Favorable Orthorhombic Phase Cobalt Diselenide Cathode for Rechargeable Mg Batteries: Elucidating the Significant Impact of Crystal Structure on Conversion‐Type Mg‐Storage Reactions

Abstract Rechargeable Mg batteries are an energy‐storage technology suitable for large‐scale applications, but the lack of high‐performance cathode materials is currently hindering their development. Conversion‐type cathodes break the limits of Mg‐intercalation principle, but existing structural design strategies mostly focus on morphology optimization to increase active reaction interfaces. The present study reveals that crystal structure also plays a significant role in the Mg‐storage activity of conversion reactions. Two types of CoSe 2 with orthorhombic and cubic phases are synthesized from ZIF‐67 and comparatively investigated as cathode materials for RMBs. Despite exhibiting similar micromorphology and a lower specific surface area, the orthorhombic phase CoSe 2 demonstrates superior Mg‐storage capacity, rate performance, lower charge transfer resistance, and higher solid‐state Mg 2+ diffusion coefficients compared to the cubic phase CoSe 2 . Mechanism studies reveal that the conversion reaction of orthorhombic CoSe 2 is more thorough and reversible, involving the redox of both cations and anions. Further theoretical computations indicate that the higher reaction activity at (010) plane of orthorhombic CoSe 2 , along with more active sites of Se‒Se bonds, facilitates the conversion Mg‐storage reaction via co‐redox of the cations and anions. This study underscores the importance of crystal structure in the design of conversion‐type RMB cathode materials.