The Out‐of‐Plane C─S Bonds Boosting Reversible Redox in Copper Sulfide Cathodes for Ultradurable Magnesium Battery

Abstract As a typical conversion‐type cathode material, CuS has shown great potential in the field of rechargeable magnesium batteries (RMBs) due to its excellent energy density, stable voltage platforms, and low cost. However, the poor phase conversion reversibility in CuS cathodes has resulted in low Coulombic efficiency and short cycling life, impeding its further development. Herein, an abundance of CS heterointerfaces is meticulously crafted by the CuS nanoparticles anchored on rGO nanosheets (CuS@G). The out‐of‐plane C─S bonds effectively reduce the activation energy of sulfur atoms within Cu‐S tetrahedrons, facilitating the formation of S─S bonds in the Cu₂S crystal structure and driving the reversible phase conversion between Cu₂S and CuS during the charge/discharge process. Furthermore, a more reversible phase conversion could diminish copper ion dissolution induced by volume expansion. Consequently, the CuS@G cathode exhibits one of the most remarkable rate performances to date (160.5 mAh g −1 at 1 A g −1 ), retaining 64.7% of its capacity after 1000 cycles. Additionally, a durable CuS@G||Mg pouch cell is successfully assembled, delivering a high capacity of 9.5 mAh. These fundamental insights provide valuable guidance for the design of high‐performance conversion cathode materials for next‐generation RMBs.