Cu 9 S 5 Nanoflower Cathode for Mg Secondary Batteries: High Performance and Reaction Mechanism

Mg secondary batteries have attracted widespread attention as a promising large‐scale energy storage system with high safety and high natural abundance, but their development is hindered by the limited cathode materials. Herein, a Cu 9 S 5 nanoflower as a high‐performance conversion cathode for Mg secondary batteries is reported. This material delivers a reversible capacity of 200 mAh g −1 at 20 mA g −1 , a good rate capability of 90 mAh g −1 at 1000 mA g −1 , and a superior long‐term cycleability over 400 cycles. The reversible conversion reaction mainly occurs between Cu 1.96 S and Cu/MgS. Significantly, the outstanding performance is ascribed to the moderate dissolution of the copper polysulfide intermediate in the electrolyte, which substitutes a fast liquid‐phase copper ion diffusion for the sluggish solid‐phase Mg 2+ ion diffusion, and thus facilitates the reaction kinetics. A scientific insight into the abnormally high performance of copper sulfide as a Mg battery cathode and a significant hint for the performance enhancement of Mg‐S batteries are provided.