or carbonates of Mg, Co, Ni, Cu, Zn and Ga. Mixtures were ground together and heated repeatedly at 700-850 °C in air with intermittent re-grinding and furnace cooling. For
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neutron diffraction samples,
