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 CoSe₂ 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 CoSe₂ demonstrates superior Mg-storage capacity, rate performance, lower charge transfer resistance, and higher solid-state Mg²⁺ diffusion coefficients compared to the cubic phase CoSe₂. Mechanism studies reveal that the conversion reaction of orthorhombic CoSe₂ 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 CoSe₂, 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.