Mg rechargeable batteries (MRBs) have attracted much attention as a post Li-ion battery owing to high capacity, low electrode potential, and safety of Mg metal anode. However, it is generally a laborious task to seek suitable cathode materials for MRBs. According to the previous report [1], Mg insertion into several kinds of spinel oxide cathodes, Mg(TM)₂O₄ (TM = Co, Mn, etc), occurs at about 2-3 V vs Mg²⁺/Mg, which causes the spinel-to-rocksalt transition. In addition, it is noteworthy that Mg extraction from MgMn₂O₄ has been shown to occur at intermediate temperatures (around 150 ℃), but the detailed mechanism (such as structure change, electrochemical reaction, valence change, etc) has been still unclarified. In this study, we aim to investigate the feasibility of electrochemical extraction of Mg cations from MgMn₂O₄ to seek a better cathode material delivering a higher voltage.

MgMn₂O₄ was synthesized by the Pechini method [2] and calcined at 425 °C in air. After mixed with carbon black and binder, MgMn₂O₄ was coated on Al foil. Electrochemical experiments were conducted in three-electrode cells at 150 ℃, where Li reference electrode was employed to compare with the Li-extraction potential from the same material; the Li reference electrode was well separated from the ionic liquid Mg_0.1Cs_0.9-TFSA bulk electrolytes with ceramic filters. Compositions and crystal structures of MgMn₂O₄ cathodes after electrochemical experiments were investigated by ICP, XRD, and XPS analyses.

Compositions of MgMn₂O₄ cathodes after charge (Mg extraction from spinel) and also discharge (Mg insertion into spinel) processes are shown in Fig. 1. The Mg composition is decreased to 0.73 at charge amount equivalent to 139 mAh/g and to 0.58 at 346 mAh/g from the initial composition (x~1.04), which indicates that Mg cations can be certainly extracted from MgMn₂O₄, however, accompanying by some degree of electrolyte decomposition. To grasp the electrochemical equilibrium properties of Mg-extraction from MgMn₂O₄, galvanostatic intermittent titration technique (GITT) was employed. Figure 2 shows that the overpotential of Mg extraction is increased on charging, the open circuit potential reaches 3.9 V vs Li⁺/Li, and it is substantially unchanged till the end of the measurement. The plateau potential indicates that two-phase equilibrium reaction occurs during the Mg extraction process. Synchrotron XRD measurement reveals that the charged samples consist of the original spinel phase and a cubic spinel phase, and XPS measurement also suggests that the valence state of Mn is shifted to a higher (binding energy) from the initial state after charge.