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 three-dimensional 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 Cu2−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 areal capacity (>500 mAh g−1 at 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.