Metastable and nanosize cation-disordered rocksalt-type oxides: revisit of stoichiometric LiMnO2 and NaMnO2

Stoichiometric LiMnO2 and NaMnO2 with a cation-disordered rocksalt-type structure as metastable polymorphs were successfully prepared by mechanical milling. Although cation-disordered rocksalt phases with a stoichiometric composition (Li : Mn molar ratio = 1 : 1) are expected to be electrochemically less active, both samples show superior performance as electrode materials when compared with thermodynamically stable layered phases in Li/Na cells. Both metastable samples deliver large reversible capacities, which correspond to >80% of their theoretical capacities, with relatively small polarization on the basis of reversible Mn3+/Mn4+ redox. Moreover, for rocksalt LiMnO2, the phase transition into a spinel phase is effectively suppressed compared with a thermodynamically stable phase. The electrode reversibility of NaMnO2 is also drastically improved by the use of the metastable phase with good capacity retention. Metastable phases with unique nanostructures open a new path for the design of advanced electrode materials with high energy density, and thus a broad impact is anticipated for rechargeable Li/Na battery applications.