Unlocking a Fast Track for Magnesium Migration in\nCu<sub>1.8</sub>S<sub>1–<i>x</i></sub>Se<sub><i>x</i></sub> via Anion-Tuning Chemistry

Rechargeable\nmagnesium batteries (rMBs) are promising candidates\nfor next-generation batteries in which sulfides are widely used as\ncathode materials. The slow kinetics, low redox reversibility, and\npoor magnesium storage stability induced by the large Coulombic resistance\nand ionic polarization of Mg<sup>2+</sup> ions have obstructed the\ndevelopment of high-performance rMBs. Herein, a Cu<sub>1.8</sub>S<sub>1–<i>x</i></sub>Se<sub><i>x</i></sub> cathode\nmaterial with a two-dimensional sheet structure has been prepared\nby an anion-tuning strategy, achieving improved magnesium storage\ncapacity and cycling stability. Element-specific synchrotron radiation\nanalysis is evidence that selenium incorporation has indeed changed\nthe chemical state of Cu species. Density functional theory calculations\ncombined with kinetics analysis reveal that the anionic substitution\nendows the Cu<sub>1.8</sub>S<sub>1–<i>x</i></sub>Se<sub><i>x</i></sub> electrode with favorable charge-transfer\nkinetics and low ion diffusion barrier. The principal magnesium storage\nmechanisms and structural evolution process have been revealed in\ndetails based on a series of ex situ investigations. Our findings\nprovide an effective heteroatom-tuning tactic of optimizing electrode\nstructure toward advanced energy storage devices.