Facile Synthesis of Metastable Mg-Mn Spinel Nanoparticles for Magnesium-Ion Batteries

摘要

High energy and high density rechargeable batteries are indispensable for widespread portable electronic devices and the spreading of electric vehicles. Rechargeable Mg-ion batteries have gained much attention as promising alternatives to Li-ion batteries due to the high natural abundance, high volumetric energy density, and no dendrite formation of the Mg metal anode. While Mg-ion batteries are nowadays progressing rapidly, their practical use is still hampered by problems related to both cathode materials and electrolytes. One of the crucial problems is the very low rate capability at the cathodes due to the slow diffusion of Mg~(2+) ions in solids, therefore Mg-ion batteries work only at low current densities or at high temperature. Among various oxide cathode candidates for Mg-ion batteries, spinel oxides have a high redox potential and a relatively high ion-diffusivity. Especially in the common electrochemical window of electrolytes, MgMn_2O_4 spinel can exhibit a high theoretical capacity of 540 mAh g~(-1) using both Mn~(3+)/Mn~(4+) and Mn~(3+)/Mn~(2+) redox reactions (λ-MnO_2 formation and rock-salt Mg_2Mn_2O_4 formation reactions, respectively). However, these reactions should exhibit large polarizations because of the less reversible tetragonal-cubic phase transitions; MgMn_2O_4 has a tetragonal spinel structure due to the Jahn-Teller effect of Mn~(3+) ions, while λ-MnO_2 and Mg_2Mn_2O_4 are cubic phase. A suppression of the lattice distortion of MgMn_2O_4 is necessary for improving its cathode performance. Although a cubic MgMn_2O_4 phase is known as a metastable phase, this phase is only obtained at high-temperature or high-pressure in bulk. In this study, we aimed at synthesizing metastable cubic MgMn_2O_4 spinel nanoparticles under ambient conditions and applying them as cathodes for Magnesium-ion batteries.