CHARACTERIZATION OF LI-1-DELTA-MN2-DELTA-O4 DEFECT SPINEL MATERIALS BY THEIR PHASE-TRANSITION, MAGNETIC AND ELECTROCHEMICAL PROPERTIES

The effects of both the initial Li:Mn ratio and the Mn valency on the electrochemical and the magnetic properties and on the structural chan ges were investigated using stoichiometric and nonstoichiometric LiMn2 O4 samples prepared by a solid-state reaction between 673 and 1173 K. A small but known additional plateau near 3.0-3.2 V was observed on di scharging the Li/LiMn2O4 cell using samples with an Mn valency < 3.60. However, no plateau at similar to 3.2 V was observed for the cell us ing samples with an Mn valency of 3.61+. Below 280 K the structure of the former samples changed from a cubic spinel structure into two phas es with a cubic and a tetragonal structure, whereas the structure of t he latter, i.e. an Mn4+-rich sample, remained cubic down to 20 K. Ther efore, the additional plateau could be correlated with the instability of the cubic spinel structure of the sample. (C) 1997 Elsevier Scienc e S.A.