Directional migration of Mg²⁺ in hexagonal Se cathode to unlock high-energy-density Mg metal batteries

Conversion-type cathodes have raised much attention in rechargeable Mg metal batteries owing to the low reaction energy barriers and high specific capacities. However, the structural collapse during the charging/discharging cycles often leads to increased electrochemical polarization and capacity degradation. In this work, we demonstrate a directional Mg²⁺ migration strategy in hexagonal selenium (H-Se) cathodes with 3D interconnected chain structures, enabling the sequential diffusion of Cu and Mg ions along the specific crystal plane and the lattice-matching phase conversions from H-Se (100) to Cu_2-xSe (220) and eventually to MgSe (220). As a result, H-Se exhibits both a high specific capacity (630 mAh g^-1), excellent rate performance (434 mAh g^-1 at 2 C), high specific capacity and areal capacity (>500 mAh g^-1 and 5 mAh cm^-2) and long lifespan (∼1000 cycles). Finally, a H-Se based prototype pouch cell with a gravimetric energy density of 50 Wh kg^-1 is achieved.