Rechargeable magnesium batteries are promising candidates for post-lithium-ion batteries, owing to the large source abundance and high theoretical energy density. However, there remain few reports on constructing practical cells with oxide cathodes and Mg anodes at room temperature. In this work, we compare the reaction behavior of various MnO 2 polymorph cathodes in two representative electrolytes: Mg[TFSA] 2 /G3 and Mg[Al(hfip) 4 ] 2 /G3. In Mg[TFSA] 2 /G3, discharge capacities of the MnO 2 cathodes are well consistent with the changes in Mg composition, where nanorod-like α-MnO 2 and λ-MnO 2 show the capacities of about 100 mA h g −1 at room temperature. However, this electrolyte has the disadvantage that the Mg anodes are easily passivated. In contrast, Mg[Al(hfip) 4 ] 2 /G3 allows highly reversible deposition/dissolution of Mg anodes, whereas the discharge process of the MnO 2 cathodes involves a large part of side reactions, in which the MnO 2 active material takes part in some reductive reaction together with electrolyte species instead of the expected Mg 2+ intercalation. Such an unstable electrode/electrolyte interface would lead to continuous degradation on/near the cathode surface. Thus, the interfacial stability between the oxide cathodes and the electrolytes must be improved for practical applications.

Mn-based spinel-oxide cathode materials are promising for achieving high-energy-density rechargeable Mg batteries (RMBs). However, Mg insertion into them often induces unfavorable phase transformation due to the poor stability of λ-MnO2, leading to capacity fading during cycling. Defect spinel ZnMnO3, which can be regarded as ZnO-stabilized λ-MnO2, is an outstanding exception that allows highly reversible Mg insertion/extraction. To further understand its phase stability, here we investigate wide-range compositions in Mg–Zn–Mn oxide systems and show that the stability of the spinel structure can be significantly improved by compositionally incorporating stable XO (X = Zn, Mg) with λ-MnO2. In particular, (i) the equimolar mixing of XO and MnO2 is critical to obtaining a single-phase cubic spinel structure and (ii) a higher Zn/Mg ratio is effective for preventing the formation of an irreversible rock salt phase to decrease the overpotential during discharge/charge cycling. Consequently, Zn-rich Mg–Zn–Mn oxides with the cubic spinel structure delivered as high as 120 mAh/g discharge capacities repeatedly at an elevated temperature of 150 °C. This work provides a fundamental understanding of the phase stability of Mg–Zn–Mn oxide materials and insights into designing high-performance cathode materials for RMBs.