The effects of both the initial Li:Mn ratio and the Mn valency on the electrochemical and the magnetic properties and on the structural changes were investigated using stoichiometric and nonstoichiometric LiMn2O4 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 discharging 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 using 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 phases with a cubic and a tetragonal structure, whereas the structure of the latter, i.e. an Mn4+-rich sample, remained cubic down to 20 K. Therefore, the additional plateau could be correlated with the instability of the cubic spinel structure of the sample.