Synthesis of Li-Deficient Type LiMn₂O₄ Spinel and Its Gradual Structural Change

Abstract

Spinel-type lithium manganese oxides with lithium-deficient compositions in the atomic ratio range of Li/Mn=y=0.475-0.485 were obtained in a single phase by sintering a mixture of Li₂CO₃ and MnCO₃ at 750°C in air, followed by quenching into liquid nitrogen. Initially, they were a cubic (Fd3m) compound like the stoichiometric LiMn₂O₄ spinel. However, it was found that the structure turned from cubic to orthorhombic (Fddd) just in storage at room temperature. The change was slow and it took several days or more depending on the sample's composition. The changes in the density, the manganese valence and the weight of the samples accompanied with their structural change were monitored. Those measurements revealed that, though Li-deficient spinels immediately after preparation took a Li (8a)-defective arrangement (Li_xMn₂O₄, x<1), they were gradually changed to a Mn-excess compound (Li_yMn_2+δO_4-δ; y/(2+δ)=x/2, y≤1). It is likely that Jahn-Teller Mn³⁺ ion increased by reduction of the compound generates the structural distortion.