Romanechite, an Asymmetric Tunnel‐Type MnO₂, for Rechargeable Magnesium Battery Cathodes

Rechargeable magnesium batteries are expected to serve as alternatives to lithium‐ion batteries due to their advantages in high volumetric energy density, low cost, and safety, associated with using the magnesium metal anode. Tunnel‐type manganese dioxides have been intensively explored as cathode materials due to their robust host structure and high operating potential. However, Hollandite (α‐MnO 2 ), a standard tunnel‐type MnO 2 having 2 × 2 channels, shows an irreversible structure change during magnesium intercalation, from a symmetric tunnel structure to an asymmetric tunnel structure, due to the deviated Mg site from the tunnel center. Herein, an asymmetric 3 × 2 tunnel‐type MnO 2 , Romanechite, is synthesized using a room‐temperature alcohol solution process. The obtained Romanechite has plate‐like nanoparticles and exhibits a discharge capacity of 130 mAh g −1 with no significant structure change during the discharge and recharge, demonstrating superior electrochemical magnesium intercalation/extraction efficiency and capacity retention at room‐temperature operation.