Accelerated Kinetics Revealing Metastable Pathways of Magnesiation-Induced Transformations in MnO₂ Polymorphs

The intrinsic potential of manganese dioxides, considered high-capacity cathodes of rechargeable magnesium batteries, was clearly exposed under conditions where the Mg migration kinetics are sufficiently enhanced. It has been reported to date that magnesium insertion into MnO2 is substantially confined to the surfaces of MnO2 particles due to its sluggish kinetics at room temperature, which leads to local overmagnesiation conditions causing conversion reactions etc. To unveil its ergodic or metastable phase-transformation pathways of MnO2 polymorphs (α, β, γ, δ, and λ) during magnesiation, this study employed intermediate-temperature electrochemical experiments (at 150 °C) using heat-tolerant ionic liquid electrolytes. Regardless of its original polymorphic structure, each MnO2 polymorph was found to transform into a Mg-including spinel and then to a rocksalt-like phase by magnesiation. Given this tendency of transformation, the defect spinel λ-MnO2 phase possessing the coherent framework of spinel/rocksalt structures is expected to follow a topotactic transformation pathway, but thermally unstable λ-MnO2 underwent spontaneous reduction into Mn3O4 before magnesiation in an electrolyte. Instead, α-MnO2 was found to be robust enough among MnO2 polymorphs to exhibit reversible magnesium intercalation at 150 °C under limiting capacity conditions. This result highlights that reversible magnesium intercalation in oxide cathodes is feasible for structures that are kinetically resistant to irreversible transformation pathways to spinel and rocksalt structures.