The effect of 3d substitutions in the manganese sublattice on the charge transport mechanism and electrochemical properties of manganese spinel

In this paper we present structural, XPS, thermal (DSC and TG) and electrical (conductivity and thermoelectric power (TEP)) measurements of series of manganese spinel samples with manganese substituted to different degree (x=0-0.5) with other 3d metals LiMxMn2-xO4 (M=Cr, Fe, Co, Ni, Cu). Electrochemical studies of such characterized cathode materials were conducted in a Li/Li+/LiyMxMn2-xO4 type cell. Substitution of manganese causes disappearance of the phase transition characteristic of a stoichiometric spinel. The small polaron mechanism of conductivity is preserved for substituted materials but with different activation energies. Studies of electrical properties reveal that Cr, Co and Ni doped ions do not participate in charge transport at low temperatures. In the charge curves of Li/Li+/LiyMxMn2-xO4 cells there are two visible plateaux, separated with distinct potential jump (0.5-0.7V), which position on Li content perfectly matches the Mn3+ content in the doped cathode material. The lower plateau is related to the Mn3+-->Mn4+ oxidation, while the next of higher voltage, to the Mm+ dopant oxidation. The schematic diagrams of relative Mn-M electronic levels alignment are proposed. (C) 2004 Elsevier B.V. All rights reserved.