Reductive solvothermal synthesis of MgMn₂O₄ spinel nanoparticles for Mg-ion battery cathodes

Rechargeable Mg-ion batteries have gained significant attention as promising alternatives to Li-ion batteries. Owing to its high theoretical energy density and relatively high Mg-ion diffusivity, spinel oxide MgMn₂O₄ is a viable candidate as a cathode material; however, its poor rate capability limits its applicability. Decreasing the particle size can effectively address this problem by enhancing Mg-ion diffusion. In this paper, we demonstrate the conventional solvothermal synthesis of MgMn₂O₄ spinel nanoparticles. Solvothermal process is one of the most fundamental methods for nanoparticle synthesis because of its simple and flexible synthetic conditions. In the alcohol solvothermal conditions, spinel type MgMn₂O₄ nanoparticles of approximately 10–15 nm are successfully synthesized using amorphous MnO₂ as a precursor. We note that controlling Mg²⁺ solvation and oxidation/reduction conditions in the reaction solution is crucial for the effective intercalation of Mg²⁺ into the MnO₆ octahedral framework. Although the obtained MgMn₂O₄ nanoparticles aggregate to form submicron secondary particles, the aggregation can be suppressed by compositing them with the carbon nanotubes dispersed in the reaction solution. The composite exhibits a discharge capacity of 60 mAh g⁻¹ with maintaining 80% of capacity retention after the 10th cycle.